Chemical Reactions on Polymers - American Chemical Society

maleic anhydride to introduce the succinoyl group (1-5), and by reac- tion with ... stant ). 1 9 F-NMR spectra were determined, at 85 MHz on a JEOL FX...
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Chapter 22

Radical-Promoted Functionalization of Polyethylene Controlled Incorporation of Hydrogen-Bonding Groups

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M. F. Schlecht, E. M. Pearce, T. K. Kwei, and W. Cheung Department of Chemistry, Polytechnic University, 333 Jay Street, Brooklyn, NY 11201 The radical-promoted addition of hexafluoroacetone to polyethylene has been examined with the aim of maximizing the extent of incorporation and characterizing the functionalized polymer. Reaction conditions were found which produce samples modified to the extent of from 0.4 to 5.6 residues per hundred methylenes, as determined by elemental combustion analysis. The functionalization reaction was performed on small molecule substrates, and for comparison model compounds containing the fluoroalkanol and fluoroalkyl ether groups were prepared by independent synthetic routes. Analysis of the modified polyethylene samples by F-NMR shotted that of the pendent groups introduced, four in five are fluoroalkanol groups, and one in five is a fluoroalkyl ether. The functionalized polymers are soluble in tetrahydrofuran and in aromatic solvents, and show the expected thermal properties in proportion to the increasing percent modification, e.g. decreasing degradation temperature, increasing glass transition temperature, and decreasing melting point. Some preliminary results indicate that the functionalized polymers form miscible blends with poly(methyl methacrylate). 19

The chemical m o d i f i c a t i o n o f polymers i s a p o w e r f u l p r e p a r a t i v e method f o r o b t a i n i n g f u n c t i o n a l i z e d macromolecules from s i m p l e p r e c u r s o r polymers, w i t h o u t t h e n e c e s s i t y o f s y n t h e s i z i n g s p e c i a l t y monomers. Among t h e methods a v a i l a b l e , t h e r a d i c a l - p r o m o t e d a d d i t i o n o f f u n c t i o n a l groups i s perhaps t h e most w i d e l y used, y e t t h e l e a s t unders t o o d . The p a t e n t l i t e r a t u r e c o n t a i n s a l a r g e number o f such p r e p a r a t i o n s , and i n a d d i t i o n s e v e r a l s y s t e m a t i c s t u d i e s have been reported. Hydrocarbon polymers have been m o d i f i e d b y a d d i t i o n o f m a l e i c a n h y d r i d e t o i n t r o d u c e t h e s u c c i n o y l group ( 1 - 5 ) , and b y r e a c t i o n w i t h phosphorus t r i c h l o r i d e and oxygen t o i n t r o d u c e t h e phosp h o r y l group ( 6 ) . The r a d i c a l - p r o m o t e d a d d i t i o n o f h e x a f l u o r o a c e t o n e (7) t o v a r i o u s o r g a n i c s u b s t r a t e s , i n c l u d i n g p o l y e t h y l e n e , was 0097-6156/88/0364-0300506.00/0 © 1988 American Chemical Society

