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Chapter 20
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Controlled Synthesis of Amphiphilic Poly(methacrylate)-g-[poly(ester)/poly(ether)] Graft Terpolymers 1
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Isabelle Ydens , Philippe Degée , Jan Libiszowski , Andrzej Duda , Stanislaw Penczek , and Philippe Dubois 2
1,*
1
Laboratory of Polymeric and Composite Materials (LPCM), University of Mons-Hainaut, Place du Parc 20, B-7000 Mons, Belgium Department of Polymer Chemistry, Center of Molecular and Macromolecular Sienkiewicza 112, PL-90-363 Lodz, Poland 2
Coupling atom transfer radical polymerization (ATRP) and coordination-insertion ring-opening polymerization (ROP) allows access to well-defined poly(methacrylate)-g[poly(ester)/poly(ether)] graft terpolymers according to a two-step procedure. In the first step, the controlled copolymerization of methyl methacrylate ( M M A ) , 2-hydroxyethylmethacrylate (HEMA) and poly(ethylene glycol) methyl ether methacrylate (PEGMA) was carried out by using ethyl 2-bromoisobutyrate and NiBr (PPh ) as initiator and catalyst, respectively. The second step consisted of the ring-opening polymerization (ROP) of ε-caprolactone (CL) or L,L -dilactide (LA) initiated by poly(MMA-co-HEMA-co-PEGMA) in the presence of either tin (II) bis(2-ethylhexanoate) (Sn(Oct) ) or triethylaluminum (AlEt ). The resulting graft copolymers as well as the intermediate poly(MMA-co-HEMA -co-PEGMA) terpolymers proved to be efficient surfactants as evidenced by dynamic interfacial tension measurements. 2
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© 2003 American Chemical Society
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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Introduction In analogy to low molecular weight surfactants, amphiphilic block and graft copolymers consisting of hydrophilic and hydrophobic segments can be used for stabilizing aqueous dispersions and emulsions {1,2), for compatibilizing the interface in polymer blends and composites (3), and for tuning the wettability of polymeric materials by surface modification, especially in the biomedical field (4,5). The factors that influence the tensioactive properties of amphiphilic copolymers include composition, structure, molar mass, copolymer-solvent interactions, concentration, temperature and preparation methods. In that respect graft amphiphilic coplymers represent valuable materials with the unique ability to vary a large variety of molecular parameters provided that they are obtained by controlled polymerization techniques, for example controlled anionic (6), cationic (7), radical (5-77) or ring-opening (metathesis) polymerization (72). In a recent paper (75), we reported on the synthesis of poly(methyl methacrylate-co-2-hydroxyethyl methacrylate) copolymers prepared by atom transfer polymerization (ATRP) and used as precursors for the ring-opening polymerization (ROP) of (di)lactones. The combination of these two consecutive polymerization processes allowed to access to well-defined brush like poly(methacrylate)-g-poly(aliphatic ester) copolymers of a wide range of molar mass and composition (14,15). However, both the polymethacrylate backbone and the polyester branches were hydrophobic which prompted us to incorporate a third functional (hydrophilic) comonomer, i.e. poly(ethylene glycol) methyl ether methacrylate ( P E G M A ) . This paper thus focuses on the synthesis and characterization of poly(methacrylate)-g-[poly(ester)/poly(ether)] graft terpolymers (Figure 1) by controlled radical terpolymerization of methyl methacrylate ( M M A ) , 2-hydroxyethyl methacrylate ( H E M A ) , and poly(ethylene glycol) methyl ether methacrylate ( P E G M A ) , followed by the graft copolymerization of ε-caprolactone (CL) or L,L-dilactide (LA) initiated from the free hydroxyl groups of the H E M A units in the presence of tin (II) bis(2ethylhexanoate) (Sn(Oct) ) or triethylaluminum (Al(Et) ). Preliminary dynamic interfacial tension experiments have been carried out on the copolymers. 2
3
Experimental Section
Synthesis of P o l y ( M M A - c o - H E M A - c o - P E G M A ) A l l experiments were conducted according to the Schlenck method. NiBr (PPh ) and a magnetic bar were introduced in open air into a pre-weighed glass tube, which was then closed by a three-way stopcock and purged by three repeated vacuum/nitrogen cycles. In a 50ml flask, M M A , dried H E M A , P E G M A and the desired volume of toluene were introduced and bubbled with nitrogen for 2
3
2
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003. 3
Ο
CH3
N
CH,
ÇH,
:
J^O °& Ο
Br 2
C
H
J ^ O ^ C H / CH^T Y -(CH^
0
X
> O ^ C H
Poly(MMA-co-HEMA-co-PEGMA)-g-poly(£-caprolactone)
ι
A> A)
O
CH
Figure 1.
