or Metal

Chapter 20

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AB Block Copolymers Containing Methacrylic Acid and/or Metal Methacrylate Blocks Preparation by Selective Cleavage of Methacrylic Esters Douglas E. Bugner Eastman Kodak Company, 1999 Lake Avenue, Rochester, NY 14650 We have successfully prepared block copolymers containing blocks of methacrylic acid and/or alkali metal methacrylate from the corresponding methacrylic ester block copolymers by judicious choice of both the ester and the reaction conditions (1). Such copolymers are not readily prepared by conventional anionic techniques. Al though poly(methyl methacrylate) can be hydrolyzed with either acid or base, block copolymers containing less than 10 mol% methyl methacrylate, e.g., poly(styrene-b-methyl methacrylate) (S-b-MM), are very difficult to hydrolyze. Other, nonhydrolytic methods for the cleavage of esters are known in the synthetic literature, however. For exam ple, trimethylsilyl iodide is known to cleave alkyl esters under ambient conditions to yield the corresponding acid in high yield upon workup. Interestingly, S-b-MM is again unreactive, but the t-butyl methacrylate copolymer, S-b-tBM, is cleanly converted to the methacrylic acid copolymer, S-b-MA, under the same conditions. This re markable selectivity has been exploited to synthesize the previously unknown poly(methyl methacrylate-b-methacrylic acid) (MM-b-MA). These and other reactions are described. B l o c k copolymers c o m p r i s i n g o l e o p h i l i c and h y d r o p h i l i c b l o c k s a r e o f i n t e r e s t on b o t h p r a c t i c a l and t h e o r e t i c a l grounds (2). The p r e p a r a t i o n o f such c o p o l y m e r s , however, i s n o t an easy t a s k . The t e c h nique o f s e q u e n t i a l a n i o n i c p o l y m e r i z a t i o n i s g e n e r a l l y n o t amenable t o p o l a r and/or i o n i c monomers (3_) . Heterogeneous, f r e e r a d i c a l c o p o l y m e r i z a t i o n s have been shown t o g i v e " b l o c k y " c o p o l y m e r s , b u t the m i c r o s t r u c t u r e i s u s u a l l y i l l - d e f i n e d , and t h e p o l y m e r i z a t i o n c o n d i t i o n s a r e o f t e n s p e c i f i c t o a g i v e n p a i r o f monomers (4,5) . An a l t e r n a t e approach t o polymers c o n t a i n i n g w e l l - d e f i n e d i o n o m e r i c b l o c k s i s t o f i r s t p r e p a r e a p r e c u r s o r b l o c k copolymer by t h e s t a n d a r d a n i o n i c method f o l l o w e d by c o n v e r s i o n o f one o f t h e b l o c k s i n t o an i o n - b e a r i n g group.

0097-6156/88/0364-0276$06.00/0 © 1988 American Chemical Society

In Chemical Reactions on Polymers; Benham, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


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Brown and White employed t h i s approach t o p r e p a r e b l o c k copolymers o f s t y r e n e and m e t h a c r y l i c a c i d ( 6 ) . They were a b l e t o h y d r o l y z e p o l y ( s t y r e n e - b ~ m e t h y l m e t h a c r y l a t e ) (S-b-MM) w i t h p t o l u e n e s u l f o n i c a c i d (TsOH). A l l e n , e t a l . , have r e c e n t l y r e p o r t e d a c i d i c h y d r o l y s i s o f p o l y ( s t y r e n e - b - t - b u t y l m e t h a c r y l a t e ) (S-b-tBM) (7-10). These same workers have a l s o p r e p a r e d p o t a s s i u m m e t h a c r y l a t e b l o c k s d i r e c t l y by t r e a t i n g b l o c k s o f a l k y l m e t h a c r y l a t e s w i t h p o t a s s i u m s u p e r o x i d e (7-10). Our r e q u i r e m e n t s f o r c e r t a i n a p p l i c a t i o n s c a l l e d f o r t h e p r e p a r a t i o n o f b l o c k copolymers o f s t y r e n e and a l k a l i m e t a l m e t h a c r y l a t e s w i t h m o l e c u l a r w e i g h t s o f about 20,000 and m e t h a c r y l a t e c o n t e n t s o f about 10 mol%. In t h i s r e p o r t we d e s c r i b e the p r e p a r a t i o n and r e a c t i o n s o f S-b-MM and S-b-tBM. In the c o u r s e o f our i n v e s t i g a t i o n , we have found s e v e r a l new methods f o r the c o n v e r s i o n o f a l k y l m e t h a c r y l a t e b l o c k s i n t o m e t h a c r y l i c a c i d and/or m e t a l m e t h a c r y l a t e blocks. Of p a r t i c u l a r i n t e r e s t i s t h e r e a c t i o n w i t h t r i m e t h y l s i l y l iodide. Under the same m i l d c o n d i t i o n s , MM b l o c k s a r e c o m p l e t e l y u n r e a c t i v e , w h i l e tBM b l o c k s a r e c l e a n l y c o n v e r t e d t o e i t h e r m e t h a c r y l i c a c i d o r m e t a l m e t h a c r y l a t e b l o c k s . As a consequence o f t h i s unexpected s e l e c t i v i t y , we a l s o r e p o r t the p r e p a r a t i o n o f the new b l o c k copolymers, p o l y ( m e t h y l m e t h a c r y l a t e - b - p o t a s s i u m m e t h a c r y l a t e ) (MM-b-MA.K) and p o l y ( m e t h y l m e t h a c r y l a t e - b - m e t h a c r y l i c a c i d ) (MM-b-MA). Experimental Materials. M e t h y l m e t h a c r y l a t e was a p r o d u c t o f Rohm and Haas, and t - b u t y l m e t h a c r y l a t e was o b t a i n e d from P o l y s c i e n c e s , I n c . Potassium t r i m e t h y l s i l a n o l a t e (PTMS) was o b t a i n e d from P e t r a r c h Systems, I n c . Anhydrous l i t h i u m i o d i d e , t r i m e t h y l s i l y l i o d i d e (TMSI), and nb u t y l l i t i u m ( i n hexanes) were p u r c h a s e d from A l d r i c h C h e m i c a l Co. A l l o t h e r r e a g e n t s and s o l v e n t s were o b t a i n e d from Kodak L a b o r a t o r y Chemicals. A l l monomers were p u r i f i e d by d i s t i l l a t i o n under an i n e r t atmosphere. They were d i s t i l l e d from and c o l l e c t e d o v e r 3 A m o l e c u l a r s i e v e s j u s t p r i o r t o the p o l y m e r i z a t i o n s (11). TMSI was d i s t i l l e d under an i n e r t atmosphere and s t o r e d o v e r copper powder. I t remained c o l o r l e s s and r e t a i n e d i t s r e a c t i v i t y f o r w e l l o v e r a


