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On the Versatility of Urethane/Urea Bonds...

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On the Versatility of Urethane/Urea Bonds: Reversibility, Blocked Isocyanate, and Non-isocyanate Polyurethane Etienne Delebecq,† Jean-Pierre Pascault,‡,§ Bernard Boutevin,† and François Ganachaud*,†,‡,§ †

Institut Charles Gerhardt, UMR 5253 CNRS, Ingénierie et Architectures Macromoléculaires, Ecole Nationale Supérieure de Chimie de Montpellier, 8 rue de l’école normale, 34296 Montpellier, Cedex 05, France ‡ INSA-Lyon, IMP, UMR5223, F-69621, Villeurbanne, France § Université de Lyon, F-69622, Lyon, France S Supporting Information *

3.2.1. Bulk Cross-Linked PUs by Cast Molding (CM) and Reaction Injection Molding (RIM) 3.2.2. Reaction in Solution: Coatings, Sealants, and Adhesives 3.2.3. Reaction with Water: Preparation of Solid Foams 3.2.4. Reaction in Water: Waterborne Coatings 3.2.5. Solvent-Free Reaction: Powder Formulations 3.2.6. Isocyanate Chemistry in Self-Healing Materials 3.3. Reversibility of Urethane Bonds and Transcarbamoylation during the Processing of Polyurethanes 3.3.1. Example 1: Reversibility and Rheological Behavior 3.3.2. Example 2: Reversibility To in Situ Compatibilize Polymer Blends 3.3.3. Example 3: Reversibility To Generate “Reprocessable” Cross-Linked Polymers 3.4. Reversibility as a Means of Recycling Polyurethanes 4. Model Reversible Urea/Urethane Bonds: The Case of Blocked Isocyanates 4.1. Introduction 4.2. Overall Mechanisms for Reactions of Blocked Isocyanates 4.2.1. Elimination−Addition Mechanism 4.2.2. Addition−Elimination Mechanism 4.2.3. More Complex Mechanisms 4.3. Recording the Deblocking Reaction 4.3.1. A Point about the Measurement of the “Deblocking Temperature” 4.3.2. Techniques 4.2.3. Operating Conditions 4.3.4. Presence of Chemical Additives 4.3.5. Catalysis Action 4.3.6. Conclusion 4.4. Chemical Structure−Reactivity Relationship of Blocked Isocyanates

CONTENTS 1. Introduction 2. Background on the Isocyanate Chemistry 2.1. Structure and Reactivity of Monoisocyanates 2.2. Urethanes and Ureas 2.2.1. Preparation of Urethanes, Ureas, and Thiourethanes 2.2.2. Mechanism and Reaction Parameters 2.2.3. “Transreactions” and Reversibility 2.3. Dimer, Trimer, and Polymer 2.3.1. Preparation 2.3.2. Reversibility 2.4. Allophanate and Biuret 2.4.1. Formation 2.4.2. Rate of Formation 2.4.3. Reversibility 2.5. Oxazolidone 2.6. The Reactivity of Diisocyanates and Polyisocyanates 2.7. “Polymeric Isocyanates” 2.8. Conclusion 3. Implications of the Urethane/Urea Bond Reversibility in Preparing Polyurethane Materials 3.1. Synthesis of Polyurethane Materials 3.1.1. Polymerization and Phase Transitions 3.1.2. Thermoplastic versus Thermoset Polymers 3.2. Processing of Polyurethane Materials

B C C C C C F G G H H H H I J J K L L M M

N N N O P P

P P Q Q R R R S S S S S T T V V W X X

M N Received: May 14, 2012

© XXXX American Chemical Society

A

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Chemical Reviews 4.4.1. Alcohols and Phenols 4.4.2. Amines 4.4.3. Oximes 4.4.4. Other N-Based Compounds: Amides, Imides and Imidazole, Pyrazole, and Triazole 4.5. Improving Industrial Systems 4.5.1. Conventional Systems 4.5.2. Chemical Tricks 4.5.3. Dual Networks 4.5.4. Blocking Reaction of Amine Groups 4.6. Blocked Isocyanates as Initiators for Other Reactions 4.6.1. Urea Linkage as a Source of Amine 4.6.2. Urethane Promoting Ring-Opening Polymerization of 1,3-Benzoxazine 5. “New” Trends: Toward Non-isocyanate-Based Polyurethanes 5.1. Phosgene-Free Isocyanate Precursors 5.2. Phosgene-Free Isocyanate-Free Precursors 5.2.1. Carboxamide/Alcohol Polycondensation 5.2.2. Dicarbonate/Amine Condensation 5.2.3. The Speciﬁc Case of Cyclocarbonate/ Amine Addition 5.3. Conclusion 6. Final Conclusion: Isocyanate-Free versus Nonisocyanate Solutions in View of an Industrial Context Associated Content Supporting Information Author Information Corresponding Author Notes Biographies Acknowledgments Symbols and Abbreviations References

Review

reactivity and the large yield of reaction one achieves, even in viscous systems or at low temperatures. One major drawback is the inherent toxicity of isocyanate molecules, which in the context of REACh becomes more and more problematic.1 Sure enough, the chemistry of isocyanate is complex since it entails a variety of reactions with other simple functions, such as alcohol or amine, but also self-additions and transcondensations. Prior reviews from this journal already described to a large extent the wide spectrum of reactions that this simple function could generate.2,3 Among the most recent literature survey on polyurethanes, the widely cited review article of Krol already scanned a wide spectrum of all old and modern aspects to prepare these materials.4 A series of comprehensive papers5−7 proposed mainly a patent compilation of an industrial trend picture, the generation of “blocked isocyanates” that notably changed the method of formulation of PU materials, allowing, for example, one-component or waterborne polyurethane formulations to be commercialized. From the application point of view, the major parameter of the system is the temperature at which reactive functions are regenerated, in other words, the deblocking temperature of adduct. The rate and extent of regenerating isocyanate however depends on many factors: the chemical function containing the active hydrogen, the isocyanate and blocking molecule structures, the solvent properties, the presence of a catalyst, and the reaction temperature. Also, the presence of an external molecule able to consume regenerated isocyanate functions severely shifts the equilibrium towards the adduct deprotection (vide inf ra). This review proposes to extend this approach of the reversibility of isocyanate-based bond to all products of this chemistry, namely, urethane, urea, uretdione, biuret, allophanate, and isocyanurate. From this arbitrary starting point, diﬀerent factors leading for instance to the choice or the design of a system of isocyanate and blocking molecule or to the reprocessing of polyurethane materials are proposed. Also, to respond to the problem of toxicity of the isocyanate precursors, which could entail problems at the end of life of the materials (owing in part to its reprocessability), recent pathways that (re)discovered elegant, straightforward chemistry to avoid the use or presence of isocyanate functions in ready-to-process materials are also reported here. In writing this review, equally dedicated to academic and industrial researchers, one obviously had to set some limits. First, we exclusively focused on the academic literature; patents are hardly touched upon. If selection makes this review more restrictive, clarity shall be improved since the open literature generally compares close systems to conclude about the inﬂuence of various parameters. In the same idea, emphasis will be placed on the most industrially used isocyanates; speciﬁc isocyanates with sulfonyl, chlorosulfonyl, phosphorus, or carbonyl functions attached to the isocyanic group will not be considered here (these were however described in the old reviews). Finally, we did not try to cover a speciﬁc range of time of publication, but selected what we believe were (and still are) the most relevant articles to the subjects treated here. In a ﬁrst part, we recall some background on isocyanate chemistry, including the structures and reactivity of mono- and diisocyanates, the description of reactions leading to urethane and urea groups, and the preparation of autocondensed functions, for example, allophanate, biuret, or oxazolidone. In a second part, the chemistry of polyurethanes is approached on the starting precept of the reversibility: both thermoplastic and thermosetting polymer syntheses are reviewed, insisting on the

X Y Y

Y Y Y Z AA AA AA AA AB AB AB AB AB AB AC AG

AG AH AH AH AH AH AH AI AI AI

1. INTRODUCTION Discovered by Wurtz in 1848, the isocyanates revealed their chemistry from systematic study during the nineteenth century, for example, by Curtius or Hofmann. The discovery of polyurethanes (PU), via the reaction of a polyester diol with a diisocyanate, by Bayer and his co-workers in 1937, made the diisocyanates one of the major chemicals produced in the world. Currently, the global isocyanate market grows by 5% per year, stimulated primarily by the polyurethane output expansion. The strength of the polyurethane market surely comes from the fact that a large variety of products can be prepared from essentially simple precursors, namely, toluene diisocyanate (TDI) and methylene diphenyl isocyanate (MDI), used for the manufacture of ﬂexible and rigid polyurethane products, respectively. Depending on whether linear or crosslinked networks are prepared, thermoplastic elastomers and thermoset resins (including foams) are sold every day as major components from paints to binders to materials for the aeronautic industry, for instance. The large range of mechanical properties that can be reached within such materials arise mainly from two physical chemical processes, that is, phase separation between hard and soft segments and hydrogen bonding between carbamate (or urethane) bonds. Another important advantage of isocyanate chemistry is its very deep B

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nitrogen atom is less available because of interactions with the aromatic ring. Generally speaking, if steric factors are neglected, any electron-withdrawing groups attached on the NCO moiety increase the positive charge on the carbon atom and so enhance the reactivity. This leads to the following reactivity classiﬁcation:11 ClSO2NCO > RSO2NCO (R = alkyl, aryl) > OP(NCO)3 > aryl-NCO (p-NO2C6H4− > p-ClC6H4− > pCH3C6H4− > p-CH3OC6H4−) > alkyl-NCO.

speciﬁc use of transcarbamoylation reactions to process the materials. In the third part, the blocked isocyanate chemistry is reviewed by discussing the addition between isocyanate group and reactive hydrogen containing compounds and the thermal reversibility of the formed products to ﬁnally describe the most widespread classes of blocking molecules. Finally, new trends in the isocyanate/polyurethane chemistry complete this review before comparing, in a conclusive discussion, isocyanate-free or non-isocyanate systems in the prospects of industrial outcomes.

2.2. Urethanes and Ureas

2. BACKGROUND ON THE ISOCYANATE CHEMISTRY

2.2.1. Preparation of Urethanes, Ureas, and Thiourethanes. According to the resonance forms presented in Scheme 1, isocyanate easily reacts with active nucleophilic reagents XH; the nucleophilic center X attacks the electrophilic carbon of isocyanate. The most important reactions involving isocyanate are additions on alcohol, thiol, or amine: the isocyanate group reacts with a hydroxyl group to yield urethane (Scheme 3a),

2.1. Structure and Reactivity of Monoisocyanates

The isocyanate group is a strained linear structure with two cumulated double bonds NC and CO. As in the case of other heterocumulenes, the isocyanate group reactivity is based on the polarization induced by the high electronegativities of nitrogen and oxygen atoms, which delocalize the electron density toward the nitrogen and oxygen atoms, as shown in Scheme 1.

Scheme 3. Common Reactions Used in Polyurethane Chemistry

Scheme 1. Resonance Structures of the Isocyanate Group

Surprisingly, the electron density distribution, as well as the geometry, of the isocyanate group is still unknown. Caraculacu8 has reviewed the major results in the ﬁeld of theoretical calculations and observed that the only point on which the diﬀerent authors agree is that the carbon atom possesses the minimum electron density; the charge magnitudes, the distances, and even the angle values diﬀer from one author to another. These considerations have important consequences in understanding the isocyanate reactivity and the mechanism of urethane/urea bond formation. Sacher9 found out that in the phenyl isocyanate molecule the NCO group is perpendicular to the benzene ring plane, whereas Bondarenko et al.10 claimed a completely planar structure for the whole phenylisocyanate molecule. Scheme 2

with a thiol to produce thiourethane (Scheme 3b), and with an amine to give urea bonds (Scheme 3c). The thiol group reacts with isocyanates in the same way as its oxygen analog, with much less reactivity.3,12 The reaction with water generates unstable carbamic acid, which decomposes into gaseous carbon dioxide and a primary amine, a common way to produce a wide range of PU materials with diﬀerent densities, for example, from moisture-curing coatings to urethane foams (Scheme 3d). The propensity of inorganic acids, such as HCl or H3PO4 used during the monomer synthesis,13 to slow isocyanate reactivity is known from industry. Unfortunately, we could not ﬁnd any mention of it in the literature nor a mechanistic scheme to explain the chemistry involved. 2.2.2. Mechanism and Reaction Parameters. 2.2.2.1. Autocatalytic Model. Seminal works have been carried out to study the reaction mechanism and establish the kinetic equations. Generally speaking, it is accepted that the nucleophilic center of XH is ﬁrst added to the electrophilic carbon of the isocyanate group (Scheme 4); then hydrogen atom is transferred to nitrogen. From this mechanism, the following second-order kinetics of the ﬁnal product formation is deduced (eq 1).

Scheme 2. Geometry and Electron Density of Phenyl Isocyanate According to Sacher

shows the electron density, the distances, and the angles for this molecule according to Sacher. These diﬀerences are essential since they determine the reaction mechanism by specifying (i) the isomeric cis/trans structure; (ii) which of the double bonds, NC or CO, is the ﬁrst aﬀected in the initial stage of reaction, and (iii) whether the lone electron pair on the

v=

−d[RNCO] = k[RNCO][R′XH] dt

(1)

where [RNCO] and [R′XH] represent the concentration at reaction time t of isocyanate and nucleophile groups, respectively. C

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Scheme 4. General Mechanism of Isocyanate/Nucleophile Reaction

Scheme 5. Autocatalytic Mechanism by Hydrogen Bonding (a) between Isocyanate and Urethane/Urea or (b) between Isocyanate and RXH (X = O, S, NH)

solvent: polarity, hydrogen bonding ability, and dielectric constant. Many groups studied the association states of the reactants mainly in the case of alcohol−isocyanate reaction, particularly Baker,22−27 Entelis et al.,28,29 Ephraim, 30 Oberth and Bruenner31 and Saunders and Frisch.32 Diﬀerent parameters (hydrogen bonding, dielectric constant, or basicities of species) were proposed to explain such eﬀects with more or less incomplete data explanation. Chang33 gathered all the kinetic data and proposed a general ion-pair mechanism based on the electron donation capability of the reactants measured by the electron donor number, DN, as deﬁned by Gutmann.34 First, the monomeric alcohol forms a hydrogen bonding complex with the isocyanate; then a molecule S (i.e., any molecule present in the reaction medium, e.g., solvent, alcohol) solvates the active hydrogen in the complex to form an ion pair, which undergoes urethane attack more easily (Scheme 6).