C K C H O ^ O CH3CHP
CH3
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X
2
C
H
:
CHjOH
/(CH ) 2
286 10 minutes, before transferring the mixture into the glass tube. Then, the initiator (ethyl 2-bromoisobutyrate, (EiB)Br) (0.27 molL* in dried toluene) was added under nitrogen with a syringe. The glass tube was then immerged into an oil bath maintained at a defined temperature. After a predetermined polymerization time, the glass tube was rapidly cooled down to room temperature and its content was dissolved in T H F . The terpolymer was then selectively recovered by precipitation from heptane. The conversion in terpolymer was determined by weighing after drying at 80°C for 24h under reduced pressure. In order to remove out the nickel catalyst, the terpolymer was dissolved in tetrahydrofuran and passed through a column of basic alumina. The purified terpolymer was recovered by precipitation from heptane, filtration and drying until constant weight.
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1
Synthesis of Terpolymers
Poly(methacrylate)-g-[poly(ester)/poly(ether)]
Graft
Polymerization was carried out with purified and dried reactants, using standard high vacuum technique (/ 7). Typically, separate ampoules equipped with break-seals containing respectively, the desired cyclic ester ( L A or CL), the poly(MMA-co-HEMA-coP E G M A ) terpolymer and Sn(Oct) were sealed to a reacting glass ampoule (30 mL) together with a polarimetric (5 mL) or a dilatometric (15 mL) cell. A known volume of T H F was distilled into the reacting glass ampoule which was then sealed off. The break-seals were broken, and when all components were dissolved, the resulting solution was transferred into either the polarimetric ( L A case) or dilatometric cell (CL case), and into the reacting ampoule. The cell and ampoule containing the reacting mixture were separately sealed off. The cell was first placed into a bath kept at 80°C so as kinetic data could be collected. Then, the ampoule was allowed to polymerize at 80°C for the time required to reach maximum monomer conversion. The ampoule was opened and a drop o f the crude reacting mixture was injected in a S E C apparatus to determine the conversion and molecular weight parameters. The rest of the reacting medium was precipitated into cold methanol, separated by filtration and dried under vacuum. In an alternative procedure, the poly(MMA-co-HEMA-co-PEGMA) terpolymers were transferred in open air into a previously dried two-necked round bottom flask equipped with a stopcock and a rubber septum, dried by three successive azeotropic distillations of toluene, then dissolved in dry toluene and added with a defined volume of the activator : Sn(Oct) or Al(Et) . A s far as Sn(Oct) is concerned, the activator and the monomer were added successively to the terpolymer solution in toluene and the temperature was raised up to 110 °C. When triethylaluminum (Al(Et) ) was used as the activator, the reaction flask was equipped with an oil valve for ethane evolution and the reaction was stirred for 4 2
2
3
2
3
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
287 hours at r.t. before adding C L and heating up to desired polymerization temperature. Whatever the activator, the reaction product was recovered by precipitation from heptane, filtration and drying under vacuum until constant weight.