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y e a r under t h e s e c o n d i t i o n s . F l u o r e n e was r e c r y s t a l l i z e d from abso l u t e e t h a n o l and s t o r e d i n vacuum d e s s i c a t o r . T e t r a h y d r o f u r a n (THF) was d i s t i l l e d d i r e c t l y i n t o t h e r e a c t i o n v e s s e l from a s o l u t i o n o f sodium benzophenone k e t y l . Toluene and d i c h l o r o m e t h a n e were d r i e d w i t h 3 A m o l e c u l a r s i e v e s . A l l t r a n s f e r s o f monomers and TMSI were made w i t h s y r i n g e s o r s t a i n l e s s s t e e l c a n n u l a s . A l l polymerizations were c a r r i e d o u t under a s l i g h t p o s i t i v e p r e s s u r e o f d r y , oxygenf r e e argon.


1

A n a l y t i c a l Methods. P r o t o n magnetic resonancs s p e c t r a ( H NMR) were r e c o r d e d on a G e n e r a l E l e c t r i c QE-300 i n s t r u m e n t . Infrared spectra (IR) were o b t a i n e d w i t h P e r k i n - E l m e r 297 o r 298 s p e c t r o p h o t o m e t e r s . M i c r o a n a l y s e s and i n d u c t i v e l y c o u p l e d plasma s p e c t o s c o p y (ICP) were performed a t Kodak Research L a b o r a t o r i e s . Copolymer c o m p o s i t i o n s f o r S-b-MM and S-b-tBM were determined by t a k i n g t h e average o f two o r more measurments f o r C, H, and 0, and then n o r m a l i z i n g t h e s e a v erages t o 100%. M o l e c u l a r weight d e t e r m i n a t i o n s were made by u s i n g g e l p e r m e a t i o n chromatography (GPC) i n THF on a 5-column s e t o f / i S t y r a g e l columns which were c a l i b r a t e d w i t h p o l y s t y r e n e s t a n d a r d s . The m o l e c u l a r weights r e p o r t e d a r e thus e x p r e s s e d as p o l y s t y r e n e e q u i v a l e n t s ( p s e ) . C a r b o x y l a t e c o n t e n t s were measured by non aqueous t i t r a t i o n (NAT) i n THF u s i n g 0.1 Ν p e r c h l o r i c a c i d i n 1:25 methanol-THF t i t r a n t . T i t r a t i o n s f o r c a r b o x y l i c a c i d were done i n 1:9 water-THF u s i n g 0.1 Ν aqueous NaOH t i t r a n t . P r e p a r a t i o n o f B l o c k Copolymers. Poly(styrene-b-methyl m e t h a c r y l a t e ) and p o l y ( s t y r e n e - b - t - b u t y l m e t h a c r y l a t e ) were p r e p a r e d by p r o c e d u r e s s i m i l a r t o t h o s e r e p o r t e d f o r p o l y ( s t y r e n e - b - m e t h y l m e t h a c r y l a t e (12 ,13). Poly(methyl methacrylate-b-t-butyl m e t h a c r y l a t e ) was s y n t h e s i z e d by a d a p t a t i o n o f the method p u b l i s h e d (14) f o r s y n d i o t a c t i c p o l y ( m e t h y l m e t h a c r y l a t e ) : p o l y m e r i z a t i o n o f m e t h y l m e t h a c r y l a t e was i n i t i a t e d w i t h f l u o r e n y l l i t h i u m , and p r i o r t o t e r m i n a t i o n , t - b u t y l m e t h a c r y l a t e was added t o g i v e the b l o c k copolymer. P e r t i n e n t a n a l y t i c a l d a t a a r e as f o l l o w s . S-b-MM: H NMR (CDC1 ): δ 0.85, 1.0, and 3.59 ppm, p l u s t h e u s u a l p o l y s t y r e n e s i g n a l s , i n t e g r a t i o n o f t h e MM s i g n a l a t 3.59 ppm a g a i n s t t h e s t y r e n e a r o m a t i c r e g i o n r e v e a l s 10.5 mol% (10.1 wt%) MM; IR ( f i l m ) : ν 1730, 1273, 1242, 1193, and 1151 cm" , p l u s p o l y s t y r e n e bands; GPC (THF): Mw = 60600 p s e , Mw/Mn = 1.46; A n a l , found: 88.8% C, 7.75% H, and 3.48% O; S-b-MM (89:11) r e q u i r e s : 88.8% C, 7.78% H, and 3.39% 0. S-b-tBM: H NMR: (CDC1 ): δ 1.41 ppm, p l u s p o l y s t y r e n e s i g n a l s , i n t e g r a t i o n r e v e a l s 9.7 mol% (13 wt%) tBM (see a l s o F i g u r e l a ) ; IR ( f i l m ) : ir 2977, 1728, 1397, 1374, 1247, 1130, and 839 cm" , p l u s p o l y s t y r e n e bands (see a l s o F i g u r e 2 a ) ; GPC (THF): Mw = 31400 pse, Mw/Mn = 1.39; A n a l , found: 88.8% C, 8.13% H, and 2.96% 0; S-b-tBM (90:10) r e q u i r e s : 89.0% C, 8.03% H, and 2.96% 0. MM-b-tBM: H NMR (CDC1 ): δ 0.85, 1.02, 1.42, 1.81, and 3.60 ppm, i n t e g r a t i o n r e v e a l s 5.5 mol% (7.6 wt%) tBM and 72% s y n d i o t a c t i c i t y (see a l s o F i g u r e 3 a ) ; IR ( f i l m ) : ν 2995, 2960, 2850, 1726, 1484, 1445, 1390, 1367, 1270, 1240, 1192, 1146, 1068, 990, 972, 9 2 1 ^ 8 5 2 , 833, 816, 756, and 738 cm" (see a l s o F i g u r e 4 a ) ; GPC (THF): Mw = 16000 p s e , Mw/Mn = 1.17 (see a l s o F i g u r e 5 a ) . 1

3

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F i g u r e 2. Comparison o f IR s p e c t r a f o r S-b-tBM, S-b-MA, and S-b-MA.K.