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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r e p o r t e d by a Dupont group t o y i e l d t h e s u b s t i t u t e d b i s ( t r i f l u o r o m e t h y l )ca.rbinols d e r i v e d b y i n s e r t i o n o f t h e c a r b o n y l group o f hexaf l u o r o a c e t o n e i n t o t h e C-H bond o f t h e s u b s t r a t e ( 8 ) . The m o d i f i e d polymers which r e s u l t from t h i s t y p e o f r e a c t i o n a r e o f i n t e r e s t t o us because o f t h e pronounced changes i n p h y s i c a l and c h e m i c a l p r o p e r t i e s which a r i s e due t o t h e i n c o r p o r a t e d f l u o r o a l k a n o l g r o u p s . F l u o r o a l k a n o l s a r e r e p o r t e d t o have g r e a t l y enhanced a c i d i t y (pKa = 9.3 f o r h e x a f l u o r o - 2 - p r o p a n o l v s . pKa = 18 f o r 2-propanol ) ( 9 ), which s u g g e s t s t h a t polymers b e a r i n g such pendent groups would e x h i b i t u s e f u l hydrogen bond donor p r o p e r t i e s . I n f a c t , we have f o u n d t h a t p o l y e t h y l e n e , which i s n o r m a l l y q u i t e i n s o l u b l e , becomes r e a d i l y s o l u b l e i n t e t r a h y d r o f u r a n o r benzene w i t h even v e r y l o w l e v e l s o f incorporation. A polymer b e a r i n g f l u o r o a l k a n o l pendent groups would e x h i b i t hydrogen-bonding power i n t e r m e d i a t e between s i m p l e a l c o h o l s (e.g. p o l y v i n y l a l c o h o l ) , and a c a r b o x y l i c a c i d (e.g. p o l y a c r y l i c acid). By v i r t u e o f the s h i e l d i n g i n f l u e n c e o f t h e b u l k y t r i f l u o r o methyl s u b s t i t u e n t s i n t h e a l c o h o l pendent group, t h e s e f u n c t i o n a l i z e d macro-molecules would l i k e l y not s u f f e r the problem o f competing s e l f - a s s o c i a t i o n t o t h e same e x t e n t found w i t h t h e u s u a l hydrogenbonding m o i e t i e s . A l l o f t h e s e f a c t o r s suggest t h a t t h e s e m o d i f i e d polymers s h o u l d p r o v i d e u n i q u e m a t e r i a l s f o r f u r t h e r s t u d y . With the u l t i m a t e o b j e c t i v e o f u s i n g t h e s e m a t e r i a l s a s components o f b l e n d s , a n d i n a p p l i c a t i o n s a s polymer r e a g e n t s , we have made a thorough i n v e s t i g a t i o n o f the a d d i t i o n o f h e x a f l u o r o a c e t o n e t o X)olyethylene, a n d t o two c y c l o a l k a n e model s u b s t r a t e s , w i t h the g o a l o f o p t i m i z i n g the r e a c t i o n c o n d i t i o n s . We have p r e p a r e d , f o r c o m p a r i son, a u t h e n t i c samples o f o f t h e s m a l l m o l e c u l e model compounds b y a l t e r n a t i v e s y n t h e t i c procedures. The samples o f m o d i f i e d p o l y e t h y l ene w i t h v a r y i n g degrees o f i n c o r p o r a t i o n o f the f l u o r o a l k a n o l group have been c h a r a c t e r i z e d b y combustion a n a l y s i s , t h e r m a l a n a l y t i c t e c h n i q u e s , m o l e c u l a r w e i g h t d e t e r m i n a t i o n s , and s p e c t r o s c o p i c a l l y w i t h comparisons t o s m a l l m o l e c u l e model compounds. Experimental

Section

Materials. Low d e n s i t y p o l y e t h y l e n e (LDPE) was o b t a i n e d from Dow Chemical Co., PE 510. H i g h d e n s i t y p o l y e t h y l e n e (HDPE) was o b t a i n e d from P h i l l i p s C h e m i c a l , M a r l e x 6001. P o l y p r o p y l e n e (PP) was o b t a i n e d from Exxon Corp., Exxon PP #8216. D i - t - b u t y l p e r o x i d e , 99%, (DtBP) was o b t a i n e d f r o m P o l y s c i e n c e s . G a s e o u s h e x a f l u o r o a c e t o n e was o b t a i n e d from Nippon Mektron L t d . , Japan. Methods. E l e m e n t a l combustion a n a l y s i s was performed, b y G a l b r a i t h L a b o r a t o r i e s . I n f r a r e d s p e c t r a were d e t e r m i n e d u s i n g a Shimadzu 435 i n f r a r e d s p e c t r o p h o t o m e t e r : samples were p r e p a r e d a s h o t h y d r a u l i c p r e s s e d f i l m s f o r LDPE, a s s o l v e n t - c a s t f i l m s on NaCl windows f o r t h e m o d i f i e d polymer samples, and a s a t h i n f i l m on NaCl windows f o r t h e model compounds. A l l v a l u e s a r e i n r e c i p r o c a l c e n t i m e t e r s , c n r ; (w) = weak, (m) = medium, (s) = s t r o n g , ( b r ) = b r o a d . H-NMR s p e c t r a were determined a t 90 MHz on a V a r i a n EM390 NMR s p e c t r o m e t e r , and a r e r e p o r t e d i n ppm d o w n f i e l d from i n t e r n a l t e t r a m e t h y l s i l a n e s t a n d a r d a s f o l l o w s : c h e m i c a l s h i f t ( m u l t i p l i c i t y , i n t e g r a t i o n , c o u p l i n g cons t a n t ). F-NMR s p e c t r a were determined, a t 85 MHz on a JEOL FX90Q FT NMR s p e c t r o m e t e r and a r e r e p o r t e d i n ppm u p f i e l d from i n t e r n a l f l u o r o t r i c h l o r o m e t h a n e i n the above format w i t h the i n t e g r a t i o n g i v e n i n 1

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p e r c e n t a g e where a p p r o p r i a t e . The g e l p e r m e a t i o n chromatography (GPC!) was performed on a Waters 590 HPLC i n THF (except t h a t 1,2,4t r i c h l o r o b e n z e n e was used f o r LDPE i t s e l f ) and c a l i b r a t e d w i t h p o l y ­ styrene standards.