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Instrumentation *H N M R spectra were recorded using a Bruker A M X 3 0 0 or a Bruker D R X 500 spectrometer at room temperature in C D C I 3 . Size exclusion chromatography (SEC) was performed either in T H F at 35°C or in CH C1 . The actual numberaverage molar masses (M ) of the copolymers were determined in chloroform with a Knauer vapor phase pressure osmometer and a Knauer membrane osmometer for M < 3.5 χ 10 and > 3.5 χ 10 , respectively. Interfacial tensions were obtained using a D S A 10-MK2 tensiometer (Wilten Fysika) at 20°C according to the pendant drop method. Interfacial tension values were determined from the shape of the pendant drop by fitting the Gauss-Laplace equation to the experimental drop shape coordinates (16). A l l reported data points are average values of triplicate measurements (with a max. variation < 2%). 2
2
n
4
4
t t
Results and Discussion
Poly(MMA-co-HEMA-co-PEGMA) Poly(MMA-co-HEMA-co-PEGMA) terpolymers have been synthesized by atom transfer radical copolymerization of M M A , H E M A and P E G M A in toluene at 80 or 85 °C using NiBr (PPh ) as catalyst and ethyl 2-bromoisobutyrate ((EiB)Br) as initiator. Tables I and II show the experimental conditions for the synthesis of the terpolymers, which will be used as precursors for the grafting ROP reaction, and their molecular characteristics, respectively. The initial content of P E G M A ( M = 455) ranged from 3 to 11 mol % while the H E M A molar fraction was deliberately maintained below 11 mol % in order to prepare graft copolymers with a low branching density. Compared to the initial molar fractions in comonomers, the final composition of poly(MMA-co-HEMA-co-PEGMA) terpolymers as determined by H N M R shows that the functional monomers, i.e. H E M A and P E G M A , are preferably incorporated into the growing chains. For instance, molar fractions in T3 reach 0.75 for M M A , 0.15 for H E M A and 0.10 for P E G M A starting from 0.82, 0.10, and 0.08, respectively. 2
3
2
n
!
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
288 Table I. Experimental conditions for the synthesis of methacrylic [MMAJo fmolL') 2.56 2.61 2.62 2.59 2.59
Sample
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Tl T2 T3 T4* T5**
fPEGMAJo (molL') 0.09 0.29 0.25 0.33 0.33
[HEMAJo (molL ) 0.12 0.17 0.33 0.17 0.17 1
Conv. (96) 68 76 74 83 64
Toluene, 16h, 85°C, [Mtotal]o/[(EiB)Br]o/[NiBr (PPh ) ]o= 100/1/0.5 3
2
2
*T=80°C **T= 80°C, [Mtotal]o/[(EiB)Br]o/[NiBr (PPh3)2]o = 200/1/0.5 2
Table II. Molecular characteristics of methacrylic terpolymers Sample M (VPOf M MJM ΠΜΜΑ ΠΟΗ ΠΡΕΟΜΑ Tl 11,100 10,000 1.20 80.9 6.7 3.8 T2 16,100 13,900 1.18 77.5 8.7 13.6 T3 14,600 14,600 1.19 72.9 14.2 10.2 Τ4 13,000 14,600 1.27 73.1 7.1 9.9 Τ5 16,000 20,600 1.32 88.6 9.3 12.4 b
n
n
b
n
a
As determined by vapour phase pressure osmometry in methylene chloride.
b
As determined by SEC in THF with reference to PMMA standards.
c
nj denotes the average number of repetitive units per terpolymer chain : n H
=
0
(M (VPO)-M n
(2Id/3I ); n f
Figure!
{EiB)Br
P E G M A
)/f(2I /3I ) χ MMMA + (2I,/3I ) χ M
=n
d
0 H
f
f
P E G
M A + MHEMA]; Π ΜΑ = n Μ
0 H
x
x (2I|/3I ) (see Figure 2). f
1
Η NMR (500MHz) spectrum of a poly(MMA-co-HEMA-co-PEGMA) terpolymer (sample T3 in Tables 1 and 2).