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—i 10

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ΙΟ

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F i g u r e 3. Comparison o f H NMR s p e c t r a f o r MM-b-tBM and MM-b-MA.


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F i g u r e 5. Comparison o f GPC t r a c e s f o r MM-b-tBM and MM-b-MA ( d i a z o m e t h y l a t e d ) .


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Attempted H y d r o l y s i s o f S-b-MM w i t h TsOH. F o l l o w i n g t h e p r o c e d u r e o f Brown and White ( 6 ) , a m i x t u r e o f S-b-MM-90/10-wt (1.52 g, 1.5 meq MM), TsOH-H 0 (3.02 g, 15.9 mmol), a c e t i c a c i d (100 g ) , dimethylformamide (20 mL), and water (0.5 mL) was r e f l u x e d f o r 47 hr. The r e s u l t i n g s u s p e n s i o n was poured i n t o methanol (500 mL). The s o l i d s were c o l l e c t e d , washed w i t h water (2 χ 250 mL) and methanol (500 mL), and d r i e d i n a vacuum oven. The d r i e d polymer was d i s s o l v e d i n d i c h l o r o m e t h a n e and r e p r e c i p i t a t e d from methanol. The p r e c i p i t a t e was washed w i t h methanol and d r i e d i n a vacuum oven, y i e l d i n g 1.25 g o f p r o d u c t . Both H NMR and IR i n d i c a t e e s s e n t i a l l y u n r e a c t e d S-b-MM. 2


1

Attempted H y d r o l y s i s o f S-b-MM w i t h KOH. A s o l u t i o n o f S-b-MM-90/10-wt (1.50 g, 1.5 meq MM) i n 1:9 water-THF (50 mL) was t r e a t e d w i t h KOH (1.57 g, 28.0 mmol), and t h e m i x t u r e was r e f l u x e d f o r 47 h r . The s o l v e n t s were e v a p o r a t e d , and t h e r e s i d u e was shredded i n a b l e n d e r c o n t a i n i n g 200 mL o f water. The s o l i d s were f i l t e r e d , washed w i t h water and methanol, and d r i e d . The d r i e d polymer was d i s s o l v e d i n d i c h l o r o m e t h a n e and p r e c i p i t a t e d from methanol. The p r e c i p i t a t e was washed w i t h methanol and d r i e d i n a vacuum oven, y i e l d i n g 1.23 g o f p r o d u c t . Both H NMR and IR i n d i c a t e e s s e n t i a l l y u n r e a c t e d S-b-MM. 1

R e a c t i o n o f S-b-MM w i t h L i t h i u m I o d i d e . The r e a c t i o n was c a r r i e d out under n i t r o g e n i n a 250-mL, round-bottom f l a s k e q u i p p e d w i t h a magnetic s t i r r e r and a r e f l u x condenser. To a s o l u t i o n o f S-b-MM-90/10-wt (3.00 g, 3.0 meq MM) i n t o l u e n e (100 mL) was added L i l (0.45 g, 3.4 mmol), and t h e m i x t u r e was r e f l u x e d f o r 20 h r . The s o l v e n t was p a r t i a l l y e v a p o r a t e d , and t h e polymer was t h e n p r e c i p i t a t e d from methanol. The p r e c i p i t a t e was washed w i t h methanol, wa t e r , and a g a i n w i t h methanol. The d r i e d polymer was r e p r e c i p i t a t e d , washed, and d r i e d as d e s c r i b e d above, y i e l d i n g 2.58 g o f p r o d u c t . A n a l y s i s by H NMR shows o n l y a t r a c e o f MM s i g n a l a t 3.59 ppm; IR ( f i l m ) : ΙΊ723 and 1570 cm" ( s h o u l d e r ) ; ICP: 0.106, 0.110 wt% L i (0.156 meq L i / g ) ; e x p e c t 1.07 meq L i / g . 1

1

R e a c t i o n o f S-b-MM w i t h P o t a s s i u m T r i m e t h y l s i l a n o l a t e . The r e a c t i o n was c a r r i e d o u t i n a 250-mL, round-bottom f l a s k e q u i p p e d w i t h a mag n e t i c s t i r r e r and a r e f l u x condenser. To a s o l u t i o n o f S-b-MM (6.02 g, 6.0 meq MM) i n t o l u e n e (100 mL) was added PTMS (0.87 g, 6.8 mmol). The m i x t u r e was r e f l u x e d f o r 1 h r , and was t h e n i s o l a t e d as d e s c r i b e d above. Attempts t o r e d i s s o l v e t h e p r o d u c t i n d i c h l o r o m e t h a n e r e s u l t e d i n a t h i c k s u s p e n s i o n . T h i s was p r e c i p i t a t e d from methanol, washed, and d r i e d as b e f o r e , y i e l d i n g 4.55 g o f p r o d u c t . A n a l y s i s by H NMR shows no s i g n a l a t 3.59 ppm; IR ( f i l m ) : 1727, 1569 ( s h o u l d e r ) , 1256- 1206, and 1089 cm" , bands f o r MM no l o n g e r p r e s e n t ; ICP: 1.44, 1.38 wt% Κ (0.361 meq K / g ) ; e x p e c t 1.03 meq K/g. 1