This model is generally only veriﬁed for low-to-medium degrees of conversion. As the conversion increases, the isocyanate group is activated by the formation of a hydrogen bond in which the urethane (or urea) moiety (Scheme 5a) or even the nucleophile (Scheme 5b) acts as a basic catalyst.14 Sato15,16 was the ﬁrst to develop a kinetic equation taking into account the autocatalytic eﬀect of alcohol and urethane groups. Based on the above mechanism, the formation rate is expressed as: −

k k [RNCO][R′XH][RNHCOXR′] d[NCO] = 1 3 k 2 + k 3[RNHCOXR′] dt +

k1′k 3′[RNCO][R′XH]2 k 2′ + k 3′[R′XH]

(2)

Aromatic isocyanates are highly reactive, and thus, the rate constant k2 and k2′ are much higher than k3[RNHCOXR′] and k3′ [RXH], respectively. The rate of reaction (eq 2) becomes −

Scheme 6. Uncatalyzed Urethane Formation Mechanism

d[NCO] = K1[RNCO][R′XH][RNHCOXR′] dt + K 2[RNCO][R′XH]2

(3)

where K1 =

k1k 3 k2

and

K2 =

k1′k 3′ k 2′

Global kinetic models that consider these autocatalytic contributions were successfully applied for describing both the noncatalyzed and catalysis-mediated isocyanate/alcohol and isocyanate/water reactions. These models include second-order kinetics during the initial stages of the reaction and subsequently describe autocatalytic third-order kinetics arising from the formation of hydrogen bonds.17,18 Majoros et al.19−21 synthesized aromatic and aliphatic polyurethane prepolymers by high throughput experiments and checked that the order of reaction in nucleophile is 2. 2.2.2.2. Concentration and Solvent Eﬀects. As explained above, the reactivity toward the NCO group is not constant but strongly depends on the possible association state of the reactants. The concentration of these complexes, such as dimeric or trimeric alcohol or alcohol−isocyanate or alcohol− solvent complexes, not only depends on the concentration of the reactants but also is determined by the characteristics of the

According to Scheme 6, the rate greatly depends upon the solvation power (equilibrium b). The electron donor character of the reactant S controls the reaction rate according to two roles: (i) it can catalyze the reaction by activating the isocyanate/alcohol complex or (ii) it inhibits the reaction by forming a hydrogen bonding complex with the oxygen of alcohol. The inhibition eﬀect is due to the reduction of the concentration of the alcohol/isocyanate complex and of monomeric alcohol, which entails catalytic activity. When the reaction is carried out in alkanes, the low DN value (lower than for alcohols) gives no interaction with the reactants. Here, solvent is considered as a diluent, which ﬁrst increases the monomeric alcohol concentration and second decreases the alcohol/isocyanate complex concentration. Thus, D

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Table 1. Solvent Physical Properties and Apparent Second-Order Rate Constants Observed for Butanol−Phenyl Isocyanate Reaction at 25 °C31,34,35 kobs × 103 L/(mol·min) [butanol]/[phenyl isocyanate] entry

aprotic solvent

hydrogen bonding index, γa

donor number, DN

dielectric constant, ε

0.15 M/0.15 Mb

1 2 3 4 5 6 7 8 9

cyclohexane chlorobenzene benzene nitrobenzene ethyl acetate acetonitrile dioxane DMF DMSO

0 1.5 0 2.8 8.4 6.3 9.7 12.3 7.7

0 −0.4 0.1 4.4 17.1 14.1 14.8 26.6 29.8

2.02 5.71 2.28 36.1 6.4 38.8 2.21 36.7 48.9

39 8.0 5.8 1.8 0.18 0.15 0.08

0.229 M/0.229 Mb

4.8 0.18

276 1410

a

Measured as an IR frequency shift in deuterated methanol (see refs 36 and 37 for details). bFor reactions carried out in bulk ([butanol]/[phenyl isocyanate] = 5 M/5 M), the rate constant was determined at 40 × 10−3 L/(mol·min)

performing the reaction in alkane medium gives two antagonist eﬀects leading to no signiﬁcant variation of the reaction rate compared with bulk reactions (Table 1, line 1). The common aprotic polar solvents that form hydrogen bonds decrease the monomeric alcohol (the active species) and the isocyanate/ alcohol complex concentration; the reaction rate is reduced (Table 1, rows 2−7). On the other hand, some aprotic solvents, for example, DMSO and DMF, have higher solvation power than alcohol (higher DN), which increases the solvated ion pair concentration, counteracting the lower complex concentration to produce a huge catalytic eﬀect (Table 1, rows 8 and 9). Chlorobenzene, benzene, nitrobenzene, ethyl acetate, acetonitrile, and dioxane have a certain solvation power but lower than the monomeric alcohol; the overall eﬀect of these solvents is an inhibition eﬀect. In the third step (Scheme 6c), the high dielectric constant of the solvent (ε) helps to dissociate the O−H bond. The independence of ε from the reaction rate indicates that OH dissociation is not the controlling step. 2.2.2.3. Catalysis. Tertiary amines and organometallic compounds, such as salts of tin or iron, present catalytic activity for the reaction between isocyanate and compounds bearing a labile hydrogen atom. The catalysis role of tertiary amine is easily explained by the mechanism described in Scheme 6. Amine plays a similar role as the solvent molecules, and thus the catalytic activity strongly depends on the donor number. With very high donor value in the range of 30−50, the addition of a small amount of tertiary amine does not aﬀect the concentration of the isocyanate/ alcohol complex, but its high DN value leads to strong solvation eﬀect. Table 2 presents an example of rate constants measured

for a model isocyanate−alcohol reaction catalyzed by tertiary amines. For amine catalysis, the most widespread industrial catalyst is 2,2′-diazabicyclo[2.2.2]octane or DABCO. It is important to note that their relative reactivity depends on catalyst concentration (Table 3). Table 3. Relative Rate Constants of Catalysts in an Isocyanate−Hydroxyl Reaction49a catalyst uncatalyzed DABCO

DBTDL DBTDL + DABCO a

a

DN34

relative rate constant

uncatalyzed quinoline pyridine tributyl amine triethylamine DABCO

30.4 36.7 a 50.7 a

1 6 11 54 134 1206

relative rate constant

0.1 0.2 0.3 0.1 0.5 0.1 + 0.2

1 130 260 330 210 670 1000

Reactants not given in the publication.

Generally speaking, metal-based catalysts act as Lewis acids toward alcohol or isocyanate to initiate the reaction. It has been shown that numerous organometallic compounds, such as organo-lead,39 -tin,17 -zirconium,40,41 -magnesium,42 -bismuth, and -iron43,44 are eﬀective catalysts for the isocyanate−hydroxyl reaction. The tin compounds, among which those represented by the general formula Bu2SnX2 (X is an anion) exhibit remarkable catalytic activity, for example, the dibutyltin dilaurate, DBTL. Mercury compounds combine two industrially remarkable properties: they initially retard the reaction (good latency) and thereafter, when the reaction commences, they provide a high conversion in short time reaction.45−48 Current investigations are carried out to replace these highly toxic compounds, so far with mixed results. Concerning metal-based catalysis, recent kinetic data40,50−53 tend to prove that the isocyanate activation by tin alkoxide originally proposed by Bloodworth and Davies54 is more relevant than alcohol activation by coordinating with tin as Entelis suggested earlier.55 In the Bloodworth’s mechanism, the cycle of catalysis involves N-coordination of the isocyanate with the tin alkoxide previously formed by alcoholysis of the starting tin compound as shown in Scheme 7A. Transfer of the alkoxide anion onto the coordinated isocyanate aﬀords an N-stannylurethane, which then undergoes alcoholysis to give the urethane and the original tin alkoxide. For a mixture of aliphatic and aromatic alcohols, catalysis choice orientates the reaction

Table 2. Relative Rate Constants for the Methanol/Phenyl Isocyanate (0.24 M/0.24 M) Reaction in Di-n-butyl Ether, in Presence of 0.03 M Amine Based Catalysts at 20 °C33,38 tertiary amine

concentration (%)

Not given. E

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Scheme 7. Catalysis of Isocyanate/Alcohol Reaction (A) by Organotin According to Bloodworth and Davies51 and (B) by Carbene According to Coutelier et al.63

Scheme 8. Possible Degradation Reactions of Urethane Bond

(Scheme 4). The urethane bond formed by addition of isocyanate on alcohol presents a weak C−NH bond with an activation energy of dissociation in the range of 100−130 kJ/ mol.64,65 In a temperature range starting from 150 °C and above, four degradation pathways were identiﬁed (Scheme 8): ﬁrst, the urethane linkage can be broken into the initial reagents (isocyanate and alcohol). Other pathways, leading to small molecules such as carbon dioxide and primary and secondary amines, eventually with oleﬁn formation, are also possible but occur more slowly and at higher temperature (Scheme 8b,c). In the presence of a more nucleophilic compound, urethane can rearrange, via transcarbamoylation, into a new urethane or urea compound (Scheme 8d). The main degradation pathway depends on the equilibrium between urethane bond and the starting functional groups. When temperature increases, the reverse reaction occurs (Scheme 8a). Generally speaking, the more easily the urethanes are formed, the less stable they are. Thus, according to the reactivity classiﬁcation presented before, the following decomposition temperature order is given: alkyl-NCO/alkyl-OH (250 °C) > aryl-NCO/alkyl-OH (200 °C) > alkyl-NCO/aryl-OH (180 °C) > aryl-NCO/aryl-OH (120 °C).67 Electronic eﬀects on the substituents also inﬂuence both the kinetics and pathway of the thermal decomposition: aromatic groups tend to favor the reverse reaction (Scheme 8a), while aliphatic ones promote urethane dissociation into primary amine and oleﬁn (Scheme 8b).67 Reverse reaction occurs

toward either aliphatic or aromatic urethane. At room temperature, in presence of DABCO or even in the absence of catalyst, hexamethylene diisocyanate (HDI) reacts ﬁrst with the phenolic OH; whereas with organotin catalyst, the aliphatic OH−HDI reaction is faster. For both catalyses, the aliphatic urethane is always thermodynamically favored, so upon reﬂuxing the phenolic-based urethane rearranges to an aliphatic urethane.56 The catalyst concentration inﬂuences the relative reactivity of reaction (Table 3) and according to Richter et al.,57 the tin catalyst has ﬁrst to dissociate before complexing the isocyanate group. This equilibrium is temperature sensitive and thus, the eﬀective concentration in catalytic active species is diﬃcult to ascertain. Several studies demonstrated that for both amine58 or tin59 catalysis systems, increasing the catalyst concentration beyond a certain level does not accelerate the reaction further. Synergism between tin and amine catalyst systems has been depicted by Tarasov et al.28 for addition of butanol on mchlorophenyl isocyanate, by Sojecki and co-worker60,61 for the TDI-macrodiols reactions or by Frisch and Rumao49 (Table 3). Finally, one should note the recent study on carbene as a catalyst of isocyanate−hydroxyl reaction under mild conditions, typically below 70 °C (Scheme 7B).62,63 2.2.3. “Transreactions” and Reversibility. The individual stages of the reactions between isocyanates and protic nucleophilic reagents are all reversible and, furthermore, characterized by an intrinsic reversibility under heating F

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(Scheme 9) or by a two-step process: ﬁrst reverse reaction of the urethane linkage (carbamate, -NHCOO-) above the melting temperature of HS2 (Tm ≈ 187.6 °C) and then reaction of thus-generated -NCO and -OH groups when the sample cools to room temperature (Scheme 8a).

exclusively for N-mono substituted polyurethanes while Ndisubstituted ones mainly degrade through dissociation to primary amine, oleﬁn, and CO2.68 At a temperature as low as 120 °C, urethane linkage has been proven to break in the presence of a nucleophilic reagent (Scheme 8d). O-Alkyl carbamates are quite stable and necessitate high reaction temperature or eﬃcient catalysis. Jousseaume et al. reported transcarbamoylation reaction of Nhexyl O-methyl carbamate with n-octanol aided by 1% bismuth triﬂuoromethanesulfonate or 1,3-dibromodistannoxane, giving yields in the range of 70−90%.69,70 Depending on the substituents, exchange reactions are also possible between two urethanes (Scheme 9) in the same range of temperature as the reverse urethane formation reaction (T ≈ 180−250 °C) or just below.

2.3. Dimer, Trimer, and Polymer

2.3.1. Preparation. In addition to the reaction of addition between an isocyanate group and reactive hydrogen-containing compounds, isocyanate groups also undergo homocyclization. The Scheme 11 presents the diﬀerent possibilities of isocyanate homocyclization depending on the catalyst used. Scheme 11. Isocyanate Self-Reactions

Scheme 9. Transurethanization Reaction

Exchange reactions have been evidenced by heating up a model compound to its melting temperature.71 Diphenylmethane diisocyanate was ﬁrst reacted with butanediol (BDO) and ethanol (molar ratio 2:1:1), from which puriﬁed product, called HS2 (Scheme 10, n = 1), was analyzed by size exclusion If aliphatic isocyanates dimerize with diﬃculties,72 dimers of aromatic isocyanates are obtained by heating the monomer with a nucleophilic catalyst. Dimerization is an equilibrium reaction catalyzed by trialkylphosphine, substituted pyridines, or trialkylamines.3,72 Phosphines and especially trialkylphosphine are much more eﬃcient than pyridine in catalyzing dimer formation and must be deactivated by a stoiechiometric amount of alkylating molecule such as benzyl chloride (Scheme 12).

Scheme 10. Structure of Model Compounds Used in Scrambling Reaction Obtained from the Reaction between MDI, BDO, and Ethanola

a

In Figure 1, molecules are labeled HS1 (for n = 0) to HS4 (for n = 3).