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
289 As recently reported (13) and in perfect agreement with previous publications (18,19), the copolymerization of M M A and H E M A under very similar experimental conditions has proved to be totally controlled as evidenced by kinetic measurements (linear time dependencies o f ln([M] /[M]) where [ M ] and [M] represent the comonomer concentration at start and at a given polymerization time, respectively) and a linear increase of M as a function of conversion. The molar mass can be predicted from the initial comonomer-to-initiator molar ratio corrected by comonomers consumption degree (a) and the molar mass distribution is remarkably narrow ( M / M < 1.25), at least for α < 0.85. 0
0
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n
w
n
Poly(MMA-co-HEMA-co-PEGMA)-g-aliphatic polyesters Poly(methacrylate)-g-[poly(ester)/poly(ether)] graft copolymers have been obtained by initiating the ring-opening polymerization (ROP) of either εcaprolactone (CL) or L,L-dilactide (LA) from the hydroxyl functions pending along the poly(MMA-co-HEMA-co-PEGMA) terpolymers, previously activated by tin(II) bis(2-ethyl hexanoate), Sn(Oct) , in T H F at 80 °C for an initial concentration in Sn(Oct) of 5 10" molL" . Under such conditions, Sn(Oct) has proved to react with hydroxyl functions providing initiating species such as tin(II) alkoxides (20-22). Table III and I V list the experimental conditions and molecular characteristics of the synthesized graft copolymers. Cyclic ester conversion was determined by SEC analysis as reported elsewhere (20,21). The molar mass of both the graft copolymer and the polyester branches, as determined by osmometry and H N M R spectroscopy, respectively, are in quite good agreement with the values expected from the initial monomer-tomacroinitiator molar ratio. 2
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l
Table III. Conditions for the synthesis of poly(MM A-co-HEM A-coPEGMA)-g-aIiphatic polyester in THF at 80 °C under high vacuum technique Conv. Time [OHJo" [M] MF Sample (min.) (molL ) (moll ) (%) 256 92.3 0.0751 PI LA Tl 0.82 89.0 776 0.0104 LA Tl 1.09 P2 90.2 0.0128 1,651 P3 CL Tl 1.71 88.3 497 0.0082 P4 LA T2 1.00 75.0 264 0.0156 P5 LA T3 1.01 0
1
1
a
M and MI denote the monomer and the terpolymer macroinitiator, respectively.
b
[OH] = (1000 x m 0
Terpolymer
χ noHy(M (VPO) n
Teipolymer
χ total solution volume (ml)).
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
290 Table I V . Molecular characteristics of p o l y ( M M A - c o - H E M A-co P E G M A)-g-aliphatic polyester M„ M„ M„ M„ (NMR, MJM (calc, Sample (calcf (osmo) (SECf branch/ branch/ 1,150 1,350 1.22 18,700 PI 20,000 21,300 13,600 13,200 1.67 55,100 P2 99,600 73,300 5,800 6,700 1.99 109,500 109,600 80,400 P3 15,100 15,250 1.75 P4 148,700 97,800 89,400 13,350 13,550 1.59 69,600 P5 101,900 86,650 Downloaded by PENNSYLVANIA STATE UNIV on August 8, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch020
n
a
M (calc) = ((mMono^niTerpoiymerHl) x Μ ( ν Ρ Ο ) n
b
η
ΤαροΙ>ΠΐβΓ
χ conv.
As determined by SEC in THF with reference to PMMA standards.
c
M calc, branch = (M calc - M (VPO) erpoiymeryn HM NMR, PCL branch = ( ( 1 ^ - 2/3 I,) / ( l - I )) x M (see Figure 3) and M„ NMR, PLA branch = (I / (I - I + 2/3 I,)) χ MLA/2 (see Figure 4). n
n
n
T
0
d
n
n + f
h
f+tn
e
C L
j+c+e
Practically, the number-average molecular weight of the poly(ecaprolactone) (PCL) and poly(L,L-lactide) ( P L A ) branches were determined from the relative intensities of the protons of the repetitive units ( - C H 2 - O - C O - at 4.06 ppm for P C L and - C H - O - C O - at 5.15 ppm for P L A ) and the protons of the polyester hydroxyl end-groups ( - C H 2 - O H at 3.63 ppm for P C L and - C H - O H at 4.35 ppm for P L A ) (Figures 3 and 4). It is worth mentioning that for poly(MMAco-HEMA-co-PEGMA)-g-PCL, the assignment has been facilitated by the addition of a few drops of trichloroacetylisocyanate (TCAI) directly in the N M R tube. T C A I is known to quantitatively react with hydroxyl functions to form trichloroacetylcarbamate derivatives, thereby provoking a down-field shift of approximately 1 ppm for the α-hydroxymethylene protons. The corresponding atrichloroacetylcarbamate methylene protons are thus observed at 4.30 ppm. It must also be emphasized that the chemical shift o f terpolymer ahydroxymethylene protons H from 3.85 (see Figure 2) up to 4.28 ppm (Figure 3), attests for the grafting efficiency, meaning that every pendant hydroxyl group of the poly(MMA-co-HEMA-co-PEGMA) precursors has actually initiated the polymerization of both C L and L A . f
The kinetics of cyclic esters polymerization initiated with Sn(Oct) /macroalcohol system have been studied by measuring the monomer conversion by either dilatometry (CL) (20) or polarometry ( L A ) (21) as previously reported. 2
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
291
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tn HLC 2\
CH A 2
c+j+m
ΗΝ CljC
n+f _JuJ
Τ
δ/ppm
Τ
!
