Cellulose Technology Research - American Chemical Society

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4 Silyl Cellulose R O B E R T E . H A R M O N and K A L Y A N K.

DE

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Western Michigan University, Kalamazoo, Mich. 49001

E a r l y Works on

Silylation

of C e l l u l o s e

The silylation (the term "silylation" i s used i n the gene r a l sense of s u b s t i t u t i o n w i t h t r i o r g a n o s i l y l groups) of numerous n a t u r a l products, i n c l u d i n g v a r i o u s sugars, has been reported in the l i t e r a t u r e , and t h i s procedure has become a commonplace operation whenever the e f f e c t s of hydrogen bonding on the p h y s i c a l p r o p e r t i e s are u n d e s i r a b l e . One of the most ubiquitous and important n a t u r a l products with an abundance of f r e e hydroxyl groups which s i g n i f i c a n t l y determine its p h y s i c a l p r o p e r t i e s is c e l l u l o s e . Replacement of some or all of the hydroxyl protons of c e l l u l o s e by silyl groups can be expected to a l t e r r a d i c a l l y the p r o p e r t i e s of t h i s p o l y mer, j u s t as e s t e r i f i c a t i o n or a l k y l ether formation d r a s t i c a l l y modify the parent c e l l u l o s e . The e f f e c t s of polymer m o d i f i c a t i o n by silylation have been demonstrated in a t l e a s t two cases, eg., silyl p o l y v i n y l a l c o h o l (1) and silyl polyureas (2). The increased solubility of silylated polymers i n nonpolar solvents i s p a r t i c u l a r l y noteworthy. Silicon t e t r a c h l o r i d e and aryl- and a l k y l h a l o s i l a n e s r e a c t e a s i l y and r a p i d l y with organic hydroxyl groups (3,4,5) to y i e l d hydrogen c h l o r i d e and silicic e s t e r s . A number of patents have been issued d e a l i n g with the treatment of c e l l u l o s e with organo-silicon h a l i d e s to impart water r e p e l l e n c y (6,7,8). In one case (6) a s u r f a c e r e a c t i o n e i t h e r with adsorbed moisture or w i t h hydroxyl groups of the c e l l u l o s e was p o s t u l a t e d . However, the amount of product formed was too small to be measured or anal y z e d . J u l l a n d e r (9) has s t u d i e d the r e a c t i o n of silicon t e t r a c h l o r i d e with n i t r o c e l l u l o s e and r e p o r t s the formation of g e l s due to cross l i n k a g e . Schuyten (10,11) and Hunter (12) have reported the preparat i o n and modified p r o p e r t i e s of partially s i l y l a t e d c e l l u l o s e . The ether or e s t e r groups i n partially e t h e r i f i e d or e s t e r i f i e d c e l l u l o s e are partially replaced with R R ' S i r a d i c a l s (12), where R is aryl and R' is aryl or a l k y l , to give film-forming c e l l u l o s e 2

39

In Cellulose Technology Research; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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d e r i v a t i v e s with improved thermal s t a b i l i t y , d i e l e c t r i c strength, and moisture r e s i s t a n c e . The s i l i c o n content of t h i s product reported by Hunter was 5.44%. Schuyten has obtained trimethy1s i l y l c e l l u l o s e with as high as 2.75 t r i m e t h y l s i l y l groups per glucose u n i t . With p a r t i a l l y s u b s t i t u t e d c e l l u l o s e acetate, the t o t a l s u b s t i t u t i o n approached 3.0 (Table I ) .

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Table I.

Reactions of A l k y l c h l o r o s i l a n e s With C e l l u l o s e Acetate

A c e t y l groups/ glucose

Reagent

Before

S i l y l groups/ glucose

Total groups

After

(CH ) SiCl

2.30

2.24

0.65

2.89

(C H ) SiCl

2.30

2.24

0.58

2.82

(CH ) SiCl

2.90

2.87

0.14

3.01

3

2

3

s

3

3

3

Various other s i l a n e s were employed and s i m i l a r r e s u l t s were obtained (Table I I ) . Table I I .

Reactions of Various S u b s t i t u t e d S i l a n e s With C e l l u l o s e

Cellulose (g)

Reagent

Si (%)

S i l y l group/ glucose

(C H ) SiCl

2.15

11.55

1.27

(CH ) SiCl

1.79

2.47

a

1.96

1.33

a

1.90

1.16

a

2

5

3

3

2

2

n-C H SiCl e

l 7

CH Si(0Ac) 3

^ot

3

3

c a l c u l a t e d because of the p o s s i b i l i t y of cross l i n k a g e .