Scheme 12. Dimerization Mechanism of Aryl Isocyanate Catalyzed by Phosphine

chromatography (Figure 1) in tetrahydrofurane (THF) before and after reaching 220 °C. It clearly shows that in the liquid state, the strictly monodisperse HS length distribution of the model compound became polydisperse, with the most probable molar mass distribution expected from a step-growth polymerization. Such a “scrambling” reaction can be explained according to two mechanisms, either by transurethanization

Trimerization of alkyl and aryl isocyanates is promoted by catalysts such as alkali metal alkoxides.3 Tertiary amines and certain tin compounds also catalyze the reaction, though very slowly. The trimerization reaction is also favored by incorporating tin(II) bis(acetylacetonate) catalyst, which yields 33% of triphenyl isocyanurate.74,75 Urethane groups are known also to be good catalysts for isocyanurate formation.76 Recently, Buchmeiser’s team showed that N-heterocyclic carbene derivatives could produce 99% of phenyl isocyanate trimer but also promotes reaction of these with diols.77 Polyisocyanates are synthesized at low temperature (from −40° to 100 °C) by anionic chain polymerization. The homopolymer is not thermally stable because unzipping occurs and it forms monomers and cyclic trimers around 140 °C or

Figure 1. SEC curves of (a) the puriﬁed model compound (HS2 = 2MDI + 1BDO + 2ethanol) and (b) the mixture of compounds after heating at 220 °C. Peaks 1, 3, and 4 are equivalent in molar mass to HS1, HS3, and HS4. Reprinted with permission from ref 71. Copyright 2003 John Wiley & Sons. G

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even at room temperature in presence of basic impurities.78 For more details on polyisocyanates, readers are referred to the work of Bur et al.79 Isocyanates can also be converted into carbodiimides with elimination of carbon dioxide (Scheme 13) by prolonged

Scheme 14. Butyl Phenylcarbamate

Scheme 13. Carbodiimide Formation Starting from Isocyanates

products. For these reasons, isocyanurates are sometimes considered ﬁre-resistant additives. 2.4. Allophanate and Biuret

2.4.1. Formation. Urethane and urea adducts are active compounds thanks to the hydrogen atom linked to the nitrogen atom. Because they are less reactive than hydroxyl or amine compounds, the reactions occur especially when isocyanates are introduced in excess. By the reaction of isocyanate with urethane or urea group, an allophanate or biuret, respectively, forms, as summarized in Scheme 15. Due to the electronwithdrawing eﬀect of the carbonyl group, urethane and urea groups have a much lower reactivity than the secondary amine -N-H groups; in order to promote allophanate or biuret formation, a temperature typically greater than 110 °C or a catalyst addition is necessary. To decrease isocyanate toxicity, coating formulations favor high molar mass polyisocyanates from allophanate, biuret, and isocyanurate chemistry.91 When uretdione is heated in the presence of a nucleophile, such as an alcohol or an amine, two mechanisms were suggested to occur, one of which generates allophanate or biuret, respectively (see an example, Scheme 16, of allophanate formation); the reaction proceeds either by free isocyanate group releasing, generating new ureas, or through direct allophanate formation. The second mechanism competes also with linear polyurethanes formation, especially when isocyanate is in excess, and so, the gel point is reached at lower conversion (see section 3 of this review). Singh92 found that the reaction of TDI dimer with alcohols at 90 °C gives principally diurethanes with only traces of allophanates. Higher temperatures (in the range of 125−160 °C) and a catalyst such as triethylamine or N-methylmorpholine led to the formation of allophanate and triphenyl isocyanates. Di-n-propylamine, di-n-butylamine, and di-n-amylamine ruptured the dimer ring, whereas di-secbutylamine and dibenzylamine did not aﬀect it. 2.4.2. Rate of Formation. Formation reactions of urethane, uretdione, biuret, and allophanates compete between each other. In a system containing monoalcohol, monoisocyanate, and water in presence of catalyst93 and in a selective solvent, the following order of rate constants were found: urethane ≫ biuret > allophanate. At a given temperature, the equilibrium constant of biuret formation is higher by an order of magnitude than the equilibrium constant of allophanate formation.94 The rate of allophanate formation is much smaller than that of urethane formation; therefore allophanates are mainly formed when the overall ratio NCO/OH is higher than 1. For temperature below 60 °C, biuret and allophanate formation is very slow; no trace of these functions was found after 5 days when poly(oxypropylene)diol reacts with 4,4′diisocyanato-diphenylmethane systems.95 Schwetlich96 studied the alcohol−isocyanate reaction at 50 °C at NCO/OH ratio smaller than 1 and reported that the urethane formation lies almost completely on the product side, with or without the presence of common urethane formation catalyst (DBTDL or DABCO). Special catalysts such as amino alcohols, amidines, or alkoxide anions are needed to enhance

heating at high temperature (180−200 °C).80 Transition metals81 and especially organic compounds of main group 5, phosphine oxides,82 and nitrogen-containing molecules (such as amide, urea, anilide, or isocyanate derivatives)83,84 catalyze the condensation reaction under mild conditions. 2.3.2. Reversibility. If carbodiimide formation is clearly irreversible, homocyclization products can be split oﬀ and regenerate isocyanate at high temperature. Querat et al.73 studied by infrared spectroscopy the dissociation of 2,4-TDI dimer without catalyst. They reported ﬁrst-order kinetics up to 80% conversion at 160 °C and observed a relatively high activation energy (Ea = 142 kJ/mol). Without catalyst, the free isocyanate regeneration, which depends on the parent isocyanates, occurs only at high temperature. The uretdiones of 2,4-TDI, isophorone diisocyanate (IPDI), and HDI (for structures, see Scheme 21) decompose without catalyst at 150, 160, and 200 °C, respectively.7,85 The presence of the remaining catalyst lowers the dissociation temperature. In presence of tributylphosphine, Saunders2 reported that TDI dimers are 10% dissociated at 10 °C, 25% dissociated at 25 °C and totally dissociated at 80 °C in benzene solution. Although the isocyanates can be regenerated from their dimers by heating at moderate temperature, a higher temperature (>250 °C) leads to isocyanurates via trimerization. While usually considered thermally stable,86 isocyanurate degradation has been observed by infrared spectroscopy, thermogravimetric analysis,87 and 1H and 13C nuclear magnetic resonance (NMR).88 Kordomenos et al.89,90 studied by infrared spectroscopy the degradation kinetics of model isocyanurate synthesized by homocyclization of phenylisocyanate. A temperature above 417 °C is required to observe a degradation rate through volatilization superior to 0.3 h−1 (Table 4); this temperature Table 4. Kinetic Parameters for the Degradation Reaction of Butyl Phenyl Carbamate and Various Isocyanurates

butyl phenylcarbamate isocyanurate of phenylisocyanate poly(aromatic isocyanurate) poly(aliphatic isocyanurate)

Ea (kJ·mol−1)

A (h−1)

TD (°C) at k = 0.3 h−1

ref

161.8 252.2

7.38 × 1012 3.69 × 1016

277 417

89 89

184.1

8.46 × 1010

441

90

246.7

2.12 × 1016

410

90

must be compared with 277 °C for butyl phenylcarbamate 1 (Scheme 14). For isocyanurate-based polymer, synthesized through self-reactions between mono- and diisocyanate, they reported a higher stability for aromatic than aliphatic isocyanurate, which is explained by the strong char formation during aromatic isocyanurate degradation, which retains volatile H

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Scheme 15. Two Chemical Pathways Proposed for Allophanate and Biuret Formations

Scheme 16. Transreactions between Uretdione and Alcohol

allophanate and isocyanurate formation; nevertheless alcohol is then only partly consumed.96 At high NCO/OH ratio and high temperature, Kogon97 found that the equilibrium constant of the allophanate 4 formation (Keq) equals 0.2 at 128 °C. This constant is independent of the ratio phenyl isocyanate 2/ethylcarbanilate 3 (varying from 9 to 12) but decreases when temperature increases (Scheme 17). When the reaction is catalyzed by tertiary amine like N-methylmorpholine, triphenylisocyanurate is the only ﬁnal product formed via allophanate intermediates. In bulk, Heintz98 measured an increase of allophanate concentration with temperature. The amount of side products, namely, the percentage of nitrogen atom involved in allophanate bond, was quantiﬁed by 1H NMR as a function of time and temperature for the diol−isocyanate reaction (NCO/OH = 1.6) (Figure 2). With moderate temperature increase from 108 to 145 °C, the amount of allophanate increases from a negligible content (less than 1%) to over 10% of all nitrogen-containing compounds. Both low and high molecular species equally participate to the allophanate formation. The presence of such trifunctional groups broadens the molar mass distribution. In that case, isocyanurate concentration is much less than the allophanate one. 2.4.3. Reversibility. Allophanate and biuret bonds, as products of addition reactions between isocyanate and compounds bearing active hydrogen, are thermally reversible when heated. Their thermal stability is generally considered as lower than that of the urethane bond. The decomposition rates in halogenated solvent were reported by Kogon (Scheme 18 and Figure 3). The reaction rate increases with temperature for allophanate and biuret groups. Allophanate is less stable than biuret (comparison between 6 and 7 in Figure 3) and its dissociation rate increases with temperature but also with

Figure 2. Allophanate concentration during the reaction between a secondary hydroxyl of poly(propylene glycol) and MDI.98

Scheme 18. Allophanate (X = O) and Biuret (X = N) Decompositions

aromatic substitution (comparison between 5 and 6 in Figure 3). An interesting work of Duﬀ et al.100,101 compared the thermal stability of the diﬀerent R−NH−CO−X bonds. Such study was performed on an isocyanurate-rich polymer, obtained by homoaddition of 4,4′-methylene diisocyanate (MDI) in the presence of stannous octoate, leading to aromatic N-functions such as urea, amine, uretdione, biuret, and isocyanurate. The thermal stability of this resin was studied by recording the relative content of N-function by 15N cross-polarization/magic angle spinning spectroscopy (CP/MAS NMR) from room temperature to 500 °C (Figure 4). Upon increase of the temperature, degradation of isocyanate, uretdione, and biuret led to an increase of urea linkage concentration up to 230 °C. Then, between 240 and 260 °C, urea degradation occurred leading to amine bonds whereas isocyanurate linkage broke from 280 °C. Since allophanate has been proven to be less

Scheme 17. Allophanate Formation from Phenyl Isocyanate 2 and Ethylcarbanilate 3

I

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Scheme 19. Epoxy Isocyanate Reactions

trimerization, but not the oxazolidone formation, while tertiary amines favor the isocyanurate ring formation.106 Thus, depending on the catalyst and the reactant ratio, the crosslinking density of diepoxy/diisocyanate material can be adjusted by varying the oxazolidone/isocyanurate ratio. Okumoto et al.107 studied model reactions through ab initio calculations to determine the main reactions pathways (Scheme 20). The 2-oxazolidone 9 is thought to be produced by a dual SN2 reaction, where the catalyst (e.g., Cl−) is a nucleophile and a leaving group on the ethylene−oxide carbon. Isocyanurate 10 is generated by the stepwise association of three isocyanate molecules, where one of the molecules is initially linked with a base. The six-membered ring isocyanurate is isomerized stepwise into the isocyanate and 2-oxazolidone components. A tetrahedral type of complex 11 between the isocyanurate and a base-catalyzed ethylene oxide is the key intermediate for the isomerization.

Figure 3. First-order rate constant for allophanate and biuret decompositions (structures are given in Scheme 18).

2.6. The Reactivity of Diisocyanates and Polyisocyanates

Diisocyanates are extensively used as monomers in the manufacture of polyurethanes formed by addition reactions (vide inf ra). The use of a diisocyanate is directly proportional to the price of diamine supply and phosgenation; since inexpensive aliphatic diamines are not readily available as precursor for diisocyanates, the aromatic diisocyanates, that is, NCO groups borne by an aromatic moiety, represent more than 95% of the market. Among them, two products are especially widespread, TDI and MDI. Due to its small size and symmetry, the para-phenylenediisocyanate molecule (PPDI) provides great reactivity diﬀerence between the ﬁrst NCO to react and the remaining one. Regarding the aliphatic molecules, the main reactants are HDI and to a lesser extent IPDI and bis(4-isocyanatocyclohexyl)methane (also called hydrogenated MDI, HMDI; see Scheme 21 for the structure of this product). m-Tetramethyl-xylylene (TMXDI) is a special molecule since it allows preparation of solvent-free waterborne polyurethane dispersions due to its low reactivity toward water (for a recent review, see ref 108). For a symmetric diisocyanate, both isocyanate groups initially present the same reactivity (k1 = k2 = k). But, as the NCO group itself exhibits an activating eﬀect on the isocyanate reactivity, the second (unreacted) isocyanate group undergoes a substitution eﬀect and its reactivity can be lowered (k′) by the electronic and steric eﬀect of created urethane or urea group (see Scheme 22a as an example). The ﬁrst isocyanate group of 2,6-TDI reacts 5.8 times faster with poly(propylene) glycol at 60 °C than the second one.109 For longer distance between diisocyanates, the reactivity diﬀerence due to substitution is smaller. For HMDI, the ratio between k and k′ is reported to equal 1.4.110 It should be noted that a temperature increase

Figure 4. Content of N-functions measured by 15N CP/MAS NMR for each thermal decomposition temperature. Uretdione functions were identiﬁed but not quantiﬁable.

stable than biuret, the thermal stability of isocyanate-based bonds ensues:73,95 allophanate < biuret < urethane < urea < isocyanurate. Very recently, Kozakiewicz and co-workers showed on a model allophanate molecule (obtained from phenyl isocyanate and N-ethylurethane) that not only transurethanization but also urethane decomposition could regenerate isocyanate functions for further curing. They made use of these ﬁndings to introduce functional blocking agents for eﬃcient gelling reaction during heat curing.102 2.5. Oxazolidone

Heating an epoxy function in the presence of an isocyanate group leads to the formation of an oxazolidone ring, together with the trimerization of isocyanate and the homopolymerization of epoxy.103,104 It was found that the main reactions take place step by step while the temperature increases: (i) isocyanurate formation (Scheme 19a), (ii) epoxy−isocyanate reaction leading to oxazolidone rings (Scheme 19b), and (iii) isocyanurate decomposition by epoxy groups producing oxazolidone rings (Scheme 19c).105 Isocyanurate rings were found to be stable in the presence of epoxide and isocyanate excesses, that is, reaction b is faster than c.103 A catalyst like imidazole decreases the reaction temperature and promotes the J

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Scheme 20. Diﬀerent Reactions Theoretically Able To Occur in Model Epoxy−Isocyanate Reactions in the Presence of a Nucleophilic Catalyst

Adapted from ref 107.

Scheme 21. Names and Structures of Common Diisocyanates with Their Room Temperature State in Italicsa

a

Values in parentheses indicate the melting temperature.

An eﬃcient catalysis can compensate the reactivity diﬀerences; for example, the secondary isocyanate group of IPDI is 1.6 times more reactive with n-butanol at 50 °C than the primary NCO group. Using DBTDL, the reactivity of the secondary NCO group is 12 times superior to the primary one; while in the presence of DABCO, the primary NCO group is 1.2 times more reactive than the secondary one.72

lowers the reactivity diﬀerences. The reactivity ratio varies from 2.9 at room temperature to 1 at 70 °C for MDI reacting with nbutanol.110,111 Asymmetric diisocyanates exhibit two initial diﬀerent reactivities (k1 and k2), which are again altered by the reaction of the ﬁrst isocyanate group. Thus, a reaction between an asymmetric diisocyanate and nucleophilic compound leads to four reaction rate constants as exempliﬁed in the case of 2,4TDI in Scheme 22b. These reactivity diﬀerences strongly complicate diisocyanate kinetics study. Table 5 shows the reactivity ratios between isocyanate groups of 2,4-TDI reacted with diﬀerent types of alcohol. The discrepancy in the results is attributed to the temperature increase, which decreases the selectivity. According to the work of Burel et al.,112 the selectivity between primary and secondary isocyanate groups of IPDI (ksecondary/kprimary) decreases with temperature going from 31 at 25 °C to 15 at 90 °C.