1

A 2.09 g p o r t i o n o f t h i s p r o d u c t was d i s s o l v e d i n THF (50 mL) and t r e a t e d w i t h cone. H C l (3.33 mL, 40 meq) i n water (5 mL). The m i x t u r e was r e f l u x e d f o r 4 h r . I t was c o o l e d , f i l t e r e d t o remove i n s o l u b l e r e s i d u e , and p r e c i p i t a t e d from 1:1 water-methanol. The p r e c i p i t a t e was resuspended i n d i c h l o r o m e t h a n e , r e p r e c i p i t a t e d from methanol, washed, and d r i e d as b e f o r e , y i e l d i n g 1.27 g : IR ( f i l m ) : ν 1732, 1701, 1199, and 1083 cm" . 1


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A 2.02 g p o r t i o n o f t h e i n i t i a l p r o d u c t was d i s s o l v e d i n THF (50 mL) and t r e a t e d w i t h a s o l u t i o n o f KOH (2.02 g, 36.1 mmol) i n water (5 mL). The m i x t u r e was r e f l u x e d f o r 4 h r , and was t h e n i s o l a t e d and r e p r e c i p i t a t e d as b e f o r e , y i e l d i n g 1.66 g: IR ( f i l m ) : ν 1725, 1664, 1572 ( s h o u l d e r ) , 1274, 1204, and 1081 cm ; ICP: 1.83, 1.87 wt% Κ (0.473 meq K/g); e x p e c t 1.03 meq K/g.


-1

R e a c t i o n o f S-b-MM w i t h T r i m e t h y l s i l y l I o d i d e . The r e a c t i o n was c a r r i e d o u t under n i t r o g e n i n a 250-mL, round-bottom f l a s k equipped w i t h a magnetic s t i r r e r . To a s o l u t i o n o f S-b-MM-94/6-wt (5.01 g, 2.9 meq MM) i n d i c h l o r o m e t h a n e (50 mL) was added TMSI (1.3 g, 6.3 mmol) v i a s y r i n g e . The s o l u t i o n was r e f l u x e d f o r 22 h r . I t was c o o l e d , p r e c i p i t a t e d from methanol, washed w i t h m e t h a n o l , and d r i e d , y i e l d i n g 4.39 g. NMR, IR, and GPC a n a l y s e s were v i r t u a l l y i d e n t i c a l to those o f the s t a r t i n g m a t e r i a l . H y d r o l y s i s o f S-b-tBM w i t h TsOH. U s i n g t h e same p r o c e d u r e as t h a t d e s c r i b e d above f o r t h e r e a c t i o n o f S-b-MM w i t h TsOH, a 1.51 g p o r t i o n o f S-b-tBM-87/13-wt (1.4 meq tBM) y i e l d e d 1.20 g o f S-b-MA: H NMR (CDC1 ): s i g n a l a t 1.41 ppm s u b s t a n t i a l l y r e d u c e d i n i n t e n s i t y (see a l s o F i g u r e l b ) ; IR ( f i l m ) : vllOA and 1280 cm" , bands f o r tBM no l o n g e r p r e s e n t (see a l s o F i g u r e 2b); GPC (THF, d i a z o m e t h y l a t e d ) : Mw = 30800 pse, Mw/Mn = 1.35; t i t r a t i o n : 0.94 meq COOH/g; e x p e c t 0.98 meq/g. A 0.49 g p o r t i o n o f S-b-MA as o b t a i n e d above was t r e a t e d w i t h KOH (0.51 g, 9.1 mmol) i n 1:10 water-THF (33 mL). The m i x t u r e was r e f l u x e d f o r 4 h r , and t h e n was i s o l a t e d and r e p r e c i p i t a t e d as d e s c r i b e d above f o r t h e r e a c t i o n o f S-b-MM w i t h KOH, y i e l d i n g 0.42 g o f S-b-MA.Κ: IR ( f i l m ) : ν 1664, 1566, and 1206 c m " ( s e e a l s o F i g u r e 2 c ) ; ICP: 3.84, 3.82 wt% Κ (0.980 meq K/g); NAT: 0.91 meq CO'/g; e x p e c t 0.94 meq/g. 1

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Attempted H y d r o l y s i s o f S-b-tBM w i t h KOH. U s i n g t h e same p r o c e d u r e as t h a t d e s c r i b e d above f o r t h e attempted h y d r o l y s i s o f S-b-MM, 1.57 g o f S-b-tBM-87/13-wt (1.4 meq tBM) y i e l d e d 1.35 g o f p r o d u c t . IR a n a l y s i s i n d i c a t e d t h a t e s s e n t i a l l y no r e a c t i o n had o c c u r r e d .

1

R e a c t i o n o f S-b-tBM w i t h P o t a s s i u m T r i m e t h y l s i l a n o l a t e . U s i n g t h e same p r o c e d u r e as t h a t employed f o r t h e r e a c t i o n o f S-b-MM w i t h PTMS, 3.0 g o f S-b-tBM-87/13-wt (2.8 meq tBM) y i e l d e d 2.5 g o f p r o d uct. The IR spectrum was i d e n t i c a l t o t h a t o f t h e s t a r t i n g polymer. H NMR showed a s l i g h t r e d u c t i o n i n t h e i n t e g r a l f o r t h e a l i p h a t i c r e g i o n o f t h e spectrum. ICP i n d i c a t e d 0.36, 0.37 wt% Κ (0.093 meq K/g); e x p e c t 0.94 meq K/g. R e a c t i o n o f S-b-tBM w i t h T r i m e t h y l s i l y l I o d i d e . The r e a c t i o n was c a r r i e d o u t under c o n d i t i o n s s i m i l a r t o t h a t employed f o r t h e r e a c t i o n o f S-b-MM w i t h TMSI. A m i x t u r e o f S-b-tBM-87/13-wt (10.0 g, 9.3 meq tBM) i n d i c h l o r o m e t h a n e (100 mL, d r i e d o v e r 3 A s i e v e s ) was t r e a t e d w i t h TMSI (5.0 g, 25 mmol) and was s t i r r e d f o r 4 h r a t room t e m p e r a t u r e , r e s u l t i n g i n a dark r e d s o l u t i o n . The s o l v e n t was p a r t i a l l y e v a p o r a t e d , and t h e r e s i d u e was p r e c i p i t a t e d from methanol. The p r e c i p i t a t e was washed w i t h s e v e r a l p o r t i o n s o f methanol and was dried. I t was r e d i s s o l v e d i n 1:9 water-THF (300 mL), 3 mL o f cone. HC1 was added, and the m i x t u r e was r e f l u x e d f o r 2 h r . The s o l v e n t s