The t r i m e t h y l s i l y l c e l l u l o s e obtained might be decomposed by b o i l i n g with water or w i t h d i l u t e a c i d or base. No q u a n t i t a t i v e r e s u l t s were obtained but b o i l i n g with d i l u t e a c i d y i e l d e d a v o l a t i l e l i q u i d product which burned and deposited s i l i c a and was assumed to be t r i m e t h y l s i l a n o l . Accordingly samples of t r i m e t h y l s i l y l c e l l u l o s e c o n t a i n i n g 2.6 t r i m e t h y l s i l y l groups per glucose u n i t were placed i n open containers under three s e t s of conditions: (1) d e s i c c a t e d over phosphorus pentoxide; (2) d r i e d i n an oven at 105°; and (3) conditioned at 70°F (21.1°) and 65% r e l a t i v e humidity. The samples were weighed a t i n t e r v a l s and the percent r e t e n t i o n of added weight was p l o t t e d a g a i n s t time. The r e s u l t s are shown i n F i g u r e 1. The i n f l e c t i o n a t the beginning of the curve f o r the conditioned sample i s due to i n c r e a s e i n moisture content at 65% r e l a t i v e humidity.

In Cellulose Technology Research; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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4.

HARMON

AND

Silyl Cellulose

DE

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Hundreds of hours Figure 1. Deterioration of trimethylsilyl cellulose: A, desiccated (P 0 ); B, heated (105°); C, conditioned at 70°F and 65% rektive humidity 2

5

These data i n d i c a t e the formation of d e f i n i t e compounds o f c e l l u l o s e with s u b s t i t u t e d s i l a n e s . A 0.1 g. sample o f t r i m e t h y l s i l y l c e l l u l o s e (2.42 groups per glucose u n i t ) was placed i n 5 ml. of s o l v e n t i n a small tube and tumbled on a wheel f o r 24 hours a t room temperature. When the major p o r t i o n of the m a t e r i a l remained undissolved as determined by v i s u a l i n s p e c t i o n , i t was considered i n s o l u b l e . In some cases (designated by s w ) the sample p a r t i c l e s showed a tendency to s w e l l . No n o t i c e a b l e amount d i s s o l v e d i n the f o l l o w i n g s o l v e n t s : acetone, benzene, carbon t e t r a c h l o r i d e , chloroform, d i e t h y l f o r m amide, e t h y l acetate, e t h y l a l c o h o l , ethylene d i c h l o r i d e , ether (sw), methylene c h l o r i d e , methyl e t h y l ketone (sw), nitromethane, nitropropane, p y r i d i n e (sw), s-tetrachloroethane and xylene. The m a t e r i a l was not s o l u b l e i n the f o l l o w i n g mixtures: acetone (80%), ethanol (15%), e t h y l acetate (5%) (sw) ; toluene (80%), ethanol (20%) (sw); ethylene d i c h l o r i d e (90%), methyl a l c o h o l (10%); and benzene (66%), ethylene d i c h l o r i d e (34%) (sw). Rogovin (13,14) has reported preparing s i l y l c e l l u l o s e from a l k a l i c e l l u l o s e (1 part s u l f i t e pulp + 10 p a r t s 40% NaOH, p r e s s ed to 4 times the weight o f c e l l u l o s e ) and h a l o m e t h y l t r i a l k y l s i l a n e s . Maximum degrees of s u b s t i t u t i o n were reached when the r e a c t i o n was c a r r i e d out a t 120° f o r 20 hours i n sealed ampules. Reaction with c h l o r o m e t h y l t r i m e t h y l s i l a n e under these c o n d i t i o n s gave s i l y l c e l l u l o s e with 10.9% s i l i c o n , with i o d o m e t h y l t r i m e t h y l s i l a n e gave s i l y l c e l l u l o s e with 13.9% s i l i c o n , and with c h l o r o m e t h y l t r i e t h y l s i l a n e gave s i l y l c e l l u l o s e with 4.5% s i l i c o n . The products were i n s o l u b l e i n common organic solvents and i n cuprammonium s o l u t i o n . f f

f l

In Cellulose Technology Research; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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S i l y l a t i o n With B i s ( t r i m e t h y l s i l y l ) - A c e t a m i d e

TECHNOLOGY

RESEARCH

(BSA)