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Puri Π cation. The commercial LDPE was d i s s o l v e d i n h o t t o l u e n e and p r e c i p i t a t e d i n methanol. The r e s u l t i n g s m a l l c h i p s were d r i e d i n a vacuum oven o v e r n i g h t a t 80° C. HDPE was p u r i f i e d i n t h e same man­ n e r , e x c e p t t h a t i n t h i s case much o f t h e commercial m a t e r i a l was i n s o l u b l e i n t o l u e n e , and so s i g n i f i c a n t f r a c t i o n a t i o n a p p a r e n t l y o c c u r s d u r i n g t h e p u r i f i c a t i o n . The model compounds were p u r i f i e d by the f l a s h c h r o m a t o g r a p h i c t e c h n i q u e o f S t i l l e t a l . ( 1 0 ) . M o d i f i c a t i o n Reactions. The p r o c e d u r e o f Howard e t a l . (8) was t a k e n as a s t a n d a r d . A P a r r p r e s s u r e r e a c t o r (500 mL c a p a c i t y ) was charged w i t h 20 g o f polymer (1.43 mol o f "CH " f o r LDPE o r HDPE, o r 0.974 mol o f t h e e q u i v a l e n t f o r PP) , 200 mL o f d r y benzene, and 0.2 mL (1.09 mmolj, o r o t h e r measured amount, o f d i - t - b u t y l p e r o x i d e . The r e s u l t i n g m i x t u r e was d e g a s s e d t h r o u g h t h r e e c y c l e s o f f r e e z e e v a c u a t e - t h a w - n i t r o g e n purge. Gaseous h e x a f l u o r o a c e t o n e was passed i n t o a c a l i b r a t e d v e s s e l c o o l e d t o -78° C u n t i l 20 mL (0.159 moi) had c o l l e c t e d , and t h i s p o r t i o n was t h e n d i s t i l l e d i n t o t h e c o o l e d (-78° 0) r e a c t o r . The r e a c t o r was s e a l e d and h e a t e d t o 135° C w i t h v i g o ­ r o u s mechanical s t i r r i n g f o r 16 h ( o r o t h e r measured t i m e ) . After c o o l i n g , t h e r e a c t i o n m i x t u r e was f i l t e r e d , and t h e s o l i d s o b t a i n e d were d i s s o l v e d i n t e t r a h y d r o f u r a n (THF), p r e c i p i t a t e d i n methanol, and d r i e d under vacuum. The r e a c t i o n c o n d i t i o n s a r e d e t a i l e d i n T a b l e I f o r m o d i f i e d LDPE (samples 1 - 8 ) , f o r m o d i f i e d HDPE (sample 9) and f o r m o d i f i e d PP (sample 10). I n m o d i f i c a t i o n t r i a l s employing 1.2 mL (6.53 mmol) o f d i - t - b u t y l p e r o x i d e , t h e p r o d u c t was i n s o l u b l e i n THF and was assumed t o be c r o s s l i n k e d . The m o d i f i c a t i o n r e a c t i o n was a l s o a p p l i e d t o two c y c l o a l k a n e s a s model s u b s t r a t e s , and t h e s e were c a r r i e d o u t i n much t h e same f a s h i o n ( r e a c t i o n t i m e 3 h, 20 g o f s u b s t r a t e , 30 mL (0.239 mol) o f h e x a f l u o r o a c e t o n e , 0.6 mL (3.27 mmol) o f d i - t - b u t y l p e r o x i d e ) a l t h o u g h t h e c r u d e p r o d u c t s were n o t p u r i f i e d : c y c l o h e x a n e produced sample 11 ( 3 7 % mass r e c o v e r y ) and c y c l o d o d e c a n e produced sample 12 (220% mass r e c o v e r y ) . S e l e c t e d IR d a t a : F o r LDPE: 2