284


were e v a p o r a t e d , and the r e s i d u e was r e d i s s o l v e d i n THF (30 mL) and r e p r e c i p i t a t e d from m e t h a n o l . The p r e c i p i t a t e was c o l l e c t e d , washed w i t h m e t h a n o l , and d r i e d , y i e l d i n g 8.50 g o f S-b-MA. A n a l y s e s : H NMR and IR s p e c t r a i d e n t i c a l t o t h o s e o b s e r v e d f o r S-b-MA p r e p a r e d by h y d r o l y s i s o f S-b-MM w i t h TsOH; GPC (THF, d i a z o m e t h y l a t e d ) : Mw = 30400 p s e , Mw/Mn = 1.36; t i t r a t i o n : 0.91 meq COOH/g; e x p e c t 0.98 meq/g. A 0.55 g p o r t i o n o f S-b-MA as o b t a i n e d above was c o n v e r t e d t o t h e p o t a s s i u m s a l t i n the same manner as t h a t d e s c r i b e d above f o l l o w i n g t h e h y d r o l y s i s o f S-b-tBM w i t h TsOH, y i e l d i n g 0.43 g o f S-b-MA.Κ: IR spectrum i d e n t i c a l t o t h a t d e s c r i b e d above f o r S-b-MA. Κ ; NAT : 0.91 meq C0 /g; ICP : 4.16, 4.13 wt% Κ (1.06 meq K/g); e x p e c t 0.94 meq K/g. 1


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2

R e a c t i o n o f MM-b-tBM w i t h T r i m e t h y l s i l y l I o d i d e . The r e a c t i o n was c a r r i e d o u t e x a c t l y as d e s c r i b e d above f o r t h e r e a c t i o n o f S-b-tBM w i t h TMSI. Thus MM-b-tBM-92/8-wt (10.0 g, 5.5 meq tBM) was t r e a t e d w i t h TMSI (2.8 g, 14 mmol) i n d i c h l o r o m e t h a n e (100 mL) f o r 4 h r a t room t e m p e r a t u r e . T h i s time t h e p r o d u c t was i s o l a t e d by p r e c i p i t a t i o n from c y c l o h e x a n e , y i e l d i n g 9.41 g o f p r o d u c t . A 3.12 g p o r t i o n o f t h i s p r o d u c t was s t i r r e d i n 1 Ν HCL (240 mL, 1:1 methanol-water) f o r 2 h r . I t was f i l t e r e d , washed w i t h m e t h a n o l , and d r i e d , y i e l d i n g 2.82 g o f MM-b-MA. A n a l y s e s : H NMR (CDC1 ): s i g n a l a t 1.41 ppm now gone (see a l s o F i g u r e 3b); IR ( f i l m ) : ν band a t 1726 cm broadened and s h i f t e d t o 1717 cm" (see a l s o F i g u r e 4b); GPC (THF, d i a z o m e t h y l a t e d ) : Mw = 15000 p s e , Mw/Mn = 1.17 (see a l s o F i g u r e 5b); t i t r a t i o n : 0.583 meq COOH/g; e x p e c t 0.56 meq/g. A 0.42 g p o r t i o n o f the i n i t i a l p r o d u c t from the r e a c t i o n o f MM-b-tBM w i t h TMSI was c o n v e r t e d t o i t s p o t a s s i u m s a l t i n the same manner as t h a t p r e v i o u s l y d e s c r i b e d above f o r S-b-MA, y i e l d i n g 0.20 g o f MM-b-MA.K. A n a l y s e s : IR ( f i l m ) : ν 1729 and 1552 cm" ; ICP: 2.29, 2.24 wt% Κ (0.68 meq K/g); e x p e c t 0.54 meq K/g. 1

3

1

1

R e s u l t s and D i s c u s s i o n P r e p a r a t i o n and R e a c t i o n s o f S-b-MM. As mentioned i n the i n t r o d u c t i o n , we were i n t e r e s t e d i n b l o c k copolymers o f s t y r e n e and a l k a l i m e t a l m e t h a c r y l a t e s w i t h o v e r a l l m o l e c u l a r w e i g h t s o f about 20,000 and m e t h a c r y l a t e c o n t e n t s on t h e o r d e r o f 10 mol%. The p r e p a r a t i o n of such copolymers by the u s u a l a n i o n i c t e c h n i q u e s i s n o t f e a s i b l e . An a l t e r n a t i v e i s t o p r e p a r e b l o c k copolymers o f s t y r e n e and m e t h a c r y l i c e s t e r s by s e q u e n t i a l a n i o n i c p o l y m e r i z a t i o n , f o l l o w e d by a p o s t - p o l y m e r i z a t i o n r e a c t i o n t o produce the d e s i r e d b l o c k copolymers. The o b v i o u s f i r s t c h o i c e o f m e t h a c r y l i c e s t e r s i s methyl methacrylate. I t i s i n e x p e n s i v e , r e a d i l y a v a i l a b l e , and i t s b l o c k copolymers w i t h s t y r e n e a r e well-known. In f a c t , Brown and White have r e p o r t e d the p r e p a r a t i o n and h y d r o l y s e s o f a s e r i e s o f S-b-MM copolymers o f v a r y i n g MM c o n t e n t u s i n g p - t o l u e n e s u l f o n i c a c i d (TsOH) ( 6 ) . The r e s u l t i n g m e t h a c r y l i c a c i d copolymers were e a s i l y c o n v e r t e d t o t h e i r sodium c a r b o x y l a t e s by n e u t r a l i z a t i o n w i t h sodium hydroxide.


20.

