8 Solvent and Substituent Effects in the Anionic Polymerization of α,α-Disubstituted β-Propiolactones 1
2
ROBERT W. LENZ, EILEEN M. MINTER , DOUGLAS B. JOHNS , and SØREN HVILSTED 3
Downloaded by EAST CAROLINA UNIV on March 22, 2016 | http://pubs.acs.org Publication Date: August 16, 1985 | doi: 10.1021/bk-1985-0286.ch008
Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003
Solvent and substituent effects in the anionic polymerization of α-ethyl-α-n-butyl-β-propiolactone, EBPL, were investigated in dimethyl sulfoxide and in N-methylpyrrolidone. In dimethyl sulfoxide, DMSO, gelation of the growing polymer resulted in decreased polymerization rates and formation of bimodal molecular weight distributions, but polymer ization in N-methylpyrrolidone, NMP, continued at a constant rate despite precipitation of the growing polymer. The polymerization rate was less in DMSO than in NMP although the former has a higher di electric constant. It is suggested that solvation of the carboxylate ion endgroup by DMSO may be re sponsible for the lower rate. The propagation rate constant for EBPL in DMSO was lower than that for the α-methyl-α-butyl monomer indicating that the steric acceleration observed in the α-methyl-α-n -alkyl monomer series did not continue when two large alkyl groups are present. It is believed that living polymers are formed inallof the polymeriza tion reactions of EBPL although somewhat broad molecular weight distributions are obtained because of the heterogeneity of the reactions. The a n i o n i c p o l y m e r i z a t i o n r e a c t i o n s o f o:,û!-disubstituted-/3-propiol a c t o n e s , and t h e p r o p e r t i e s o f t h e r e s u l t i n g p o l y e s t e r s , w i t h t h e s t r u c t u r e s shown o n t h e n e x t page, have been under i n v e s t i g a t i o n i n t h i s l a b o r a t o r y f o r o v e r t e n y e a r s (1,2). 1Current address: Rochester, NY 14612 2Current address: Ethicon, Somerville, NJ 08876 3Current address: Scandinavian Paint and Printing Ink Research Institute, Denmark
0 0 9 7 - 6 1 5 6 / 8 5 / 0 2 8 6 - Ό 105$06.00/0 © 1985 American Chemical Society
McGrath; Ring-Opening Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
RING-OPENING POLYMERIZATION
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CH , C H , n - C H , n - C ^
Downloaded by EAST CAROLINA UNIV on March 22, 2016 | http://pubs.acs.org Publication Date: August 16, 1985 | doi: 10.1021/bk-1985-0286.ch008
OCH C C —
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5
3
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Unusual and unexpected s o l v e n t (3) and s u b s t i t u e n t (4_) e f f e c t s on t h e p o l y m e r i z a t i o n r a t e s have been observed i n these s t u d i e s , and q u i t e s u r p r i s i n g l y , t h e polymers o b t a i n e d from r a c e m i c monomers ( i . e . , those i n which R and R a r e d i f f e r e n t ) have a l l been found t o be c r y s t a l l i n e ( 1 , 2 , 5 ) , a l t h o u g h they a r e a p p a r e n t l y a t a c t i c {6). For t h e unexpected s u b s t i t u e n t e f f e c t s on p r o p a g a t i o n r a t e s , i t was observed t h a t t h e r a t e c o n s t a n t s f o r p r o p a g a t i o n , k , i n c r e a s e d w i t h i n c r e a s i n g s u b s t i t u e n t s i z e w i t h i n a s e r i e s o f a-methyl-a-n-alkyl-3p r o p i o l a c t o n e s ( R = C H 3 , R = C H c , C^H-j, C 4 H 9 ) , which were polymer i z e d i n e i t h e r t e t r a h y d r o f u r a n , THF, o r d i m e t h y l s u l f o x i d e , DMSO, w i t h tetraethylammonium benzoate, TEAB, i n i t i a t o r (40 . I n DMSO t h e a c t i v a t i o n e n e r g i e s a l s o i n c r e a s e d w i t h i n c r e a s i n g s i z e o f t h e na l k y l group a s might be expected because t h i s s u b s t i t u e n t on t h e α-position c o u l d cause s t e r i c h i n d r a n c e t o a t t a c k a t t h e 3 - p o s i t i o n i n t h e S 2 r e a c t i o n . That i s , n u c l e o p h i l i c s u b s t i t u t i o n n o t c a r b o n y l a t t a c k i s t h e mechanism o f t h e p r o p a g a t i o n r e a c t i o n , a s f o l l o w s : D
1
2
2
N
/WCH.C-C
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CH-,(CH ) -C - C 0
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I t i s o f i n t e r e s t t o note a l s o t h a t t h e i n c r e a s e i n a c t i v a t i o n energy was o f f s e t by an i n c r e a s e i n a c t i v a t i o n e n t r o p y w i t h i n c r e a s i n g a l k y l group s i z e a l o n g t h e s e r i e s o f e t h y l (x = 1 ) , p r o p y l (x = 2) and b u t y l (x = 3) f o r R « 2
For t h e unexpected s o l v e n t e f f e c t s r e f e r r e d t o above, i t was observed t h a t t h e r a t e o f p r o p a g a t i o n , a s i n d i c a t e d by t h e measured v a l u e o f k , was h i g h e r f o r t h e r e a c t i o n i n THF than i n DMSO, even though t h e l a t t e r i s a much h i g h e r p o l a r i t y s o l v e n t . We suggested t h a t t h e reduced r e a c t i v i t y i n DMSO c o u l d be a t t r i b u t e d t o s o l v a t i o n o f t h e c a r b o x y l a t e a n i o n by t h i s s o l v e n t w i t h f o r m a t i o n o f a s t a b l e , r e l a t i v e l y u n r e a c t i v e s o l v a t e d i o n p a i r by t h i s endgroup n u c l e o p h i l e . p
The p r e s e n t r e p o r t d e s c r i b e s a c o n t i n u a t i o n o f these r a t e i n v e s t i g a t i o n s w i t h a n o t h e r monomer i n t h i s s e r i e s and a l s o t h e p r e p a r a t i o n o f polymers w i t h o t h e r a - s u b s t i t u e n t s .
McGrath; Ring-Opening Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
L E N Z E T AL.
8.
Solvent and Substituent Effects in Anionic Polymerization 107
R e s u l t s and D i s c u s s i o n Anionic Polymerization of a-Ethyl-a-Alkyl-3-Propiolactones. For comparison w i t h t h e p r e v i o u s l y s t u d i e d s e r i e s o f a - m e t h y l - a - a l k y l s u b s t i t u t e d monomers and polymers, t h r e e monomers c o n t a i n i n g a - e t h y l a - n - a l k y l s u b s t i t u e n t s (R