[80° C ] : 2980 ( s ) , 2840 ( s ) , 2700 (w), 1480 i s ) , 1380(s) cur ι ; [120° C, d i f f e r e n c e ] : 1700 (m) cm" . For Sample 1: [room temp.]: 3600-3300 ( b ) , 2920 ( s ) , 2840 ( s ) , 1440 ( s ) , 1180 ( s ) cnr * ; [80° C, d i f f e r e n c e ] : 3630 (w), 2700 (w), 1700 ( s ) cm-1 ; [120 degrees C, d i f f e r e n c e ] : 2700 (m) cm" . For Sample 5: [room temp.]: 3600 (w), 3300-3550 (w b r ) , 2930 ( s ) , 2860 ( s ) , 1710 (w), 1620 (ww) cm- ; [330° C, d i f f e r e n c e ] : 3600 (ww), 3300-3550 (ww b r ) , 1620 ( l o s t ) c n r * ; [410° C, d i f f e r e n c e ] : 3600 ( l o s t ) , 3300-3550 ( l o s t ) c n r * . F o r Sample 6: 3640 (w), 3600-3300 (w b r ) , 2940 ( s ) , 2880 ( s ) , 1705 (w), 1480 ( s ) , 1370 ( s ) , 1290 ( s ) , 1220 ( s ) , 1130 ( s ) , 1100 ( s ) c u r * . For Sample 7: 3630 (w), 3600-3300 (s b r ) , 2940 ( s ) , 2880 ( s ) , 1710 (ww), 1480 ( s ) , 1370 ( s ) , 1290 ( s ) , 1220 ( s ) , 1130 ( s ) , 1100 ( s ) cnr * . For Sample 10: 3640 (w), 3600-3300 ( s b r ) , 2990 ( s ) , 2960 1

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( s ) , 2940 ( s ) , 2900 ( s ) , 1680 (m), 1600 ( s ) , 1470 ( s ) , 1380 ( s ) , 1360 ( s ) , 1270 ( s ) , 1220 (s), 1160 ( s ) , 1100 (s) c n r * . S e l e c t e d i^F-NMR s p e c t r a ( d e - t o l u e n e ) : Sample 4: 73.86 ( 8 0 % ) , 74.18 (20%). Sample 5: 74.00 ( 8 0 % ) , 74.28 ( 2 0 % ) . Sample 6: 73.97, 74.1 (shoulder). Sample 11: 73.52 ( 6 7 % ) , 74.83 ( 3 3 % ) . Sample 12: 74.61 ( 8 8 % ) , 75.32 ( 1 2 % ) . S y n t h e s i s o f Model F l u o r o a l k a n o 1 s 13 a n d 14. A l t h o u g h t h e a d d i t i o n o f secondary G r i g n a r d r e a g e n t s t o h e x a f l u o r o a c e t o n e i s known t o p r o ­ ceed i n low y i e l d (11-12), t h i s was deemed t h e most e x p e d i e n t s y n t h e ­ s i s o f 13 and 14. A t h r e e - n e c k 250 mL f l a s k equipped w i t h a dewar condenser, two r u b b e r s e p t a and a magnetic s t i r b a r , was f l a m e - d r i e d under n i t r o g e n f l u s h , and was c h i l l e d t o -78° C i n a. d r y i c e / a c e t o n e bath. The f l a s k was charged w i t h 30 mL o f a 3 M s o l u t i o n o f c y c l o hexy]magnesium c h l o r i d e i n e t h e r (90 mmol), and t h i s was s t i r r e d whi Le gaseous h e x a f l u o r o a c e t o n e was condensed into the solution. A f t e r 15 m, t h e o r i g i n a l l y t r a n s l u c e n t s o l u t i o n t u r n e d c l e a r a n d t h e system developed p o s i t i v e p r e s s u r e , s o t h e f l o w o f h e x a f l u o r o a c e t o n e was s t o p p e d . The a p p a r a t u s was purged w i t h n i t r o g e n , and warmed t o room t e m p e r a t u r e , and t h e r e a c t i o n m i x t u r e was poured i n t o 1 L o f 5% aqueous h y d r o c h l o r i c a c i d . T h i s was e x t r a c t e d w i t h t h r e e 100 mL p o r t i o n s o f e t h e r , and t h e combined e t h e r p o r t i o n s were d r i e d o v e r sodium s u l f a t e and c o n c e n t r a t e d t o g i v e 15 g o f l i g h t y e l l o w o i l . P u r i f i c a t i o n by chromatography (75 g o f s i l i c a / p e t r o l e u m e t h e r w i t h i n c r e a s i n g methylene c h l o r i d e ) y i e l d e d 1.923 g (9%) o f 2 - e y o l o h e x y l 1,1,1,3,3,3-hexafluoropropan-2-ol., 13, a s a c l e a r o i l : IR ( t h i n f i l m ) 3560 (m), 3450 (m b r ) , 2920 ( s ) , 2850 ( s ) , 1450 ( s ) , 1390 (w), 1345 (m), 1280 ( s b r ) , 1220 ( s b r ) , 1185 ( s ) , 1130 ( s ) , 1110 ( s ) cnr . H-NMR