AB Block Copolymers

BUGNER

CH

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-(CH^H^CHi-tDownloaded by NANYANG TECHNOLOGICAL UNIV on October 5, 2015 | http://pubs.acs.org Publication Date: December 22, 1988 | doi: 10.1021/bk-1988-0364.ch020

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In t h e i r s t u d y , m o l e c u l a r w e i g h t s were k e p t q u i t e low ( c a . 1200), and t h e l o w e s t MM c o n t e n t used was about 13 mol%. There was a l s o some q u e s t i o n as t o t h e e f f e c t o f t h e t a c t i c i t y o f t h e MM b l o c k on t h e e x t e n t o f h y d r o l y s i s . MM b l o c k s have r e p o r t e d l y been d i r e c t l y c o n v e r t e d i n t o t h e p o t a s s i u m m e t h a c r y l a t e (MA.K) b l o c k s by r e a c t i o n w i t h p o t a s s i u m s u p e r o x i d e (7-10). The s y n t h e t i c d e t a i l s o f t h i s t r a n s f o r m a t i o n , however, were s k e t c h y . We d e c i d e d t o r e i n v e s t i g a t e t h e s e and o t h e r methods f o r t h e p r e p a r a t i o n o f t h e b l o c k copolymers as d e f i n e d above. S-b-MM was p r e p a r e d a c c o r d i n g t o t h e p u b l i s h e d p r o c e d u r e s ( 4 - 6 ) . M o l e c u l a r w e i g h t s i n t h e d e s i r e d range and w i t h narrow, unimodal d i s t i b u t i o n s were o b t a i n e d w i t h o u t r e s o r t i n g t o e x t e n s i v e monomer p u r i f i c a t i o n (ljL) o r c a p p i n g o f t h e s t y r e n e b l o c k w i t h d i p h e n y l e t h y l e n e (4,5,7-10). The S-b-MM c o n t a i n e d about 10 mol% MM, and was c o n v e n i e n t l y c h a r a c t e r i z e d by H NMR and IR s p e c t r o s c o p y . The m e t h y l e s t e r g i v e s r i s e t o a f a i r l y sharp s i n g l e t a t 3.59 ppm, and t h e e s t e r c a r b o n y l e x h i b i t s an i n f r a r e d band a t 1730 cm . The MM c o n t e n t o f t h e copolymer i s e a s i l y a s c e r t a i n e d by i n t e g r a t i o n o f the H NMR spectrum and may be c o r r o b o r a t e d by e l e m e n t a l a n a l y s i s . Attempts t o h y d r o l y z e S-b-MM under e i t h e r a c i d i c o r b a s i c c o n d i t i o n s were u n s u c c e s s f u l . R e a c t i o n o f S-b-MM w i t h TsOH under t h e same c o n d i t i o n s d e s c r i b e d by Brown and White (6) r e s u l t e d i n no r e a c t i o n as e v i d e n c e d by comparison o f t h e H NMR and IR s p e c t r a o f the p r o d u c t w i t h t h a t o f t h e s t a r t i n g m a t e r i a l . A t t h e m o l e c u l a r w e i g h t s and MM c o n t e n t s t h a t we were w o r k i n g w i t h , t h e S-b-MM was i n s o l u b l e i n t h e r e a c t i o n medium, w h i c h may e x p l a i n i t s l a c k o f r e activity. A l t h o u g h t h e h y d r o l y s i s o f m e t h y l m e t h a c r y l a t e under b a s i c c o n d i t i o n s i s known t o be q u i t e s l u g g i s h , we n e v e r t h e l e s s sub j e c t e d S-b-MM t o two days o f r e f l u x i n aqueous THF i n t h e p r e s e n c e o f e x c e s s KOH. As e x p e c t e d , t h e p r o d u c t was s p e c t r o s c o p i c a l l y i d e n t i c a l t o t h e s t a r t i n g copolymer. Two a l t e r n a t i v e s t o c o n v e n t i o n a l a c i d / b a s e h y d r o l y s e s f o r c l e a v i n g e s t e r s a r e Sn2 d i s p l a c e m e n t o f t h e c a r b o x y l a t e group by r e a c t i v e n u c l e o p h i l e s and n u c l e o p h i l i c a t t a c k a t t h e c a r b o n y l c a r b o n . In t h i s l a t t e r c o n t e x t we i n v e s t i g a t e d t h e r e a c t i o n o f S-b-MM w i t h potassium t r i m e t h y l s i l a n o l a t e , a s o - c a l l e d potassium superoxide e q u i v a l e n t (_15). One advantage t h a t t h i s r e a g e n t has o v e r p o t a s s i u m 1