2.7. “Polymeric Isocyanates”

During the course of dimer and trimer preparations, MDI and HDI, symmetric diisocyanates with close reactivity for both groups, give both the targeted product and oligomers of low molar masses.113 Dimerization of MDI (12) in bulk was studied in a wide range of temperature, in both the solid and liquid state (Scheme 21 and Figure 5). In an eﬀort to keep MDI under its monomeric state, storage temperature is thus set either at very low temperature or just above the melting point, where the rate of formation of dimer is the lowest.114 K

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Scheme 22. Consecutive Reaction Steps in the Case of Diisoocyanate/Alcohol Reaction Starting from (a) 2,6 TDI and (b) 2,4TDI

Table 5. Typical Example of Reactivity Ratios of an Asymmetric Diisocyanate (2,4-TDI) against Hydroxyl Groups, Taking the Lowest Reaction Rate as the Reference (k′2) diisocyanate

alcohol

2,4-TDI

n-butanol, 25 °C8 poly(propylene glycol), 60 °C109

a

Scheme 23. Example of Pentamer Produced from the SelfAddition of Isophorone Diisocyanate

r1 = k1/k′2a r2 = k2/k′2b r′1 = k′1/k′2 18.1 15.8

4.5 2.6

4.2 2.1

NCO group in para position. bNCO group in ortho position.

formation of low molar mass allophanate species.117 High molar mass polymer bearing several isocyanate groups can also be prepared by copolymerizing isocyanate bearing monomers such as 2-isocyanatoethyl methacrylate118 or 3-isopropenylα,α′-dimethybenzyl isocyanate.119−121 2.8. Conclusion

The high reactivity of the isocyanate group toward any nucleophilic compound promotes its reaction with both the reactant and the formed product from isocyanate/nucleophile reaction, if it is suﬃciently nucleophilic. Generally speaking, the nucleophilicity of active hydrogen compounds governs the reactivity toward isocyanate. Table 6 presents generally accepted relative reaction rate as a rule of thumb. All these reactions compete among each other and all are reversible. Moreover, for diisocyanate, the situation is even more complicated by the intrinsic diﬀerence of reactivity between the isocyanate groups. Thus, studying these systems and above all kinetics from a fundamental prospect is extremely complex. On the other hand, the ﬂexibility of the isocyanate chemistry is a convenient tool to stem from a wide range of compositions in an industrial way, as shown in the following section.

Figure 5. Rate of dimer formation as a function of temperature. Reprinted from ref 114 with permission. Copyright 2009 BASF company.

Greater diﬀerence of reactivity between the two isocyanate groups facilitates the control of reactions.115 In the case of the asymmetric IPDI, the primary isocyanate group reacts preferentially, but reaction with the secondary isocyanate group likely occurs (r = 2−3).85 Thus, the trimerization reaction of isophorone diisocyanate leads to species from isocyanate trimer to tridecamer, trimer and pentamer (which structure is given in Scheme 23) representing 71 and 19 wt % of the full mixture, respectively.116 One should note that polymeric isocyanates strongly increase the viscosity of solution. Another way of increasing the viscosity consists in reacting diisocyanate with monoalcohol to favor the

3. IMPLICATIONS OF THE URETHANE/UREA BOND REVERSIBILITY IN PREPARING POLYURETHANE MATERIALS The synthesis of PU is carried out by one or few of the reactions described in Scheme 3. If the functionality of the hydroxy-containing compounds or the isocyanate is increased L

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proportional to the diﬀerence between the solubility parameters, δ, of the soft and hard blocks) and the degree of polymerization (N). It is called reaction (or polymerization) induced phase separation. The role of hydrogen bonding has extensively been discussed, and it has been demonstrated that hydrogen-bonded species grow at the same time that phase separation evolves.122−124 At the end, the resulting microphase separation morphology is composed of soft-rich and hard-rich phases. Depending on the composition, the sizes of the hard domains are in the range of 10−50 Å, but also cocontinuous or inverted structures can be obtained. Usually the SSs are amorphous with a glass transition Tg lower than room temperature, and hard domains are semicrystalline with a Tg in the range of 50−100 °C and melting temperatures from 120 to 180 °C. As the result of phase separation, a physical network is formed. The other independent transformation that can take place during network formation but also during the synthesis of a linear polymer in bulk is vitriﬁcation. This transition occurs at the particular extent of reaction where the increasing glass transition temperature, Tg, of the reacting system equals the instantaneous value of the cure temperature. At this time, the macroscopic behavior of the system changes from a liquid (for a linear or a growing network) or rubber (for a growing network) to a glass. This means an increase of several decades in the value of the storage modulus. Several equations have been proposed to describe the increase of Tg with extent of reaction; it may, however, simply be predicted by characterizing the initial and ﬁnal materials using diﬀerential scanning calorimetry.126 3.1.2. Thermoplastic versus Thermoset Polymers. For linear segmented TPU, phase separation is the essential parameter for controlling the ﬁnal thermomechanical behavior. These polymers oﬀer unique possibilities for tailor-made materials through the variation of the block lengths and composition. They are widely used for high-performance applications, especially when high tear and tensile strengths or good wear and abrasion resistances are required. For some applications, if the soft segment or the chain extender have a functionality higher than 2, the same process can be used to synthesize cross-linked cast PU. For thermosetting polymers, two structural transformations are likely to occur during network formation. At the end of the reaction, practically all constituent units are covalently bonded into an inﬁnite three-dimensional structure. This means that during the polymerization, the system evolves from a collection of molecules of ﬁnite size to an inﬁnite network, proceeding through the gel point at which the inﬁnite network structure

Table 6. Relative Reactivity of Active Hydrogen Compounds against Isocyanatea hydrogen active compound

formula

relative reaction rate (noncatalyzed, 25 °C)

primary aliphatic amine secondary aliphatic amine primary aromatic amine primary hydroxyl water secondary hydroxyl urea tertiary hydroxyl phenolic hydroxyl urethane

R−NH2 R2NH Ar−NH2 RCH2−OH HOH R2CH−OH R−NH−CO−NH−R R3C−OH Ar−OH R−NH−COOR

1000 200−500 2−3 1 1 0.3 0.15 0.005 0.001−0.005 0.001

a

Isocyanate structure was not given. Data from ref 110 were normalized according to the rate of the water-isocyanate reaction.

beyond 2, branched and possibly cross-linked polymers are produced. The nature of the polyol (polyether, polyester, polycarbonate, or polyoleﬁn) and of isocyanate components can vary widely, making PUs one of the most versatile polymers to produce a wide variety of materials such as high performance elastomers and tough thermoplastic, foams (ﬂexible and rigid), coatings, adhesives, sealants, or ﬁbers. In the following, we summarize the main steps occurring while synthesizing and processing PU copolymers, before showing some examples of the use of the reversibility in real industrial cases. 3.1. Synthesis of Polyurethane Materials

3.1.1. Polymerization and Phase Transitions. To obtain TP or cast PU elastomers, one or two diols can be reacted with the isocyanate. When two diols are used, the ﬁrst one is a macrodiol with a molar mass in the range (5−10) × 103 g·mol−1, and the second one is a short diol, typically 1,4butanediol. The PU may be prepared either by the one-shot process (three components reacting together) or by the prepolymer approach (a prepolymer is prepared ﬁrst and then reacted with the short diol as the chain extender) (Scheme 24). During the reaction in bulk, the hard segments (HSs) become incompatible with the soft segments (SSs), so that phase separation takes place during polymer-forming urethane or urea reactions. Small angle X-ray scattering, at a certain stage of the reaction, shows the apparition of a peak that marks the onset of phase separation.122−125 The driving force for microphase separation is thermodynamic in nature; it is governed by the product of the interaction parameter (χ,

Scheme 24. Schematic Synthesis of Linear Segmented Polyurethane

M

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appears for the ﬁrst time. Before the gel point, the material is a liquid with a ﬁnite value of viscosity. As the reaction proceeds, the viscosity increases and it tends to inﬁnite at the sol−gel critical transition, when a giant macromolecule percolates through the sample. Gelation can be roughly estimated as the time (extent of reaction) at which the elastic modulus becomes higher than the loss modulus, or more precisely, at which the ratio of both moduli (tan δ) becomes independent of the frequency of the experimental run. The extent of reaction at the gel point depends on the functionality (number of reactive sites) of precursors: the higher the functionality, the lower the gel conversion. After gelation, the mass fraction of the insoluble giant structure increases continuously and so does the elastic modulus of the sample.126 For thermosetting polymers, shaping of the part has to be performed before the gel point where the material is still in the liquid state. Note ﬁnally, that in the case of PU, the boundary between thermoplastic and thermosetting is not so clear. Particularly, linear segmented PU can be processed as TP granules or as cast PU. In addition when some urea links are introduced in the linear chains, the polymer became intractable and has to be processed like a thermosetting polymer.

between SSs and HSs occur; the part solidiﬁes and is then ejected. Contrary to RIM, a “two-step” process is often used for cast molding. At the end of the prepolymer synthesis (Scheme 24), the diisocyanate excess depends on the initial excess and also on the reactivity ratio between the two isocyanate groups. As sustainable development is the major trend for the future, the diisocyanate molecules in excess can be a problem to solve. There are diﬀerent routes to decrease the toxicity of volatile low molar mass diisocyanates. The ﬁrst route is to increase the boiling temperature and to decrease the vapor tension by making adducts (as examples trimers of IPDI or adducts of HDI + TMP). Other possibilities are to distillate the diisocyanate excess after the synthesis of the prepolymer or to play with the diﬀerent reactivities of the diisocyanate (like TDI 2,4 or PPDI) to prepare “diisocyanate-free” formulations (vide inf ra). Finally, in addition to cast and reaction injection molding, thermoplastic polyurethanes are sometimes processed in the melt state where the urethane bond reversibility behavior is exacerbated (see section 3.3). 3.2.2. Reaction in Solution: Coatings, Sealants, and Adhesives. Thanks to their excellent adhesives properties, good weather stability, and versatility of formulation, thermosetting PUs are currently the preferred materials for coatings, sealants, and adhesives. The majority of conventional formulations are solvent-borne, traditionally containing about 25% solids. Organic solvents are used to disperse the diﬀerent constituents of the formulation and to provide the low viscosity needed for conventional application methods (spray, roller coating, etc.). Solvent-based PU ﬁlms are formed by simultaneous cross-linking of polymer network precursors and solvent evaporation. This is a complex process in which the cross-linking rate and the development of the network depend on the content, nature, and volatility of the solvent.127 Because coating and paint producers as well as end-users wish to reduce the volatile organic compound (VOC) emissions, progress is being made to increase the solid contents of solvent-based liquid coatings.128 3.2.3. Reaction with Water: Preparation of Solid Foams. PU foams are an important commercial application of PU chemistry. They are prepared from basically the same raw materials as elastomers by the “one-step” method (Scheme 24) and with the use of a RIM machine. Here however, instead of the chain extender, water is used to react with isocyanates and form an amine and CO2, which results in foaming. The diamine thus created can then react with isocyanate groups giving polyurea short segments. A catalyst can be used; both reactions are exothermic and proceed simultaneously to generate a cross-linked network, which is blown into a cellular structure by the heat expanded CO2 and entrapped air. A silicone-based surfactant is also introduced as a cell control agent. Self-assembly of the HSs leads also to the formation of hard domains.124 The main diﬀerence between rigid and ﬂexible foams is the molar mass of the polyol component, less than 103 g/mol for rigid foams and between (2 and 8) × 103 g/mol for the ﬂexible ones, and also the average functionality of the polyol, much higher than 2 for rigid foams. Industrially, the catalysts not only bring faster rates for the reaction but also inﬂuence the selectivity and thus permit control of the balance between the following reactions: (i) water + isocyanate producing urea linkages and CO2 as blowing molecule; (ii) alcohol +

3.2. Processing of Polyurethane Materials

3.2.1. Bulk Cross-Linked PUs by Cast Molding (CM) and Reaction Injection Molding (RIM). Because isocyanate/ alcohol and isocyanate/amine reactions are very fast, give no byproduct, and attain high degree of conversion, they are well suited for direct polymerization of two reactive liquids, A and B, in a mold. Part A is the isocyanate-based component, and part B is composed of the diols plus a catalyst (or diamines) together with usual additives like antioxidants, pigments, and so on. PU formulations for reactive molding follow the same strategy as the one described in Scheme 24. Two cases are distinguished: (i) all precursors bear the same functionality of 2 and then an excess of isocyanate is used to generate allophanate, resulting in further cross-linking; (ii) some precursors of higher functionality are introduced, for example, reactive oligomers like polycaprolactone with functionality equal to 3 or 4, isocyanates, such as liquid MDI with f ≥ 2.3 or polymeric MDI, or chain extenders like trimethylol propane, TMP. When increased rigidity and high-temperature performance are desired, further cross-linking may be accomplished via isocyanurate formation, using aromatic isocyanates rather than aliphatic ones (Scheme 11). In this case, base catalysts, such as alkoxides, quaternary ammonium, or phosphonium, promote this reaction. The main diﬀerence between cast molding and RIM is to be found in the reactivity. For cast molding, the pot-life is usually higher than 5 min, whereas for RIM it has to be less than 1 min, which means that isocyanate is reacted either with amine or with alcohol in presence of catalyst. RIM is similar to thermoplastic injection molding except for the curing reaction in the mold. Two reactive monomeric liquids, A and B, are mixed together by impingement and injected into the mold by the so-called “one-step” method (Scheme 24). It involves the high-speed mixing of two or more reactive chemicals, just as they are injected into the mold. The low-viscous mixture ﬁlls the mold at relatively low temperatures and pressures. The reactions must be synchronized with the process so that gelation does not occur during the ﬁlling time. In the mold, polymerization and usually reaction induced phase separation N

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Scheme 25. Model Reactions Involving Isocyanate Group in Polyurethane Foams

Table 7. Rate Constants of the Diﬀerent Reactions Involved in Polyurethane Foam Synthesis Using Various Catalysts cross-linking activityb

catalyst triethylamine (TEA) 2,2′-diazabicyclo[2.2.2] octane (DABCO) trimethylamino-ethylethanolamine DBTDL a

urethane formation or gelling activity

urea formation or blowing activity

allophanate formation

k1,a L2/(g·mol·h)

k2,a L2/(g·mol·h)

k3, L2/(g·mol·h)

11.6 109

6.0 14.5

28.9

43.3

380

48

4.8

30

17

144

0.72 0.32

isocyanurate formation

biuret formation k4, L2/(g·h)

k5, L2/(g·mol·h)

0.1 0.18

k6, L2/(g·h)

k7, L2/(g.mol.h)

K−1[BH]. It is important to note that, in that case, no curing reaction occurs except if the blocking molecule is eliminated. The rate is expressed by eq 6:

(elimination step). Then, the free isocyanate reacts with a substrate containing nucleophile to form thermally more stable bonds (addition step). The overall reaction is summarized in Scheme 28. Scheme 28. (a) Blocking and Deblocking Reaction of Isocyanate and (b) Curing Reaction

rate =

Scheme 30. Addition−Elimination Mechanism

4.2. Overall Mechanisms for Reactions of Blocked Isocyanates

Optimizing a controlled blocked isocyanate system goes through a compulsory step of studying the actual thermal behavior. However, studying mechanisms and kinetics is hard work owing to numerous reactions that occur at the same time, most of which are reversible. 4.2.1. Elimination−Addition Mechanism. A blocked isocyanate is an adduct containing a weak bond formed by the reaction with a hydrogen active compound. At high temperature, the adduct splits oﬀ to regenerate the free isocyanate group (elimination step), which in turns reacts with a nucleophilic substrate (addition step). This elimination− addition mechanism is shown in Scheme 29.