Figure 3. HNMR (500MHz) spectrum ofpoly(MMA-co-HEMA-co-PEGMA)-gpoly(s-caprolactone) with the polyester ohydroxyl end-groups transformed into trichloroacetylcarbamate (sample P3 in Tables III and IV).
Figure 5 shows semilogarithmic plots for various polymerizations carried out in T H F at 80 °C with poly(MMA-co-HEMA-co-PEGMA) as multifunctional macroinitiator activated by Sn(Oct) . For the sake of comparison, the kinetics of L A polymerization initiated by η-butyl alcohol ([BuOH] = 1.5 lO^molL" ) in the presence of Sn(Oct) ([Sn(Oct) ] = 5 10" molL' ) is also presented. After an induction period, linear time dependencies of ln([M]o/[M]) are observed whatever the (macro)initiator and monomer which demonstrate that both C L and L A polymerizations proceed without significant change in the number of active species. 2
1
0
3
2
2
1
0
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
292 e
CH
e 3
CH
e
« κ
ÇH,
H C-ç4cH-ç4ïfcH -ç4ifcH -Ç-[ Br 3
CH CH 3
2
2
2
r
0
0
^CH
3
*CH
2
" Τ
*CH *H C
2
ο *γ
b+i
Η
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2
- 0H„ «CH *
τ
Γ
Figure 4. 'HNMR (500 MHz) spectrum ofpoly(MMA-co-HEMA-co-PEGMA)g-poly(L,L-lactide) (sample P4 in Tables III and IV).
In the B u O H and T3 coinitiated L A polymerizations ((P5 in Tables III and IV), starting concentrations of L A ( [ L A ] = 1 molL" ), Sn(Oct) ([Sn(Oct) ] 5 ΙΟ" molL" ) and hydroxyl groups ([OH] = 1.5 10* molL' ) are identical but the polymerization rate for T3 is significantly depressed. As all O H groups pendant along the polymethacrylic backbone have proved to initiated the lactone polymerization, the decreased polymerization rate can be attributed to a lower accessibility of the O H groups in the macroinitiator, and then in the growing polyester pendant chains, in perfect agreement with previously reported data involving poly(MMA-co-HEMA) macroinitiator (13). It has to be stressed that the specific effects of the hydroxyl groups concentration, the macroinitiator composition and the molar mass on the polymerization rate have not been investigated in the present contribution, the main issues being the controlled synthesis of poly(methacrylate)-g-[poly(ester)/poly(ether)] graft copolymers and their use as potential tensioactive agents. 1
β
0
3
1
2
1
0
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
2
0
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293
300
400
500
600
700
Time (min.)