-1

1

1


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superoxide i s i t s g r e a t e r s o l u b i l i t y i n organic s o l v e n t s . I t s r e a c t i o n w i t h S-b-MM i n r e f l u x i n g t o l u e n e was q u i t e r a p i d . In less t h a n an h o u r , t h e H NMR spectrum o f t h e p r o d u c t showed complete l o s s o f the methyl e s t e r . The IR spectrum, however, s t i l l e x h i b i t s a s t r o n g band a t 1727 cm" and o n l y a weak band f o r t h e c a r b o x y l a t e s t r e t c h near 1560 cm" . A n a l y s i s f o r p o t a s s i u m by i n d u c t i v e l y cou p l e d plasma s p e c t r o s c o p y (ICP) i n d i c a t e d o n l y a f r a c t i o n o f t h e e x p e c t e d amount. The polymer was a l s o p o o r l y s o l u b l e i n most o r g a n i c solvents. F u r t h e r t r e a t m e n t o f t h e polymer by e i t h e r HC1 o r KOH i n r e f l u x i n g aqueous THF f a i l e d t o e l i m i n a t e t h e IR band a t 1727 cm" . In t h e IR spectrum o f t h e a c i d i f i e d polymer, t h e weak c a r b o x y l a t e band near 1560 cm" was r e p l a c e d w i t h a new, weak c a r b o n y l band a t 1701 cm" , a t t r i b u t a b l e t o m e t h a c r y l i c a c i d r e s i d u e s . A p p a r e n t l y a s i d e - r e a c t i o n , p r o b a b l y i n v o l v i n g some c r o s s l i n k i n g o f t h e copolymer and/or f o r m a t i o n o f a new e s t e r , was o c c u r r i n g i n a d d i t i o n t o cleavage o f the methyl e s t e r . A t t h i s p o i n t a comparison o f t h e s e o b s e r v a t i o n s w i t h t h o s e r e p o r t e d by A l l e n , e t a l . (7-10), f o r t h e r e a c t i o n o f p o l y ( s t y r e n e - b i s o b u t y l m e t h a c r y l a t e ) w i t h p o t a s s i u m s u p e r o x i d e s h o u l d be made. They t o o o b t a i n e d a p r o d u c t which was p o o r l y s o l u b l e i n THF and many other s o l v e n t combinations. A c i d i f i c a t i o n w i t h HC1 i n THF improved the s o l u b i l i t y , b u t t h e p r o d u c t d i s p l a y e d an IR band a t 1730 cm" i n a d d i t i o n t o a m e t h a c r y l i c a c i d a t 1700 cm" . They a t t r i b u t e d t h e band a t 1730 cm" t o u n r e a c t e d i s o b u t y l m e t h a c r y l a t e g r o u p s , b u t i n l i g h t o f o u r o b s e r v a t i o n s , i t may be due t o t h e same t y p e o f s i d e reaction. A n o t h e r a t t r a c t i v e r e a g e n t i s l i t h i u m i o d i d e , which i s known t o produce t h e l i t h i u m c a r b o x y l a t e d i r e c t l y i n t h e case o f nonp o l y m e r i c e s t e r s by an Sn2 mechanism (16) . A l t h o u g h p y r i d i n e i s u s u a l l y t h e s o l v e n t o f c h o i c e f o r t h i s r e a c t i o n , we found o n l y p a r t i a l c l e a v a g e (by H NMR) o f t h e m e t h y l e s t e r f o r S-b-MM a f t e r e x tended p e r i o d s o f r e a c t i o n w i t h L i l i n r e f l u x i n g p y r i d i n e . A better s o l v e n t appeared t o be r e f l u x i n g t o l u e n e . A f t e r 20 h r , t h e s i g n a l f o r the m e t h y l e s t e r i n t h e NMR spectrum was e l i m i n a t e d . Analogous to t h e r e a c t i o n w i t h p o t a s s i u m t r i m e t h y l s i l a n o l a t e , however, t h e IR spectrum s t i l l d i s p l a y e d a c a r b o n y l band a t 1723 c m a l o n g w i t h a s h o u l d e r f o r t h e c a r b o x y l a t e i o n a t 1570 cm" . A n a l y s i s f o r L i (ICP) showed o n l y about 15% o f t h e e x p e c t e d amount. I t thus appeared t h a t q u i t e s i m i l a r c h e m i s t r y was o c c u r r i n g w i t h t h e s e two r e a g e n t s , t h e d e t a i l s o f which a r e c u r r e n t l y under s t u d y . A n o t h e r n o n s a p o n i c a t i v e method f o r c l e a v i n g e s t e r s employs t r i m e t h y l s i l y l i o d i d e (TMSI) (17,18) . U n f o r t u n a t e l y , r e a c t i o n o f S-b-MM w i t h TMSI i n r e f l u x i n g d i c h l o r o m e t h a n e f o r 22 h r gave back unchanged s t a r t i n g m a t e r i a l . I n t e r e s t i n g l y , t h e l i t e r a t u r e r e p o r t s t h a t t - b u t y l and b e n z y l e s t e r s a r e s i g n i f i c a n t l y more r e a c t i v e t o ward TMSI than m e t h y l e s t e r s (Γ7) . T h i s r e a c t i v i t y p a t t e r n i s s i m i l a r t o t h a t o b s e r v e d f o r a c i d - c a t a l y z e d e s t e r h y d r o l y s i s (7-10). T h e r e f o r e , confounded by o u r i n a b i l i t y t o e f f i c i e n t l y c l e a v e m e t h y l m e t h a c r y l a t e copolymers, i t was d e c i d e d t o i n v e s t i g a t e t h e p r e p a r a t i o n and r e a c t i v i t y o f t h e analogous t - b u t y l m e t h a c r y l a t e b l o c k copolymers. 1

1

1


1

1

1

1

1

1

1

1

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P r e p a r a t i o n and R e a c t i o n s o f S-b-tBM. S-b-tBM copolymers were p r e p a r e d i n e x a c t l y t h e same manner as t h a t d e s c r i b e d above f o r S-b-MM. C h a r a c t e r i z a t i o n by *K NMR c o n f i r m e d t h e p r e s e n c e o f t h e t - b u t y l