Because the initial stage corresponds to the nucleophile attack on the blocked isocyanate, this mechanism can be thought of as chemical deblocking. The rate of cured material formation is (eq 7) rate =

Scheme 29. Elimination−Addition Mechanism

(4)

where [NuH], [BI], and [BH] stand for the concentration of nucleophile, blocked isocyanate, and blocking group, respectively. The eq 4 shows two limiting cases given below. If the curing reaction is much faster than the blocking reaction, K2[NuH] ≫ K−1[BH], then, the rate is independent of the nucleophile content (eq 5): rate = K1[BI]

K3[NuH][BI] K −3[TI]

(7)

where [TI] is the tetrahedral intermediate concentration. 4.2.3. More Complex Mechanisms. For some systems, both mechanisms cohabit depending on the system composition (solvent, isocyanate, blocking molecule, coreactant, etc.) and on the temperature. At lower temperatures, the addition− elimination pathway is favored, while at higher temperature, the elimination−addition mechanism is prominent.151,152 Furthermore, the kinetic models presented above do not consider many eﬀects such as secondary reactions taking place in the whole process (allophanate, biuret, isocyanurate, etc.).153 To take into account these limitations, it would be advisable to use a more general model such as model-ﬁtting approaches or even a model-free kinetic approach,154 of which details are given in Supporting Information.

Considering that the rate constant K−2 is negligible, the ﬁnal product formation, under steady-state hypothesis, is written as in eq 4: K1K 2[NuH][BI] K −1[BH] + K 2[NuH]

(6)

With this system, a part of the intermediate isocyanate reverts to starting materials, resulting in an overall slower rate of reaction, as in the case of blocked isocyanate reacting with alcohols. 4.2.2. Addition−Elimination Mechanism. While the elimination−addition mechanism is mostly mentioned in the literature, the reverse mechanism, proceeding through a nucleophile molecule addition on the blocked isocyanate followed by the blocking molecule elimination (as shown in Scheme 30) has been proven to apply to some systems where the kinetics were speciﬁcally studied.

Since the adduct is less (or even not) sensitive toward nucleophilic molecules and in particular toward water, blocked isocyanates make it possible to prepare a one-component reactive system with long pot life and lower toxicity and to drastically reduce or even suppress solvent use.

rate =

K1K 2[NuH][BI] K −1[BH]

4.3. Recording the Deblocking Reaction

Because of the discrepancy between the analytical techniques and for the same technique, between the sample preparations and the analytical methods, comparison between two deblocking temperatures from distinct publications is tricky, if not impossible. For instance, whatever the chosen analytical method, the dissociation temperature can be measured during the pure dissociation reaction (without a coreactant) or during a reaction (with coreactant), which by competing with the

(5)

This situation occurs for instance in the case of blocked isocyanate reacting with an amine. The reaction is described by an overall ﬁrst-order reaction, zero-order in amine and ﬁrstS

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blocking equilibrium decreases the apparent dissociation temperature. This section aims at bringing an overview of the diﬀerent aspects to take into account while measuring deblocking temperature. The most traditional techniques are described here, e.g. spectroscopic techniques (FTIR, NMR...), thermal ones (DSC, TGA) or titration methods. 4.3.1. A Point about the Measurement of the “Deblocking Temperature”. Choosing a blocked isocyanate system requires taking into account several aspects. Obviously, the adduct must be extremely stable at room temperature and very reactive at high temperature. In the patents as well as in the academic literature, a system isocyanate-blocking molecule is commonly described by its deblocking temperature. However, exactness would require giving a reaction rate for a particular temperature since the deblocking reaction is associated with a kinetic mechanism, so all the rate constants are governed by the temperature through the Arrhenius relationship (eq 8). k = A e−Ea /(RT )

The onset temperature for the diﬀerent systems is taken as the temperature at which the rate of dissociation is not nil anymore. Caprolactam 24 blocked isocyanate 21 regenerates a free NCO group more slowly and at higher temperature (T = 180 °C) than the other systems due to its weak activation parameters (Figure 7). Methyl t-butyl ketoxime 23 blocked

(8) Figure 7. Dissociation rate constants of blocked isocyanate complexes in presence of amine as a function of temperature calculated from activation parameters measured in toluene (data from ref 150).

According to the above equation, whatever the temperature, reaction occurs, albeit at various rates. The exponential factor (A) is increased by a temperature increase, and for a given temperature variation, the higher the activation energy (Ea), the more important the change. To get an accentuated threshold temperature between blocked and free isocyanate, one should increase the activation energy. However a compromise is necessary, because for economical reasons elevated dissociation temperature does not comply with industrial requirements. Moreover, large activation energy makes the unblocking reaction too slow unless A is large enough to counteract Ea. Because the pre-exponential factor is a measurement of the eﬀects of both the internal degrees of freedom of the reacting species and the necessary redistribution of that energy so that the reaction ensues, the ideal reaction should have a large positive entropy change, such as the cleavage of a ring system to a ring-opened system, to take advantage of the temperature eﬀect. In practical applications, it is however possible to generate a “deblocking temperature” according to the onset temperature of dissociation. As an example, diﬀerent oxime- and caprolactam-blocked isocyanates were heated in toluene in the presence of amine (1.2−6 equiv relative to blocked isocyanate; Scheme 31), and aliquots were analyzed by gas chromatography to record the rate of appearance of urea.150

cyclohexylisocyanate (T = 200 °C) presents superior activation energy, and global reaction rate is rapidly multiplied by a temperature increase. Tetramethylcyclobutanone ketoxime 22 blocked cyclohexylisocyanate (T = 240 °C), the highest activation energy system, is extremely sensitive to the temperature; the high pre-exponential factor oﬀsets the low value of the exponential factor. 4.3.2. Techniques. FTIR has been successfully employed to characterize adducts and their minimum deblocking temperatures. The NCO group strongly absorbs between 2250 and 2270 cm−1, the disappearance of this band indicates the completion of the blocking reaction (Table 8). The CO bond frequencies can be shifted to lower vibration frequencies depending on the hydrogen bonding.155 Typically, the variations of absorbance are normalized to the C−H stretching at 2946 cm−1 to compensate the thickness changes during the reaction course. Isocyanate conversion is then calculated as follows (eq 9). α=1−

(ANCO /A ref )t (ANCO /A ref )t = 0

(9)

Although it is less employed than FTIR, UV spectroscopy can be used to determine the NCO content. The n−π* transition of CO absorbs at 282−283 nm. Tondeur et al.28,52,53 studied cyclopentanol−phenylisocyanate kinetics of reaction by calculating the conversion ratio according to eq 10:

Scheme 31

α=

A 282 − [NCO]0 εure / [NCO]0 (εNCO − εure)

(10)

where A282 stands for absorbance at 282 nm and εNCO and εure stand for the extinction coeﬃcients of phenylisocyanate and urethane, respectively. When heat is applied to blocked isocyanate adducts, the urethane linkage scission leads to an endothermic transition in the DSC curve, possibly preceded by a sharp transition that corresponds to the adduct melting. Figure 8 presents the thermogram of the toluene 2,4-TDI dimer dissociation; the ﬁrst peaks at 162 °C is attributed to the adduct melting followed by T

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Table 8. IR Band Frequency of Common Groups Involved in the Blocking or Curing Reactions

TDI evaporation occur. A weight loss of 95% was recorded after the DSC scan ended at 250 °C. For a highly volatile blocking molecule, the deblocking reaction can also be recorded in an open pan thanks to a thermal gravimetry analysis. Although 1H and 13C NMR are also widely used to characterize the blocked isocyanate and the ﬁnal products, few studies reported the kinetic recording of the deblocking reaction. This technique gives extremely accurate results about deblocking and side reactions but necessitates high boiling point deuterated solvents, such as dimethylformamide (DMF) or dimethylsulfoxide (DMSO). The isocyanate content can also be quantitatively determined through titration. Aliquots are withdrawn from the ﬂask and quenched with di-n-butylamine. The isocyanate content is determined by back-titration with an acid, such as HCl or H2SO4 and a color indicator (methyl red11 or bromocresol green156). Titration method via amine quenching is widespread

Figure 8. DSC thermogram of the melting and the dissociation of TDI dimer. Reprinted with permission from ref 73. Copyright 1996 John Wiley & Sons.

the blocking molecule−isocyanate dissociation at 185 °C. Above 200 °C, possible degradations of isocyanate group and

Figure 9. Normalized intensity of isocyanate IR band for the deblocking reaction of 3,5-dimethylpyrazole blocked HDI (a) as a function of temperature (1 °C/min)160 and (b) during isothermal measurements. U

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Figure 10. ATR spectra of deblocking reaction of methyl ethyl ketoxime blocked HDI isocyanurate as a function of time at 140 °C (a) without a cover slide and (b) with a cover removed after 40 min as indicated by the arrow. Reprinted with permission from ref 7. Copyright 2001 Elsevier.

Scheme 32. Deblocking Reaction and 1H NMR Shifts Associated with the Chemical Functions at 25 °C in DMSO

and permits the study of biuret and allophanate groups thanks to selective degradation with butylamine.95 Indeed, Furukawa and co-workers157 showed that aliphatic primary amine selectively degrades allophanate and biuret and left urethane and disubstituted urea unmodiﬁed. Gas chromatography may also be used either to conﬁrm the elimination−addition mechanism by recording the presence of dissociated reactants158 or to check the kinetic data determined with other techniques.150,159 4.2.3. Operating Conditions. While most of patents claim a precise “dissociation” or “deblocking” temperature, one should keep in mind that the “deblocking temperature” does not have any physical meaning; not only can it be measured at diﬀerent stages of the reaction, but it also depends on the recorded physical parameter and the technique sensitivity to detect a certain extent of the reaction. In other words, since the reaction is equilibrated, the measurement conditions or experimental set up greatly inﬂuence that temperature. First, isothermal or nonisothermal conditions give clearly diverging results in the extent of reaction. Isocyanate groups are only partly regenerated depending on the temperature. For nonisothermal measurement, the extent of reaction increases with temperature until potential degradation occurs (Figure 9a). While for isothermal measurements, equilibrium is rapidly reached (Figure 9b) whereas relatively few isocyanate groups are freed. The reaction extent is far below the one of the nonisothermal experiment. Second, the equilibrium reaction can be driven toward free isocyanate (via the deblocking reaction) through the elimination of released blocking molecules. Evaporation turns out to be a critical point that is inﬂuenced by the measurement techniques. In DSC, the sample is analyzed either in a sealed pan if the boiling point of the blocking molecule is higher than

the temperature range or in a pan with a hole to prevent the blocking molecule evaporation from blowing up the cell. In the open pan, the blocking molecule evaporation will pull the reaction toward free isocyanate formation. TGA measurements are evidently performed in an open pan. Because this method is based on blocking molecule evaporation, TGA is inappropriate for blocking molecule possessing a boiling point above the deblocking temperature. Concerning infrared analyses, the sample can be analyzed in the solid phase as a KBr pellet or in the liquid phase (the blocked isocyanate solution is placed between two KBr plates73,161 or on a single KBr plate162). As an example, the inﬂuence of blocking molecule evaporation was perfectly shown by Wicks (Figure 10). Moreover, when the measurement cell containing the KBr pellet is evacuated to remove disturbing air components such as CO2 and water, the deblocking reaction is also favored.162 4.3.4. Presence of Chemical Additives. In addition to inherent discrepancies due to the numerous techniques and their experimental setups, the presence of solvent or coreactant also induces strong disparities in the deblocking temperature for a particular system. Contrary to DSC and TGA, NMR techniques obviously require working in a solvent. Only IR and UV are ﬂexible enough to tolerate measurements in the presence or in the absence of solvent. The solvent choice aﬀects the deblocking reaction. As already discussed (vide supra), the mechanism of isocyanate addition on a nucleophile is not completely elucidated, but strong evidence supports the idea that the solvent greatly inﬂuences the kinetics of polyurethane synthesis. If blocked isocyanate should be synthesized in an apolar solvent, the deblocking reaction is favored by strong nucleophilic solvent.163 Moreover, the equilibrated deblocking reaction is also assisted by all nucleophiles present. For instance, the NMR V

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decarboxylates into an amine and carbon dioxide. CO2 is extracted with a carbon dioxide free, dry nitrogen purge and made to react with sodium hydroxide165 or barium hydroxide.43 The minimum temperature at which detectable turbidity from carbonate precipitation appears is taken as the minimum deblocking temperature. Ho et al.165 followed the kinetics of deblocking by weighing the evolved CO2 in the U shaped tube. The CO2 evolution methods are only qualitative and relatively subjective. 4.3.5. Catalysis Action. The mode of catalysis is well understood in the nucleophile−isocyanate reaction, whereas in the deblocking reaction it is not fully comprehended. Generally speaking the same catalysts as those used in isocyanate− nucleophile reaction are chosen, though at higher concentration. The catalytic activity is still under debate: catalyst acts either on the deblocking mechanism or in the reaction between free isocyanate and coreactant. In the presence of nucleophile, the catalyst is supposed to ﬁrst complex or coordinate with hydroxyl compound to generate the active form of the catalyst that ﬁts between the blocking molecule and isocyanate.166 Obviously, deblocking temperature depends on the catalyst (Figure 12) and on its concentration (Figure 13). To study the

signals of 3,5-dimethylpyrazole 25 (DMP) blocked isocyanate recorded as a function of temperature demonstrate that ﬁrst the isocyanate group is regenerated, and then it reacts with water to form a urea bond and CO2 (Scheme 32).164 But, as soon as water is totally consumed, the free isocyanate content reaches a constant value (Figure 11).