Figure 5. Kinetics of LA and CL polymerizations initiated by Sn(Oct) /ROH system. Experimental conditions are listed in Table III except for Sn(Oct)^BuOH initiated LA polymerization ([LA] = 1 molL" , [BuOH] = J.5 10' molL and fSn(Oct)2j = 5 Iff molL ). 2
1
2
0
1
0
3
1
0
However, under the aforementioned conditions, it must be noticed that molar masses distributions can be relatively broad with polydispersity indices which can be as high as 2 (sample P3 in Table IV). Smaller values were obtained when C L was substituted for L A (P2 and P3 in Tables III and IV) and the initial hydroxyl/Sn(Oct) molar ratio increased up to 15 (PI in Tables III and IV). The broadening of molecular weight distribution is thus likely consistent with the occurrence of transesterification reactions that can be prevented, or at least limited, by using lower (catalytic) amounts of activators as exemplified hereafter. In this respect, the ROP of C L starting from poly(MMA-co-HEMA-coP E G M A ) terpolymers has been carried out in toluene at 60 or 110 °C by varying the nature and the relative content of the activator, i.e. Al(Et) or Sn(Oct) . Tables V and V I show the experimental conditions and the molecular characteristics of the synthesized graft terpolymers. A s well-known, hydroxyl groups readily react with Al(Et) to form mono-, di-, and/or trialkoxides depending on the reaction stoechiometry. In the current study, when Al(Et) was 2
3
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3
3
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
294 Table V . Conditions for the synthesis of poly(MMA-co-HEMA-coPEGMA)-g-poly(B-caprolactone) in toluene using either Sn(Oct) or AlEt as activators (X). Time Conv. Τ [OH] [M]o Sample MF X (min) (molL ) (molL ) iOHJo eg (96) T4 95 40 Al(Et) 60 1.1 P6 2.03 0.0140 2
3
c
b
0
1
1
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3
Al(Et)
P7
T4
P8
T4
3.53
0.0832
P9
T4
7.78
0.0901
Sn(Oct) Sn(Oct)
P10
T5
7.76
0.0961
Sn(Oct)
2.01
a
0.0139
1/21
3
60
930
97
861
65
2
1/105
110
2
1/231
110
946
64
2
1/232
110
946
85
b
MI = macroinitiator [OH] = (1000 x m χ n )/(MJV?0) solution volume (ml)). As determined by gravimetry 0
Tcrpolynîer
OH
Tetpol:fmer
χ total
c
Table VI. Molecular characteristics of poly(MMA-eo-PEGMA-co HEMA)-g-poly(s-caprolactone) M„ M„ (NMR Sample M„ (calcf MJM„ (calc, M„S£C* branchf branchf ND 105,800 2.25 15,700 P6 124,500 b
P7
126,500
175,700
1.37
16,000
11,500
P8
21,200
30,300
1.32
1,200
1,400
P9
58,500
95,300
1.32
6,400
7,200
P10
89,500
181,900
1.29
7,900
ND
A
6
b
M (calc) = M ( V P O ) + ( n x M calc branch ). As determined by SEC in THF with reference to PMMA standards. M (calc, branch) = ([CL] /[OH] ) x conv χ M L N
n
tefpolymer
0H
N
c
N
d
M (NMR, branch) = [(Ip L2.3 pm x 3 χ n n
C
P
0
PEGMA
)/(Ip
EGM
0
C
A 3.35 m x 2 χ n )] x M PP
OH
C L
.
used in slight excess with respect to the number of hydroxyl groups along the polymethacrylic backbone ([AlEt ]/[OH] = 1.1), the size exclusion chromatogram displayed a rather broad molecular weight distribution (M /M„ = 2.25) (P6 in Tables V and VI). As previously reported by some of us (23, 24), such a behavior can be explained by the formation of ethylaluminum dialkoxide together with the expected diethylaluminum monoalkoxide, particularly when the hydroxyl functions are localized close to each other. A s a result, two different active species coexist and can promote the growth of polyester grafts with different kinetics, which could lead to very broad molecular weight distribution. In contrast, when a catalytic amount of Al(Et) ([AlEt ]/[OH] =4.8 10" ) is used to initiate the R O P (P7 in Tables V and VI), the polymerization o f CL is slower but the molar masses distribution is narrower ( M / M = 1.37). 3
w
2
3
3
w
n
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
295 Such a behavior can be explained by a faster alcohol-alkoxide interchange reaction than the propagation so that one type of active aluniinium alkoxide actually initiates and propagates the ROP. Similarly, the polymerization of C L has been carried out in toluene at 110 °C in the presence of a catalytic amount of Sn(Oct) (4.3 10*