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AB Block Copolymers

BUGNER

287

p r o t o n s a t 1.41 ppm c o i n c i d e n t w i t h the s i g n a l f o r the backbone methylene p r o t o n s ( F i g u r e l a ) . The IR spectrum d i s p l a y e d the e s t e r c a r b o n y l s t r e t c h a t 1728 cm- , as w e l l as t - b u t y l C-H v i b r a t i o n s a t 2977, 1397, 1374, and 1247 cm' , and the C-O-C s t r e t c h i n g band a t 1130 cm" ( F i g u r e 2 a ) . The m o l e c u l a r w e i g h t d i s t r i b u t i o n was a g a i n narrow, u n i m o d a l , and i n t h e e x p e c t e d range. We f i r s t attempted t o h y d r o l y z e S-b-tBM w i t h TsOH under the same c o n d i t i o n s w h i c h were u n s u c c e s s f u l f o r S-b-MM. T h i s t i m e , a l though the polymer was a g a i n i n c o m p l e t e l y s o l u b l e i n t h e r e a c t i o n m i l i e u , the t - b u t y l m e t h a c r y l a t e b l o c k appeared t o be q u a n t i a t i v e l y hydrolyzed. The t - b u t y l bands l i s t e d above a r e no l o n g e r o b s e r v e d i n the IR spectrum ( F i g u r e 2 b ) . The c a r b o n y l band i s broadened and s h i f t e d t o 1704 cm" , and a C-O-H s t r e t c h i s o b s e r v e d a t 1280 cm" . A weak, b r o a d band a t 2625 cm" and a s h o u l d e r a t 1735 cm" can be a t t r i b u t e d t o hydrogen-bonded O-H and C=0 s t r e t c h e s , r e s p e c t i v e l y . Such bands a r e n o t o b s e r v e d i n random copolymers o f s t y r e n e and m e t h a c r y l i c a c i d ( G u i s t i n a , R. Α., Eastman Kodak, p e r s o n a l communi c a t i o n , 1981). The s i g n a l a t 1.41 ppm i n t h e NMR spectrum i s c o n s i d e r a b l y r e d u c e d , a l s o c o n s i s t e n t w i t h the l o s s o f the t - b u t y l group ( F i g u r e l b ) . A GPC t r a c e o f t h e d i a z o m e t h y l a t e d p r o d u c t i n d i c a t e s t h a t no s i g n i f i c a n t c h a i n d e g r a d a t i o n has o c c u r r e d . The s l i g h t d e c r e a s e i n t h e o b s e r v e d Mw i s e x p e c t e d s i n c e t h e t - b u t y l group o f the s t a r t i n g copolymer has been r e p l a c e d w i t h a much s m a l l e r m e t h y l group. T i t r a t i o n w i t h 0.1 Ν sodium h y d r o x i d e i n aqueous THF i n d i c a t e s 0.94 meq COOH/g, i n good agreement w i t h the e x p e c t e d 0.98 meq/g o f m e t h a c r y l i c a c i d , based on the tBM c o n t e n t o f the p r e c u r s o r . f

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S-b-MA. Κ N e u t r a l i z a t i o n w i t h KOH i n aqueous THF gave the d e s i r e d p o l y ( s t y r e n e - b - p o t a s s i u m m e t h a c r y l a t e ) (S-b-MA.K). The c a r b o n y l band i n the IR spectrum i s r e p l a c e d w i t h a s t r o n g , b r o a d c a r b o x y l a t e a b s o r p t i o n c e n t e r e d near 1566 cm" ( F i g u r e 2 c ) . The c a r b o x y l a t e and p o t a s s i u m c o n t e n t s were a s s a y e d by non-aqueous t i t r a t i o n and ICP, respectively. The r e s u l t i n g v a l u e s o f 0.91 meq C0 "/g and 0.98 meq K/g i n d i c a t e e s s e n t i a l l y q u a n t i t a t i v e c o n v e r s i o n t o the p o t a s s i u m methacrylate. S-b-MA.Κ o b t a i n e d i n t h i s manner i s e a s i l y d i s s o l v e d i n s o l v e n t s such as THF and d i c h l o r o m e t h a n e , i n c o n t r a s t t o the 1

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p r o d u c t o f o f t h e r e a c t i o n o f S-b-MM w i t h p o t a s s i u m trimethylsilanolate. We n e x t i n v e s t i g a t e d t h e d e a l k y l a t i o n o f S-b-tBM w i t h TMSI. U n l i k e t h e r e a c t i o n w i t h S-b-MM, i t r e q u i r e d o n l y 4 h r a t room tem p e r a t u r e t o c o m p l e t e l y c l e a v e t h e t - b u t y l e s t e r . Work-up under a c i d i c c o n d i t i o n s gave S-b-MA which was v i r t u a l l y i d e n t i c a l by NMR, IR, GPC, and t i t r a t i o n w i t h t h a t j u s t d e s c r i b e d above. Likewise, n e u t r a l i z a t i o n w i t h KOH r e s u l t e d i n q u a n t i t a t i v e c o n v e r s i o n t o S-b-MA.K. A l t h o u g h t h e i n i t i a l l y formed p r o d u c t o f t h e r e a c t i o n o f a l k y l e s t e r s w i t h TMSI i s presumably t h e t r i m e t h y l s i l y l e s t e r (17), we were n o t a b l e t o i s o l a t e o r c h a r a c t e r i z e t h i s copolymer. It is known t h a t t r i m e t h y s i l y l m e t h a c r y l a t e and i t s polymers s p o n t a n e o u s l y h y d r o l y z e even i n m o i s t a i r (19). Any t r a c e s o f water i n t h e methanol used t o p r e c i p i t a t e t h e r e a c t i o n m i x t u r e would thus p r e clude i s o l a t i o n o f the intermediate t r i m e t h y l s i s l y l e s t e r . We a l s o e x p l o r e d t h e d i r e c t c o n v e r s i o n o f S-b-tBM t o S-b-MA.Κ. H y d r o l y s i s under b a s i c c o n d i t i o n s (KOH i n r e f l u x i n g aqueous THF) was a g a i n r e s u l t e d i n unchanged S-b-tBM. The r e a c t i o n w i t h p o t a s s i u m t r i m e t h y l s i l a n o l a t e f o r 1 h r i n r e f l u x i n g t o l u e n e gave v e r y l i t t l e reaction. Only 10% o f t h e e x p e c t e d amount o f p o t a s s i u m was found by ICP, and the NMR and IR s p e c t r a were l i t t l e changed from those o f the s t a r t i n g copolymer. T h i s d i f f e r e n c e i n r e a c t i v i t y between S-b-MM and S-b-tBM p a r a l l e l s t h a t o b s e r v e d f o r t h e r e a c t i o n o f a l k y l m e t h a c r y l a t e b l o c k s w i t h p o t a s s i u m s u p e r o x i d e (7-10). The P r e p a r a t i o n o f MM-b-MA and MM-b-MA.K. I n s p i r e d by t h e unex p e c t e d s e l e c t i v i t y o f t h e r e a c t i o n o f TMSI w i t h S-b-MM and S-b-tBM, we d e c i d e d t o attempt t h e p r e p a r a t i o n o f p o l y ( m e t h y l m e t h a c r y l a t e - b t - b u t y l m e t h a c r y l a t e ) (MM-b-tBM) and i t s unprecedented c o n v e r s i o n t o MM-b-MA. CH

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