Klebe and coworkers (15-18) have prepared a number of t r i a l k y l s i l y l d e r i v a t i v e s of c e l l u l o s e by using s i l y l amides. The f a v o r a b l e r e s u l t s obtained w i t h b i s ( t r i m e t h y l s i l y l ) - a c e t a m i d e (BSA) as a s i l y l a t i n g agent f o r v a r i o u s c l a s s e s of compounds w i t h r e a c t i v e protons (19) prompted them to attempt the s i l y l a t i o n of c e l l u l o s e with t h i s reagent and to r e i n v e s t i g a t e the p r o p e r t i e s of the product. The s i l y l a t i o n of c e l l u l o s e with BSA turned out to be s t r a i g h t forward once a s u i t a b l e s o l v e n t was found. I t i s c h a r a c t e r i s t i c of h i g h l y hydrogen-bonded m a t e r i a l s of high molecular weight that t h e i r s i l y l a t i o n i s very s l u g g i s h , even when the m a t e r i a l i n the s i l y l a t e d form i s q u i t e s o l u b l e i n the s o l v e n t used f o r the r e a c t i o n . Solvent systems i n which the parent polymer i s somewhat s o l u b l e or i n which a t l e a s t some s w e l l i n g occurs are p r e f e r a b l e even i f the s o l u b i l i t y of the s i l y l a t e d product i s o n l y marginal i n the p a r t i c u l a r s o l v e n t . S i l y l a t i o n s have been c a r r i e d out by suspending v a r i o u s grades of n a t u r a l c e l l u l o s e i n p o l a r s o l v e n t s such as dimethyls u l f o x i d e (DMSO), dimethylformamide (DMF), N-methylpyrrolidone (NMP), and hexamethylphosphoramide (HMPA), w i t h a d d i t i o n of an excess of 20-30% over the c a l c u l a t e d amount of BSA and on heating the a g i t a t e d mixtures under anhydrous c o n d i t i o n s a t temperatures of 100° to 150°C. HMPA and NMP were found p a r t i c u l a r l y useful. In the l a t t e r s o l v e n t the f i b e r s of a wood pulp c e l l u l o s e turned i n t o a transparent tan-colored g e l w i t h i n one hour at a temperature of 150°C. T h i s NMP-insoluble g e l y i e l d e d a v i s c o u s s o l u t i o n upon a d d i t i o n of xylene or benzene to the mixture. The choice of the grade of c e l l u l o s e proved to be of some consequence. Depending on the p r i o r h i s t o r y of the c e l l u l o s e , the s i l y l a t i o n s go more or l e s s r e a d i l y to completion. In some cases, although most of the s t a r t i n g m a t e r i a l appeared to have reacted, some i n s o l u b l e g e l remained. Some commercially a v a i l a b l e wood pulp c e l l u l o s e s (types V-60 and V-90, Buckeyee C e l l u l o s e Corp., Memphis, Tennessee) are very r e a d i l y s i l y l a t e d . Cotton l i n t e r pulps from the same source were g e n e r a l l y of higher molecular weight and, although they l e f t no unreacted i n s o l u b l e m a t e r i a l , gave extremely v i s c o u s s o l u t i o n s . The nature of the hydroxyl groups i n c e l l u l o s e suggests that any r e s i s t a n c e to s i l y l a t i o n may be due more to s t e r i c reasons and l a c k of s o l u b i l i t y than to i n t r i n s i c " c h e m i c a l difficult i e s i n d i s p l a c i n g these p a r t i c u l a r protons by t r i m e t h y l s i l y l groups. Weaker s i l y l a t i n g agents l i k e s i l y l a m i n e s could be expected to s u f f i c e once s u i t a b l e r e a c t i o n c o n d i t i o n s were found. Treatment of c e l l u l o s e i n NMP with N - t r i m e t h y l s i l y l p i p e r i d i n e at 140-150°C f o r 3-5 hours gave v i s c o u s s o l u t i o n s with v e r y l i t t l e insoluble residue. The polymeric product prepared with any of these s i l y l a t i n g agents and s o l v e n t s could be recovered e i t h e r by vacuum d i s t i l l a 1 1

In Cellulose Technology Research; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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t i o n of v o l a t i l e matter, or more conveniently, by p r e c i p i t a t i o n w i t h p o l a r solvents l i k e acetone, a c e t o n i t r i l e , or a l c o h o l s which y i e l d e d the polymer i n the form of white f i b e r s . The products contained 2-3 t r i m e t h y l s i l y l groups per repeating u n i t and were a l l cases s o l u b l e i n aromatic, c h l o r i n a t e d , and a number of a l i phatic solvents.