Figure 11. NMR signal integrations and percentage of free blocking molecule, expressed as I5.8ppm/(I6.2ppm + I5.8ppm), and various derivatives.

Most of the works measured the deblocking reaction in the presence of a nucleophile such as amines or alcohols. The relative reactivity of all reactants makes the comparison between the diﬀerent systems and publications quite tricky. The deblocking temperature followed by IR corresponds either to the disappearance of the blocking molecule−isocyanate bond73,161 or to the ﬁrst appearance of the curing molecule− isocyanate band.162 Obviously, some diﬀerences in the results are found between the two methods: ﬁrst, an undeﬁned interval between the disappearance of the blocked isocyanate bond and the appearance of the curing bond leads to a mistaken temperature in the case of a temperature ramp experiment. Second, during this period, secondary reactions may artiﬁcially decrease the concentration of free isocyanate and curing bond. Finally, due to the high reactivity of amine, the presence of free isocyanate is not likely. Other deblocking temperature measurements exclusively operate in the presence of nucleophile, for instance, titration. In this method, the CO2 created by the deblocking reaction is analyzed. The blocked isocyanate is slowly heated in the presence of either an anhydride (Scheme 33a) or molecular sieves saturated with moisture (Scheme 33b), which react with the deblocked isocyanate. In Scheme 33b, the isocyanate group reacts with water to form an unstable carbamic acid intermediate, which, as explained before, spontaneously

Figure 12. Deblocking temperature of 1,2-pyrazole-blocked phenyl isocyanate with 10 mol % catalyst determined by DSC in dry DMSO.41

eﬀect of concentration on the side reaction, Petrak et al.41 heated pyrazole blocked isocyanate 26 in the presence of catalyst and quantiﬁed by HPLC the amount of isocyanate hydrolysis product (diphenylurea 27, Scheme 34). Amine

Scheme 33. (a) Isocyanate−Anhydride Reaction and (b) Reaction between Isocyanate and Water

Figure 13. Hydrolysis reaction of 1,2-pyrazole-blocked phenyl isocyanate as a function of Zr(acac)4 catalyst concentration.41 W

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Scheme 34. Hydrolysis Reaction of 1,2-Pyrazole-Blocked Phenyl Isocyanate in the Presence of Zr(acac)4

1:33:100. The deblocking reaction reactivity obeys the opposite order: the secondary alcohol−isocyanate adduct dissociates at a lower temperature than primary alcohol blocked isocyanate. Although, tertiary alcohol blocked isocyanates 12 should be more reactive, they cannot be employed as blocked NCO, since they do not regenerate the isocyanate group but decompose into alkenes 14, carbon dioxide, and amine 13 (Scheme 35).

catalysis is not suitable for the deblocking reactions that entails a proton transfer from the NCO nitrogen to the nitrogen of a blocking agent (e.g., oxime, amine, vide inf ra). In the case of oxime-blocked isocyanate reacted with dibutylamine, DABCO was found to have no eﬀect on the deblocking reaction;156,167,168 for the deblocking reaction of pyrazole-blocked isocyanate, addition of a strong base like DABCO greatly reduced the reaction rate.169 4.3.6. Conclusion. In view of the numerous techniques, methods, and parameters available to compare the “deblocking temperature” of diﬀerent blocked isocyanates, care must be taken in the evaluation of a given system. Deblocking temperature depends on the analytical measurements, that is, the measured physical phenomenon (deblocking, curing reaction or both of them), the experimental set up (heating rate, gas ﬂow, sensibility, sealed reactor cup), and the reaction conditions (presence of coreactant or solvent, product volatility, etc.). To get an idea of the complexity of comparing methods, please refer to some examples quoted in the Supporting Information of which examination/interpretation are out of the scope of this review. One should nevertheless recall that single “deblocking temperature” does not have a physical sense, because it depends on the sensibility of an analytical method to detect some extent of the deblocking or curing reaction. To best reﬂect the curing phenomenon on the system studied, one would preferentially choose the analytical technique close enough to the application. Industrially, the reaction rate is indeed at least as important as the deblocking temperature.

Scheme 35. Decomposition of t-Butanol Blocked Isocyanate2,11

Generally speaking, the carbonyl group of the blocked isocyanate has partial positive charge, so the bond between the carbonyl carbon and the blocking molecule is labile. The lower the charge diﬀerence, the lower the strength of the bond. Thus, decreasing the nucleophilicity of the blocking molecule reduces the charge density, making the bond more labile. Consequently, due to the electron-withdrawing eﬀect of the aromatic ring, phenol-blocked isocyanate deblocks at a lower temperature than alcohol. This eﬀect is enhanced by all the electronwithdrawing substituents on the aromatic ring, as shown in Figure 14. This system is one of the most extensively described

4.4. Chemical Structure−Reactivity Relationship of Blocked Isocyanates

We have seen all the parameters to take into account while studying blocking/deblocking reactions. Nevertheless, the temperature of dissociation obviously relies in the ﬁrst place on the structure of the isocyanate and of the blocking molecule. Because only a few isocyanates are industrially available, the deblocking temperature is mainly controlled through the blocking molecule structure. This part deals with the major classes of blocking agents and for each blocking group, the deblocking reaction mechanism will be presented to introduce the inﬂuence of substituents (and mainly their electronic eﬀect) on the deblocking temperature. For intensive description of each system, the reader is redirected to the reviews of Wicks et al.5,6 4.4.1. Alcohols and Phenols. As already discussed, alcohols and phenol react with isocyanate group to form urethane linkages, the thermal dissociation temperatures roughly vary in the following order:148n-alkyl-NHCOO-nalkyl ≈ 250 °C; aryl-NHCOO-n-alkyl ≈ 200 °C; n-alkylNHCOO-aryl ≈ 180 °C; aryl-NHCOO-aryl ≈ 120 °C. Because their deblocking temperature is quite high, alcohols are not commonly used, except in water-borne coating where adducts must exhibit a very high stability. Rand39 demonstrated that in urethane-forming reactions, the competitive rates of tertiary, secondary, and primary alcohols approximated to

Figure 14. Deblocking temperature of the TDI adducts presented in Scheme 36 measured by carbon dioxide test in 2-methoxyethyl ether with water-saturated molecular sieves (■)172 and measured by CO2 test with water in poly(ethylene glycol) (●).170

in the open literature.163,170,171 Phenols substituted by stronger electron-releasing groups such as methoxy (32) and to a lesser extent methyl groups (33) (Scheme 36) deblock at higher temperature whereas electron-withdrawing substituents drop the temperature of isocyanate regeneration. Thus, to a certain extent the deblocking temperature can be correlated with the pKa value of the phenol-based blocking group. Scheme 36. Some Phenol Derivatives Used as Blocking Agents

X

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amine-blocked isocyanates.175 Oxime-blocked isocyanates are easily synthesized at moderate temperature in THF with dibutyltin dilaurate as catalyst.118 Levine et al.156,167 studied the benzophenone oxime blocked TDI 44 derivative deblocking in presence of dibutylamine in toluene. They reported blocking molecule elimination as the ﬁrst and rate-determining step followed by the NuH molecule addition 45 (Scheme 39). The mechanism, similar to the deblocking reaction of amineblocked isocyanate, leads to the following observations: (i) the intramolecular activated complex decreases the deblocking temperature, compared with alcohol-blocked isocyanate; (ii) the reaction is favored by all the electron-releasing substituents X on the blocking molecule, contrary to the alcohol-blocked isocyanates; these substituents increase the base strength of the nitrogen electron lone pair as presented in Scheme 40 and Figure 15. 4.4.4. Other N-Based Compounds: Amides, Imides and Imidazole, Pyrazole, and Triazole. Among the Ncompounds, amide is commonly used as blocking molecule. This type of molecule does not form a cyclic intramolecular transition state like oximes and, consequently, behaves similarly to the hydroxyl compound; the electron-withdrawing substituents accelerate the deblocking reaction (Scheme 41 and Figure 16). Pyrazole 61 is another interesting class of blocking molecule combining excellent reactivity and good latencies. The blocking molecule elimination goes through a ﬁve-center complex similarly to oximes (Scheme 42). Muehlebach measured the reactivity of these pyrazole-blocked isocyanates mixed with trifunctional amine-terminated prepolymer (Jeﬀamine T403) and reported the following reactivity order: 63 > 62 > 61. The electron-releasing substituents decrease the deblocking temperature (Scheme 42).169 Blocked MDI was systematically found to be more reactive than blocked HDI. Other compounds containing at least two nitrogen atoms, such as 1,2,4-triazole 60 and imidazole 57 have been proven to form the same ﬁvecenter complex facilitating the deprotonation step during the deblocking reaction.177

4.4.2. Amines. Primary amines react so fast with isocyanate that the counter-equilibrium reaction cannot be seen (the temperature of degradation of the complex is below the deblocking temperature). Secondary amines (e.g., piperidine derivatives) are very reactive nucleophilic molecules that react easily with isocyanate without external catalysis. Because reactivity increases with the blocking molecule nucleophilicity, the blocking reaction rate of isocyanates with amines is always higher than with alcohol. This high reactivity of isocyanate toward amines should induce a very high deblocking temperature of the adduct. However, in the case of aromatic amines (e.g., aniline), an autocatalytic eﬀect is observed; the tertiary nitrogen atom of the blocking molecule forms a four-centered complex through an intramolecular hydrogen bond with the urea hydrogen (Scheme 37). This transition state, proven by Scheme 37. Formation of a Four-Centered Transition States during Deblocking Reaction of Aniline−MDI Adduct

NMR,173 weakens the bond between the carbonyl carbon and the nitrogen atom of the blocking molecule. The N−H breaking is thus the rate-determining step of the deblocking reaction. Nasar158 and Gnanarajan174 studied substituted aromatic secondary amine as blocking molecule (37−41) for TDI (Scheme 38). When the deblocking reaction is carried out in presence of hydroxyl-terminated polybutadiene (42, Scheme 38), the recording through gel time measurements at 180 °C gave the following reactivity order: 37 > 38 > 39.158 Similar compounds were heated with pyromellitic dianhydride (43, Scheme 38) by TGA and in that case, the deblocking temperature increased as 40 > 37 > 41.174 As predicted by the proposed mechanism (Scheme 37), the electron-withdrawing groups (phenyl, naphthyl group attached on the nitrogen atom, or nitro group on the aromatic ring) drain the electrons from the nitrogen atom hindering the hydrogen abstraction. Conversely, the electron-releasing groups such as methyl and nitro group strengthen the electron density on nitrogen and so enhance the autocatalytic eﬀect. 4.4.3. Oximes. The third widespread blocking molecule is oxime type compounds for which the deblocking reaction follows a similar deblocking mechanism to the one reported for

4.5. Improving Industrial Systems

4.5.1. Conventional Systems. The most used blocking molecule in coating and painting industries is ε-caprolactam (Scheme 43). However, these blocked isocyanate-powder coatings have to be deblocked at rather high temperature, and they release blocking agents during the hardening process that are undesired both ecologically and economically. A low boiling point compound facilitates the deblocking reaction and acts as a solvent to level oﬀ the coating during the cross-linking reaction. However, this type of compound brings potential

Scheme 38. Substituted Aromatic Secondary Amines Used To Block TDI (left) and Chemical Additives Used as Coreactant during the Deblocking Reaction (right)

Y

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Scheme 39. Mechanism of Benzophenone Oxime Blocked Isocyanate Deblocking in the Presence of Dibutyl Amine

Scheme 40. Substituted Benzophenone Oxime

Figure 16. Deblocking temperature of amide− and imidazole− diisocyanate adducts. Those marked with “a” were measured by DSC;176 those marked with “b” were measured by TGA.177,178

Scheme 42. Deblocking Reaction of Pyrazole Blocked Isocyanate through a Five-Center Complex Formation169

Figure 15. First-order rate constant for the reaction between various blocked isocyanate molecules (Scheme 40) and dibutylamine (reactant ratio 1:1 at 70 °C).167

toxicity problems. On the other edge, a compound with higher evaporation temperature remains in the ﬁlm and plays the role of a plasticizer. Methanol blocked m-TMXDI reacts with acrylic polyol slightly faster than the ε-caprolactam blocked mTMXDI, which is explained by the higher diﬀusion and evaporation rate of methanol.179 The gelation times, inversely proportional to the reactivity, increase in the following order: methyl, ethyl, and isopropyl alcohol-blocked TMXDI in presence of an acrylic polyol.180 4.5.2. Chemical Tricks. In a view of improving polyurethane formulations, a strategy to increase the reactivity has been proposed by Carter and Pappas.181 After the deblocking, the blocking molecule can undergo a chemical reaction rendering it less reactive (or even unreactive); the back reaction is then suppressed. The oxime-blocked isocyanate system cured with alcohol is known to be much slower than that cured with amines because the back reaction (isocyanate− oxime) dominates the isocyanate−alcohol curing reaction. Ethyl acetoacetate oxime 67 was found to be a good candidate:

Scheme 41. Structures of Some N-Based Compounds Used As Blocking Molecules

Z

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Scheme 43. ε-Caprolactam-Blocked Trimer of (A) Isophorone Diisocyanate and (B) Hexamethylene Diisocyanate

from a mixture of isocyanate prepolymer blocked with N(11′-dimethyl-3-oxobutyl) acrylamide oxime, chain extender, vinyl monomers, and catalysts.186,187 Vinyl isocyanate monomers such as isocyanate ethyl methacrylate or m-isopropenylbenzyl isocyanate are also available and were copolymerized with methyl methacrylate or ethyl acrylate. They allow acrylate polymer to be cross-linked by isocyanate groups or acrylate− urethane interpenetrated networks.118,188 Due to the processing window (between the preparation and ﬂow of the powder and the cross-linking reaction), the use of powder coatings at temperatures lower than about 130 °C is limited, a fact that precludes their use on heat-sensitive substrates like many plastic substrates. Eﬀorts to develop radiation-curable powder coatings are ongoing. UV-cross-linked polyurethane acrylates (PUA) are very attractive materials that combine fairly good mechanical performances (ﬂexibility, abrasion resistance, and toughness), high chemical resistance, and adhesion properties. For their synthesis, the chain extender of the second step in Scheme 24 is replaced by hydroxyl ethyl (meth)acrylate, HE(M)A leading to α−ω (meth)acrylates, which are able to react by free radical polymerization. But up to now, due to the presence of inhomogeneities in cross-linking density, the mechanical properties such as elongation at break are less than the ones of the corresponding PU.189 Very recently, some authors proposed to functionalize the blocking agent with self-reacting groups, such as trimethoxysilane or (meth)acrylate functions, to promote an anchoring of the blocking molecules during the curing.102 4.5.4. Blocking Reaction of Amine Groups. To increase the pot-life of cast molding formulations, another strategy used masked amines, such as methylene dianiline (MDA) sodium chloride complex dispersed in dioctyl phthalate. MDA is commonly used to cure urethane prepolymers based on diisocyanate, TDI, or MDI and polyether polyols. At room temperature, the MDA/NaCl salt is virtually nonreactive, whereas when heated to 115−160 °C, the complex unblocks and the freed MDA rapidly cures the prepolymer to lead to the ﬁnal polyurethane.190,191 Another way to mask amines is through aldimine groups, which regenerate amines and aldehydes by hydrolysis.192 Interestingly, blocking the amine group rather than the isocyanate function enables polyureas as one-pot formulations.

after unblocking, an intramolecular cyclization reaction converts the free oxime 67 into a non-nucleophilic (unreactive) product 3-methylisoxazol-5-one 68 (Scheme 44). Scheme 44. Deblocking and Cyclization Reactions of Oxime of Ethyl Acetoacetate Blocked Isocyanate

The insolubility of the blocked isocyanate and the curing molecule at ambient temperature and the slow endothermic dissolution process at higher temperature enable extension of the stability, as shown for the mixture of pyrazole-blocked isocyanate and amine-terminated prepolymer.169 To increase the storage stability, some polyurethane formulations include blocked amines as coreactants in addition to blocked isocyanates. Suitable amines are ketimines and enamines, which react with moisture to regenerate amine functions.182 Finally, an interesting study showed that oxime-blocked isocyanates induce cross-linking by generating amines and hydrazines under UV radiation.183,184 4.5.3. Dual Networks. Tassel et al.185 synthesized a new blocking agent, 2-formyloxyethyl methacrylate (69, Scheme 45), bearing two functions: a labile hydrogen and a methacrylate double bond, which can homopolymerize leading to polyurethane and polyacrylate interpenetrated networks. In the same way, polyurethane−urea/polyvinyl simultaneous interpenetrating polymer networks were prepared starting Scheme 45. Blocking Agent Acting As a Comonomer after Deblocking Reaction

4.6. Blocked Isocyanates as Initiators for Other Reactions

4.6.1. Urea Linkage as a Source of Amine. Although a complex mechanism occurs, dicyandiamide is one of the most popular cross-linkers for epoxy resin in industrial applicaAA

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tions.193 To lower the reaction temperature, this reaction is generally promoted by tertiary amines and imidazoles. A way to improve the latency exploits blocked isocyanates to generate in situ amines (Scheme 46). Upon heating, substituted ureas 70

lation is commonly used for synthesizing polyurethane from polyol and urethane diol.197 Other strategies consist in preparing precursors without facing the hazards of phosgene handling. Here, we have classiﬁed these new systems according to whether they release isocyanate during the polymerization. One should however never forget that, due to the reversibility of urethane linkage, isocyanate functions may reappear in the material at a later stage, by, for example, thermal treatment.

Scheme 46. In situ Generation of Catalyst for Dicyandiamide Epoxy Reaction

5.1. Phosgene-Free Isocyanate Precursors

Klinger et al.198 showed that, by heating between 50 and 70 °C during 24 h, the acylazide groups in a polymer undergo a Curtius rearrangement (Scheme 49) and thereby generate isocyanato groups in situ and N2. Another convenient method (Scheme 50) for the transformation of amines into ﬁrst blocked isocyanates and then free isocyanates employs N,N′-carbonyldiimidazole. The blocking reaction of an isocyanate function with maleimide derivatives is not feasible for reactivity purposes. One key to nevertheless prepare these products consists in the reaction of the maleic anhydride with (substituted) urea(s) and cyclization with water release (Scheme 51).200 Such derivatives were shown to deblock at relatively low temperature (about 100 °C in DMF) and react readily with primary alcohols to generate N-carbanylmaleamates. Other authors reported succcinimide or maleimide derivatives of HDI in the same range of temperature (110 and 130 °C, respectively).176,201

dissociate into isocyanate 72 and dimethylamine 71. The latter reacts with an epoxy group to give a tertiary amine 73, which catalyzes the reaction of dicyandiamide with epoxy groups. The isocyanate moiety reacts with another epoxy unit, forming an 1,3-oxazolidin-2-one.193 A similar approach was used to generate a tertiary amine catalyst from blocked isocyanate to promote the epoxy isocyanate reaction.106 4.6.2. Urethane Promoting Ring-Opening Polymerization of 1,3-Benzoxazine. 1,3-Benzoxazines are cyclic N,O-acetal-type monomers that polymerize by ring-opening at T > 180 °C without catalyst to give the corresponding phenolic or phenoxy polymers (Scheme 47).194,195 Lower curing temperature is highly needed, and attempts to reduce the curing temperature have been carried out, with diﬀerent degrees of success, by using various catalysts, such as acidic or basic compounds as well as metal-containing catalysts. Among them, phenol has proven to be eﬃcient, and urethane reversibility has been used to create in situ the active molecule. Phenol activates benzoxazine by protonation, the produced species undergoes a ring-opening reaction. Then, the imminium moiety reacts readily and irreversibly with phenoxides (Scheme 48a). Moreover, Sudo196 claimed that the isocyanate moiety also induces polymerization by stabilizing the propagating zwitterionic species and avoiding suppression of the reverse reaction possibility (Scheme 48b).

5.2. Phosgene-Free Isocyanate-Free Precursors

5.2.1. Carboxamide/Alcohol Polycondensation. From tetramethylene urea and phenyl chloroformate, Ubagh synthesized N-(hydroxyalkyl)-2-oxo-1,3-diazepan-1-carboxamide 81 as a polycondensation monomer to lead to polyurethane.202,203 Below 100 °C, the equilibrium (Scheme 52) is completely shifted to the left; and thus, the O-phenyl urethane is considered as an activated urethane and the 1,3diazepan-2-one ring as an intramolecular blocked isocyanate. In the presence of amino alcohol, the urea ring remains intact because the nucleophilic amino group reacts selectively with the electrophilic carbon of the O-phenyl group. Then, this monomer is polymerized in the presence of catalyst such as DABCO at 150 °C to form poly(urea-urethane) 82 (Scheme 53). Another reaction involves carbonylbiscaprolactam 83 (CBC, Allinco from DSM) with a nucleophile to provide caprolactamblocked diisocyanate as an isocyanate- and phosgene-free route.204 Depending on the nucleophile, the temperature, and the catalyst, two reactions were identiﬁed: ring-opening (RO) and ring elimination (RE) mechanisms (Scheme 54). Whereas secondary amines do not react at high temperature or in presence of catalyst, the reaction of CBC with primary amines proceeds exclusively by the RE mechanism. At 70 °C, the mono-RE mechanism is quantitative without catalyst to form caprolactam-blocked isocyanate.205,206 At higher temperature, RO competes with RE mechanism, and a mixture of blocked isocyanate and urea is then obtained at 170 °C. Primary and secondary alcohols and phenols react with CBC by ringopening and ring elimination depending on the conditions (temperature and catalyst) to aﬀord carbonate, urea ester, and urethanes without isocyanate intermediates. Metal alcoholate catalysts favor the RO pathway. 5.2.2. Dicarbonate/Amine Condensation. A promising approach toward phosgene-free polyurethane is the carbonate−

Scheme 47. Monomer N-Methylbenzoxazine 75 and Phenolic and Phenoxy Structures of Poly(benzoxazine) 76

5. “NEW” TRENDS: TOWARD NON-ISOCYANATE-BASED POLYURETHANES Recently, several reactions, essentially based on transcarbamoylation, attracted researchers interest due to the possibility to form polyurethanes without isocyanate monomers in the formulations, which complies with the new environment and safety requirements. Nowadays industrially, the transcarbamoyAB

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Scheme 48. Mechanism for the Promoting Eﬀect of Urethanes on the Polymerization of Benzoxazine

Scheme 49. Curtius Rearrangement

Scheme 51. Formation of Maleimide-Blocked Isocyanate by a Phosgene-Free Reaction and Further Deblocking Reaction199

amine reaction that leads to β-hydroxyurethane. Depending on the reaction conditions and nucleophile hardness, dimethylcarbonate (DMC) acts either as a carboxymethylating or as a methylating agent of amines. In the absence of base, aliphatic amines such as benzylamine 84 react with DMC 85 to give both alkylation (86) and carboxylation (87) products without any selectivity (Scheme 55).208 Due to the lower nucleophilicity of aromatic amines, these do not react at 90 °C in the absence of a base. Basic catalysts, such as the organic superbase guanidine 1,5,7-triazabicyclododecene (TBD, see Figure 17), increase the rate and the selectivity of aliphatic amine−carbonate reaction.209 Urethane bonds are formed by dicarbonate−aliphatic or aromatic amine reaction catalyzed by bases such as sodium methoxide at 90 °C. The best results for the carboxymethylation of aniline with dimethylcarbonate in terms of selectivity (urethane/N-methyl derivative) are achieved with zinc or tin catalysts.210 Uncatalyzed urethane formation can also be performed under high pressure (8 kbar).211 Very recently, Tang et al. showed that a superbase catalyst conventionally used in ring-opening polymerization (ROP) of various cyclic monomers (including cyclocarbonates), namely, TBD, promotes transurethanization reaction between dicarbamates and diamine functionalized molecules to generate bio-based, isocyanate-free polyurethane of similar structuration and mechanical properties as conventional TPUs.209

5.2.3. The Speciﬁc Case of Cyclocarbonate/Amine Addition. As early as 1957, Groszos et al.212 patented PU preparation using a non-isocyanate route based on the reaction between cyclocarbonate and polyamine urea. Recently, this synthesis strategy has attracted more and more interest since no volatile compounds are released during the reaction. Cyclic biobased alkylenes are reactive intermediates known for more than 50 years,213 but since the 1990s, seminal work has been done to explore the high reactivity of ﬁve-membered alkylene carbonate. Primary or secondary amine has long been shown to open the cyclocarbonate at room temperature without catalyst.213−215 Using diamine and bicyclocarbonate allows synthesis of original polymers that could not be obtained by common isocyanate− alcohol addition, for instance, amorphous polyhydroxyurethane.216 A very recent review proposes practical aspects of NIPU preparation and applications.217 5.2.3.1. Synthesis of Pentacyclocarbonate. Several methods were used to synthesize ﬁve-membered cyclic carbonates as

Scheme 50. Formation of Isocyanate from Amine and Carbodiimidazole199

AC

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Scheme 52. Equilibrium between Activated Urethane/Intramolecular Blocked Isocyanate and the Mono- or Diisocyanate forms

Scheme 53. Synthesis of Poly(urea-urethane) from 81

Scheme 54. Reaction Pathway of CBC 83 with Nucleophiles

Scheme 55. Reaction of Benzylamine with DMC in the Absence of Base

summarized in Scheme 56. These include the reaction between an epoxy group and β-butyrolactone218 and the reaction between carbon dioxide and 1,2-diols in presence of catalysts.219 A recent approach involves the formation of a cyclocarbonate from a ketal formed from ethan-1,2-diol and cyclohexanone in the presence of supercritical CO2.220 Reaction between urea and alcohol is another method for the synthesis of cyclocarbonates in presence of catalysts.221,222 Phosgenation of polyols is interesting due to a high yield in cyclocarbonate but the high toxicity of phosgene is the main drawback.223−225 The transesteriﬁcation of 1,2-diols with ethylene carbonate leads to cyclocarbonates by a group exchange reaction in presence of catalyst.226 Finally, the most common method remains the addition of carbon dioxide on cyclic ethers,227−229 for which the catalysts and synthetic methods were nicely described by Daresbourg.230

Figure 17. Synthesis of isocyanate-free polyurethane via the transurethanization between dicarbamates and diamino-terminated polymers catalyzed by organic superbase guanidine 1,5,7-triazabicyclododecene. AD

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Scheme 56. Synthetic Routes to Five-Membered Cyclocarbonates

with carbon dioxide.239 Note that a tristar poly(ethylene glycol) terminated by cyclocarbonate 92 is commercialized by the company Macromer Ltd., Russia, under the trade name Laprolate L.803 (Scheme 58).240

Various techniques are likely available to synthesize dicyclocarbonates: (i) reaction of diglycidyl ether of bisphenol A (BADGE) and CO2 in presence of sodium iodide and triphenylphosphane at 100 °C;231 (ii) transesteriﬁcation of polyols such as 3,4-O-isopropylidene-D-mannitol with ethylene carbonate with yields around 65%;232 (iii) highly selective reactions of n-(tert-butoxycarbonyl)imidazole233 or triphosgene234 onto various tetrol; (iv) carbonated methyl lineolate obtained from the bis-epoxy precursor using CO2.227 The most reported technique consists of the esteriﬁcation of carboxylic acid groups of benzene-1,4-dicarboxylic acid with the alcohol function of commercial 4-(hydroxymethyl)-1,3-dioxolan-2-one 90 (Scheme 57).235 This method was also reported by others to

Scheme 58. Chemical Structure of Laprolate L.803

Scheme 57. Synthesis of Dicyclocarbonate Using Glycerin Carbonate

5.2.3.2. Cyclocarbonate/Amine Reaction. 5.2.3.2.1. Isomers. Endo et al.237,241 studied a model reaction between monofunctional cyclic carbonate 93 and benzylamine 94 (Scheme 59). After 20 h, no aminolysis product (Nbenzylbenzamide 97 in Scheme 59) but two isomers of ringopening reaction were found. The secondary hydroxyl containing compound (96 in Scheme 59) predominates in the medium (96/95 = 4.4:1); theoretical calculations conﬁrmed the stability of 96 (ΔHf = −9.77 kcal/mol) over 97 (ΔHf = −8.10 kcal/mol). The ratio between secondary and primary hydroxyl compound is determined by substituents introduced at the αmethylene of the bicyclocarbonate 98: a more electronwithdrawing group draws the reaction toward secondary hydroxyl containing product 99 (Scheme 60).242 Chemical structure and molar mass govern the amine reactivity:

lead to diﬀerent cyclocarbonates, symmetric or asymmetric.216,236 The same product was also obtained by Steblyanko from CO2 and diglycidyl terephthalate, previously synthesized from 2-oxiranylmethanol and terephthaloyl dichloride.237 Very recently, this method, in combination with ROP of the same cyclocarbonate, was applied to prepare cyclocarbonate-terminated polycarbonates.238 Carbon dioxide was also added to other glycidyl ethers to synthesize polyfunctional cyclocarbonates.227 Koen-Raadt reported the preparation of tris(2-oxo-1,3-dioxolanyl-4methyl)isocyanurate by reaction of triglycidyl isocyanurate AE