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S i l y l a t i o n With

Trimethylchlorosilane

The composition of the s o l u b l e t r i m e t h y l s i l y l c e l l u l o s e obtained by Klebe and coworkers (15-18) must be v i r t u a l l y i d e n t i c a l (according to the elemental a n a l y s i s ) with the t o t a l l y i n s o l u b l e product which Schuyten and coworkers (10,11) obtained by t r e a t ment of c e l l u l o s e with t r i m e t h y l c h l o r o s i l a n e i n p y r i d i n e . F r e s h l y d i s t i l l e d commercial t r i m e t h y l c h l o r o s i l a n e and a sample of the c e l l u l o s e which had y i e l d e d s o l u b l e product with BSA a l s o gave an i n s o l u b l e product. The c r o s s - l i n k i n g of the polymer t h e r e f o r e occurred as a r e s u l t of e i t h e r some secondary mode of r e a c t i o n of t r i m e t h y l c h l o r o s i l a n e i t s e l f or the presence of some impurity i n the c h l o r o s i l a n e . The most l i k e l y i m p u r i t i e s i n t h i s commerc i a l product are higher c h l o r i n a t e d s i l a n e s . Dimethyldichloros i l a n e and m e t h y l t r i c h l o r o s i l a n e b o i l at 70° and 66°C, respect i v e l y , and i t should be very d i f f i c u l t to remove by f r a c t i o n a t i o n traces of these compounds from t r i m e t h y l c h l o r o s i l a n e w i t h a b o i l i n g point of 57°C. Therefore, pure t r i m e t h y l c h l o r o s i l a n e was prepared by r e a c t i o n of anhydrous hydrogen c h l o r i d e with hexamethyldisilazane and allowed to r e a c t with c e l l u l o s e suspended i n a mixture of p y r i d i n e and xylene. A completely s o l u b l e product was indeed obtained a f t e r a r e a c t i o n time of 4 hours at 110°C. Thus, the i n s o l u b i l i t y of the product i n the e a r l i e r experiments had e v i d e n t l y been caused by i m p u r i t i e s i n commercial trimethylchlorosilane. The l i k e l y c u l p r i t s i n t h i s undesirable c r o s s - l i n k i n g r e a c t i o n , d i m e t h y l d i c h l o r o s i l a n e and m e t h y l t r i c h l o r o s i l a n e , are known to be considerably more r e a c t i v e than t r i m e t h y l c h l o r o s i l a n e . A d d i t i o n of a small amount of sugar to the mixture of commercial t r i m e t h y l c h l o r o s i l a n e and p y r i d i n e and heating f o r a few minutes p r i o r to the a d d i t i o n of the c e l l u l o s e proved to be s u f f i c i e n t to remove the i m p u r i t i e s from the s o l u t i o n i n the form of an e a s i l y f i l t e r a b l e brown lump of s o l i d . Cellulose added at t h i s point was s i l y l a t e d to a completely s o l u b l e product. T h i s ' sweetened procedure provides a more economical synthesis of s i l y l c e l l u l o s e than the s i l y l a t i o n s with BSA. 1

11

Trimethylsilyl Ethyl Cellulose Klebe and coworkers s u c c e s s f u l l y made m o d i f i c a t i o n of c e l l u l o s e d e r i v a t i v e s by r e p l a c i n g r e s i d u a l hydroxy protons by s i l y l groups. I t has been p o s s i b l e to t r i m e t h y l s i l y l a t e under mild c o n d i t i o n s an e t h y l c e l l u l o s e containing an average of 0.7 - 0.8

In Cellulose Technology Research; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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hydroxyl groups per anhydroglucose u n i t . The product which con­ tained 0.6 - 0.7 t r i m e t h y l s i l y l group per r i n g was i n s o l u b l e i n a l c o h o l s but s o l u b l e i n a l i p h a t i c hydrocarbons i n c o n t r a s t to the unsilylated ethyl cellulose.

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S u b s t i t u t i o n With Other S i l y l Groups The two methods developed by Klebe and coworkers f o r t r i m e t h y l s i l y l a t i n g c e l l u l o s e provided general routes to other s i l y l c e l l u l o s e s as w e l l . Reaction of c e l l u l o s e with N-(dimethyl- "V c y a n o p r o p y l s i l y l ) a c e t a m i d e i n N-methylpyrrolidone or with the corresponding s i l y l c h l o r i d e i n p y r i d i n e allowed the s u b s t i t u t i o n of 80-90% of the hydroxyl protons by dimethyl- ί -cyanopropyl­ s i l y l groups. In the same f a s h i o n , an average of 2.5 of the three hydroxyl protons of the anhydroglucose u n i t was r e p l a c e d by d i m e t h y l p h e n y l s i l y l and m e t h y l d i p h e n y l s i l y l groups, whereas sub­ s t i t u t i o n w i t h t r i p h e n y l s i l y l groups d i d not prove to be pos­ s i b l e by e i t h e r method. S i l y l a t i o n With Hexamethyldisilazane Harmon and coworkers (20,21) have developed a method f o r p e r s i l y l a t i o n of c e l l u l o s e using hexamethyldisilazane as the s i l y l a t i n g reagent and formamide as s o l v e n t . C e l l u l o s e d i s s o l v e s completely i n formamide by heating a t 70°C f o r 1-2 hours g i v i n g a homogeneous c l e a r v i s c o u s s o l u t i o n . Excess of h e x a m e t h y l d i s i l a ­ zane was added to the s o l u t i o n and the r e a c t i o n mixture was heat­ ed at 70-80°C f o r 2 hours when the s i l y l a t i o n was completed. The product was p u r i f i e d by repeated washings with anhydrous acetone. The s i l y l c e l l u l o s e , obtained, contained 22.1% s i l i c o n i n d i c a t i n g p e r s i l y l a t i o n (3 s i l y l groups per anhydro glucose u n i t ) . Nagy and coworkers (22) have s t u d i e d the e f f e c t s of a number of s o l v e n t s i n the s i l y l a t i o n of c e l l u l o s e with h e x a m e t h y l d i s i l a ­ zane. The s i l y l a t i o n of c e l l u l o s e with hexamethyldisilazane was p o s s i b l e i n the presence of some s o l v e n t s , e.g., p y r i d i n e , d i ­ methyl s u l f o x i d e and dimethyl formamide, which r a p i d l y formed un­ s t a b l e complexes which i n i t i a t e d the s i l y l a t i o n r e a c t i o n . Other s o l v e n t s , e.g., a c e t o n i t r i l e , nitromethane or nitrobenzene, d i d not form complexes or formed them slowly, and had no e f f e c t on the s i l y l a t i o n r e a c t i o n . S i l y l a t i o n With T r i m e t h y l s i l y l a c e t a m i d e Bredereck and coworkers (23) have reported p e r s i l y l a t i o n of c e l l u l o s e using trimethylsilylacetamide. T r i m e t h y l s i l y l c e l l u ­ l o s e c o n t a i n i n g 22.1% s i l i c o n was obtained by melt p o l y m e r i z a t i o n at 170-180°C f o r 6 hours.

In Cellulose Technology Research; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

4.

HARMON

Silyl Cellulose

AND DE

P r o p e r t i e s of S i l y l

45

Celluloses

Downloaded by SUNY STONY BROOK on October 24, 2014 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0010.ch004

The p r o p e r t i e s of the s i l y l c e l l u l o s e d e r i v a t i v e s have been studied e x t e n s i v e l y by Klebe and coworkers (18). The p r o p o r t i o n of the s i l y l groups i n the s i l y l polymer ranges from about 55 wt-% f o r t r i m e t h y l s i l y l c e l l u l o s e to about 75 wt-% f o r m e t h y l d i p h e n y l s i l y l c e l l u l o s e . I t i s not s u r p r i s i n g then, that the prope r t i e s e x h i b i t e d by the v a r i o u s s i l y l c e l l u l o s e s are l a r g e l y determined by the nature of the s i l y l s u b s t i t u e n t s . T r i m e t h y l s i l y l C e l l u l o s e . The polymer i s c o l o r l e s s and i s s o l u b l e i n aromatic and c h l o r i n a t e d s o l v e n t s and a l s o i n a number of a l i p h a t i c hydrocarbons. I t i s i n s o l u b l e i n a l c o h o l s , ketones, e s t e r s , n i t r i l e s , and other p o l a r s o l v e n t s . Solutioncast f i l m s are c l e a r , f l e x i b l e , and moderately strong (Table III). The polymer does not melt. Decomposition s t a r t s around 300°C i n air. E l e c t r i c a l measurements show a low d i e l e c t r i c l o s s (Table IV). The high corona r e s i s t a n c e i s noteworthy. I t i s c h a r a c t e r i s t i c of polymers with a r e l a t i v e l y high s i l i c o n content. Table I I I .

Silyl

Substitution (%)

Substituent Me Si Me(C H ) Si

83

NC(CH ) Me Si

88

2

5

2

3

2

NC(CH ) Me Si

1

Me(C H ) Si

3

NC(CH ) Me Si

1

Me(C H ) Si

1

NC(CH ) Me Si

3

Me(C H ) Si

1

2

Tensile strength psi

Elongation to break %

4500

30

1500

198

3200

20

2500

30

89

3

6

Celluloses

3

2