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Scheme 59. Ring-Opening Model Reaction of Cyclocarbonate by Amine237

aromaticity, α- and β-electron-withdrawing substituents, and high molar mass lower the reactivity.240

mass of the molecules increases; catalyst or higher temperature are then required. Typically with diethylene triamine (DETA), 8 days at room temperature is required for a quantitative reaction, whereas using a catalyst and working at 60 °C, 4 days is enough. This shows that the carbonate/amine reaction is less eﬃcient than the isocyanate/alcohol one and should rather be compared with the epoxy−amine reaction in a higher range of temperature. Kotzev proposed, as shown in Scheme 61, a scale of reactivity that summarizes perfectly this trend. 5.2.3.2.3. Larger Cycles. Endo223−225 has prepared and compared the reactivity of C5 and C6 cyclic carbonates, prepared from epoxy/CO2 and malonate/triphosgene reaction, respectively. The C6 cycle is about 20 times more reactive at 30 °C and 10 times more at 70 °C, than C5 while reacting them with 4,9-dioxadodecane-1,12-diamine. Guillaume et al. have prepared various cyclocarbonates in C7 starting from levunilic and itaconic acids but engaged these molecules exclusively in

Scheme 60. Polyhydroxyurethane Synthesis

5.2.3.2.2. Reactivity. Kotzev240 studied in 2004 the reaction between the Laprolate and various amines, which structures are given in Scheme 61. The reactivity decrease for bulk reaction is explained by the amine concentration dropping oﬀ as the molar

Scheme 61. Scale of Reactivity of Various Amines toward a Model Pentacyclocarbonate

AF

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5.3. Conclusion

ring-opening metathesis polymerization (ROMP) to prepare polycarbonates.243 5.2.3.3. Polymer Synthesis and Properties. Kihara and Endo synthesized linear polyhydroxyurethanes from reaction between diamines and dicyclocarbonates obtained from epoxy molecules.231 Burgel and Fedtke also prepared linear polyhydroxyurethanes from dicyclocarbonates and diamines.244 Finally, Lee et al.245 designed new linear polyhydroxyurethanes by polyaddition between dicyclocarbonates and various amines. Dicyclocarbonates were obtained from bisphenol S of diglycidyl ether monomer and carbon dioxide.245 The work was also extended to polymers functionalized with bicyclocarbonate, which react with polyamines to yield cross-linked polyhydroxyurethanes.246 Various types of catalysts such as acid (glacial acid acetic, methanesulfonic acid), base (triethylamine, piperazine, tetrabutyl ammonium bromide), or metal (metatin type catalyst) increase the reaction rate.240 Because the molar mass obtained by this process is usually below 10,000 g/mol and ultimately less than 20,000 g/mol, this reaction is essentially used to synthesize prepolymer247 or to cross-link cyclocarbonate bearing polymer248 or for chain extension of preformed polymers. The Tg of this material ranges between −30 and 100 °C, depending on the chemical structure of reactants. Neither volatile nor nonvolatile byproducts are produced by the reaction between cyclocarbonates and amines resulting in pore-free polyurethanes. The inﬂuence of hydroxyl functions on thermal properties of poly(hydroxyurethane) (PHU) is remarkable. Indeed, the intramolecular hydrogen bonding between the alcohol and urethane functions (Scheme 62) seems to be responsible for lowering

The carbonate/amine reaction produces polyurethane materials overcoming the use of toxic isocyanates. However, catalyst addition is required to enhance the reaction rate and the selectivity. Cyclocarbonates, especially ﬁve-membered cyclocarbonates, drew additional interest compared with carbonates since no volatile compounds are released during the polymer formation. The obtained products from these cyclic carbonates bear a hydroxyl group at the β-carbon atom of the urethane moiety, which brings improved properties such as pore-free PU. The thermal stability of nonisocyanate PU is also claimed to be improved compared with conventional PU. However, this parameter has only been checked through thermogravimetric analyses; the molar mass stability when heating should also be veriﬁed. Indeed, the reversibility of the β-hydroxyurethane bond would produce isocyanate and an excess of OH groups, which upon recondensation could induce a strong molar mass decrease. An opposite possibility would be exclusive exchange reactions without depolymerization; if that occurs, it would be possible to prepare malleable materials from permanent organic networks, as proposed recently for conventional epoxy−acide chemistry.256

6. FINAL CONCLUSION: ISOCYANATE-FREE VERSUS NONISOCYANATE SOLUTIONS IN VIEW OF AN INDUSTRIAL CONTEXT As stated in the Introduction, urethane/urea bonds have proven their versatility. One clear advantage of the NCO chemistry is the high reactivity of this function, even at room temperature. This allows fast curing of cast-molded materials or coatings, as well as a facility to prepare solid foam (chemical reaction rate competes with gas release speed, thus facilitating trapping of it in the network). Additionally, the reversibility of covalent links based on isocyanate chemistry is an open door to combining processability, reparability, and high elastomeric or rigid performance in a variety of applications. Reversibility of urethane bonds is the key to blocked isocyanate technology. Blocking the isocyanate function is a way of getting rid of isocyanate groups in a formulation, but then curing requires high temperature to deblock the chains. Another disadvantage of blocked isocyanates is the loss of (high) reactivity of the −NCO groups, as we have seen in this review. This approach is then particularly preferred when one is seeking a one-pot, slow polymerization process, as in coatings for instance. One solution to enhance the reactivity consists in preparing prepolymers, which react by transcarbamoylation with polyols to cross-link the materials; such a trick is regularly used in industry. A major problem of concern in isocyanate chemistry is the high toxicity of the precursors, that is, diisocyanates. Upon the most conventional formulation available nowadays, the industry proposes formulations where telechelic isocyanate prepolymers are prepared beforehand by reaction of a diol oligomer with an excess of diisocyanate and distillation of the remaining small reactant. The polyurethane materials are then prepared by adding a short diol or upon action of water in a one-pot formulation. This process, called “diisocyanate-free polyurethane” technology, allows the same reactivity as in the conventional diisocyanate chemistry to be retained, while the recent regulation rules, such as REACh, which requires discarding MDI or TDI hazardous molecules, are respected. Still, the presence of isocyanate functions at the end of

Scheme 62. Hydrogen Bonding in a Hydroxyurethane Function249

the susceptibility of the backbone to hydrolysis resulting in substantial increase of the chemical resistance.249 The chemical resistance of materials containing intramolecular hydrogen bonds is 1.5−2 times superior to materials with a similar chemical structure without such bonds.250 Tomita has shown that the water absorption percentages and thermal decomposition temperatures of the ﬁlm cast from synthesized PHU were reduced by 30% and from 32 to 88 °C higher, respectively, than those from commercial PU.223−225 5.2.3.4. Bio-Based Materials from this Chemistry. As a current trend, researchers tend to go further by producing nonisocyanate PU from renewable resources. In that frame, vegetable oils as feedstocks for polymeric materials attract increasing attention, because they tend to be very inexpensive and available in large quantities. They are used either as a source of polyol251 or as polycyclocarbonate obtained from epoxidized oils.252,253 As an example, Boyer et al.254 synthesized internal carbonated fatty acid diester and terminal carbonated fatty acid diester from sunﬂower oil. Interestingly, carbonate and dialkyl carbonates can be prepared from bio-based products, such as dianhydrohexitol,255 leading to green, biosourced polyurethane. AG

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oligomers can bring toxicity to the end-user, necessitating special security requirements during implementation. The carbonate−amine reaction seems to be the best way to solve these toxicity problems, because it avoids the use of isocyanate functions. The range of materials issued from this technology are called “non-isocyanate polyurethanes” (NIPUs). Up to now, the rate of this reaction has been shown to be quite slow compared with the direct −NCO + −OH reaction; still, methods are available nowadays to circumvent this lack of reactivity. For instance, working with less stranded cyclocarbonate or tailoring the nature of the catalyst may enhance the reactivity of the system. Linear NIPU have found various applications given the pending regulations.257,258 However important issues that remain concern especially the mechanical properties and chemical resistance of these materials to alkali and acid solutions. Also, the elasticity of NIPU does not permit elastomer applications. A way to circumvent all these drawbacks makes use of hybrid NIPU, based on the epoxy−amine− cyclocarbonate oligomers to build a network structure.217 Though quite old, urethane/urea chemistry deﬁnitively proposes many future new challenges in chemistry and material sciences.

mechanical properties of silicone materials, as well as the synthesis of functional polydimethylsiloxane.

Professor Jean-Pierre Pascault is Emeritus from 2005 at National Institute of Applied Science (INSA), Lyon, France. He was Professor in the same Institute from 1983 to October 2005, Director of the Laboratory of Macromolecular Materials (Associated with CNRS) from 1982 to 1998, Director of a CNRS Polymer Network Group (FR CNRS) from 2000 to 2006, and President of the French Polymer Group, GFP, and of the Polymer Division of the French Chemical Society, SFC, from 2001 to 2004. He has authored over 320 scientiﬁc publications including several book chapters and two books (Thermosetting Polymers and Epoxy Polymers: New Materials and Innovations) and 32 patents. His main research themes concern polyaddition/polycondensation reactions and new green polyurethane/epoxy materials.

ASSOCIATED CONTENT S Supporting Information *

Comparison of deblocking temperatures using diﬀerent techniques, comparison within blocker families, model-free kinetics equations for describing complex reactions, and the case of β-dicarbonyl compounds as blocking agents. This material is available free of charge via the Internet at http:// pubs.acs.org.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: +33 4 72 43 85 27. Notes

The authors declare no competing ﬁnancial interest. Biographies

Bernard Boutevin earned his Chemical Engineering degree from the School of Chemistry in Montpellier (France) and then received his Ph.D. degree in 1975 from Montpellier University, where he became a CNRS researcher. The topics of his research ﬁrst concerned telomerization, and later he investigated all of the methods of living radical polymerization (NMP, ATRP, ITP, RITP, and RAFT) in order to prepare original architectured polymers such as block, grafted, gradient, or alternated. Since 2005, he shifted his main theme of research towards green and sustainable polymer chemistry. He and his co-workers now synthesize bio-based materials such as epoxy resins obtained from natural polyphenols (ﬂavonoids, gallic acid and derivatives, and cardanol) and isocyanate-free polyurethanes through the carbonate−amine reaction. He has supervised more than 100 Ph.D. students, published 2 books, more than 500 peer-reviewed articles, and 25 reviews or chapters of books. He is the coinventor of more than 120 patents.

Etienne Delebecq was born in Lille, France, in 1984. He received his B.S. degree in Chemistry in 2005 from Ecole Nationale Supérieure de Chimie de Montpellier (ENSCM, France). In 2008, he received a M.S. degree from Université Montpellier and a Ingénieur Chimiste degree from ENSCM. In 2011, he earned his Ph.D. in Material Science at ENSCM, working under the direction of Dr. François Ganachaud. His research activities are focused on the improvement of thermal and AH

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MDA MDI NIPU OMO PHU PPDI PU PUA REACh RO/RE RO(M)P SN2 RIM TBD TDI TEA TGA THF tIPDI TM-DSC

François Ganachaud studied chemistry at CPE Lyon before graduating with a Ph.D. in 1997 from University Claude Bernard in Lyon. After 1 year postdoctoral work in Sydney at the KCPC, he took a CNRS research position ﬁrst in Paris (1999−2003) and then in Montpellier (2003−2011, habilitation in 2004). Back to Lyon from September 2011 as a senior CNRS scientist, his current research interests are (i) ionic polymerization in aqueous media, (ii) functional silicones and related materials, and (iii) emulsiﬁcation by the Ouzo eﬀect.

ACKNOWLEDGMENTS E.D. thanks FCI automotive for sponsoring his Ph.D. grant. J.P.P. and F.G. thank Françoise Méchin for several remarks and references brought to the paper.

TMP TMXDI TP(U) TS VOC

SYMBOLS AND ABBREVIATIONS ε dielectric constant γ hydrogen bonding index χ interaction parameter δ solubility parameters Mw mass-average molar mass N degree of polymerization Tcl ceiling temperature Tg glass transition Tm melting temperature 1K/2K one-pack/two-pack (reaction) acac acetylacetonate BDO butanediol CBC carbonylbiscaprolactam (CP/MAS) NMR (cross-polarization/magic angle spinning) nuclear magnetic resonance DABCO 2,2′-diazabicyclo[2.2.2]octane DBTDL dibutyltin dilaurate DBU 1,8-diazabicyclo[5.4.0]-7-undecene DETA diethylene triamine DMC dimethylcarbonate DMDEE 2,2′-dimorpholine diethyl ether DMF dimethylformamide DMP 3,5-dimethylpyrazole DMSO dimethylsulfoxide DN electron donor number FT-IR Fourier transform infrared (spectroscopy) HDI hexamethylene diisocyanate HE(M)A hydroxyl ethyl (meth)acrylate HMDI bis(4-isocyanatocyclohexyl)methane or hydrogenated MDI HPLC high-performance liquid chromatography HS/SS hard segment/soft segment IPDI isophorone diisocyanate

methylene dianiline methylene diphenyl isocyanate non-isocyanate polyurethanes dioctyl 4,4-methylenebis(phenyl carbamate) poly(hydroxyurethane) paraphenylene diisocyanate molecule polyurethane polyurethane acrylates Registration, Evaluation, Authorisation and Restriction of Chemical substances ring-opening/ring elimination (mechanisms) ring-opening (metathesis) polymerization nucleophilic substitution type 2 reaction injection molding 1,5,7-triazabicyclododecene toluene diisocyanate triethylamine thermal gravimetry analysis tetrahydrofurane trimer of isophorone diisocyanate temperature-modulated diﬀerential scanning calorimetry trimethyl-ol propane m-tetramethyl-xylylene thermoplastic (polyurethane) thermosetting (material) volatile organic compound

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