20 Anionic Polymerization of Acrolein Kinetic Study andthePossibility of Block Polymer Synthesis: Mechanisms of Initiation and Propagation Reactions
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D. GULINO, Q. T. PHAM, J. GOLÉ, andJ.P. PASCAULT Laboratoire de Materiaux Macromoleculaires, ERA 745, Batiment 403, Institut National Des Sciences Appliquées, 69621 Villeurbanne, France
Acrolein i s a very reactive monomer with a high tendency to polymerize. Free-radical and cationic polymerizations lead to insoluble polymers even if the conversions are low. On the other hand, anionic polymerization gives soluble polyacroleins under well-adapted conditions. Because of the difunctional nature of this monomer, the chains of polyacroleins contain different types of units:
Chains of polyacroleins contain different functional groups. Because of this, these polymers are among the most reactive and may be transformed into a variety of derivatives. Particularly, polyacroleins are radiation-sensitive polymers and can be used to prepare printing plates. However, the elucidation of the structure of the polyacroleins i s made extremely d i f f i c u l t by their lack of s t a b i l i t y . Indeed, these polymers eventually become i n soluble some time after their synthesis. Several authors (1,2,3,4) have tried to determine the polyacrolein microstructures by chemical analysis.. The results are very often contradictory: only Gole, et a l . (5) have shown the presence of (1,4) units. In a previous a r t i c l e (6), we have identified the different types of units using H and C NMR. These methods have enabled us to analyze all anionically prepared polyacroleins. 1
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0097-6156/81/0166-0307$05.00/0 © 1981 American Chemical Society
In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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Koton, et a l . (7,8) have s t u d i e d the a c r o l e i n propagation i n i t i a t e d by Na-Naphthalene complex or by t - b u t y l l i t h i u r a in t e t r a h y d r o f u r a n (THF) a t v a r i o u s temperatures. From these r e s u l t s , a mechanism has been deduced, but it ignores the t r a n s f e r r e a c t i o n s and the d i f f e r e n t complexations of the l i v i n g end. In t h i s a r t i c l e , we have summarized the main r e s u l t s of our k i n e t i c study ( p a r t i c u l a r l y t r a n s f e r and propagation rate con s t a n t s ) obtained in THF with L i and N a Naphthalene complex as i n i t i a t o r s . We have a l s o given the m i c r o s t r u c t u r e of polyacro l e i n s obtained w i t h the same i n i t i a t o r s . Furthermore, by studying butadiene - a c r o l e i n block polymeri z a t i o n s , we have t r i e d t o understand the mechanism of the a c r o l e i n polymerization.
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Synthesis The u s u a l techniques of the a n i o n i c p o l y m e r i z a t i o n have been employed f o r the solvent p u r i f i c a t i o n and the syntheses. The monomer is d i s t i l l e d three times from calcium hydride and then once more j u s t before the p o l y m e r i z a t i o n . To synthesize horaopolyacroleins, the naphthalene complexes ( L i , Na , K ) have been used in t e t r a h y d r o f u r a n (THF). Then, l i v i n g oligomers of butadiene, i n i t i a t e d by the naphthalene com p l e x systems were reacted w i t h a c r o l e i n t o b u i l d the block p o l y mers. Other l i v i n g oligomers of butadiene were first f u n c t i o n a l i z e d w i t h ethylene oxide before r e a c t i n g w i t h a c r o l e i n . In t h i s case, the v i s c o s i t y is very high and the a d d i t i o n of a c r o l e i n is conducted a t room temperature. On the other hand, the same reac t i o n is made a t -40°C... f o r the o l i g o b u t a d i e n e s . The f u n c t i o n a l l z a t i o n with a c r o l e i n was s t u d i e d w i t h two d i f f e r e n t l i v i n g ends: the carbanions of butadiene a t -40°C... and the oxanions of ethylene oxide a t 20°C... (Table 2 ) . The same c o n c e n t r a t i o n of butadiene u n i t s has been employed f o r the chromatograms of each block p o l y m e r i z a t i o n stage and the observed v a r i a t i o n s between the molecu l a r weight d i s t r i b u t i o n s are q u a n t i t a t i v e . Molecular Weights Molecular weight and p o l y d i s p e r s i t y determinations were ob t a i n e d by GPC with THF as e l u t i o n s o l v e n t with s t y r a g e l columns (60, 100, 500, Ι Ο , Ι Ο , 1 0 and 3.10 A ) . F l u t e d macromolecule was observed simultaneously by r e f r a c t i v e index and by u l t r a v i o l e t absorption. F o r block polymers, the determination of the molecular weight d i s t r i b u t i o n has been r e a l i z e d on a Waters apparatus with f i v e columns ( t h r e e Waters columns: 500 Â and 2 χ 100 Â and two Shodex columns S 802 and S 803. To e a s i l y e x p l a i n the r e s u l t s of the molecular weight d i s t r i b u t i o n s of the block polymers, it is necessary to synthesize i n i t i a t i n g polymers having molecular weights lower than 5000. Indeed, the r e s o l u t i o n of our apparatus Is very high +
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In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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f o r molecular weights ranging between 1000 up to 5000. Therefore, a small s h i f t in the chromatograms a f t e r the a c r o l e i n a d d i t i o n can be observed. H-NMR Spectroscopy We have used the VARIAN XL 100 (100 MHz), VARIAN DA 60 I L (60MHz) and CAMECA (350 MHz) as apparatus. The spectra have been obtained in deuterated chloroform at room temperature with t e t r a m e t h y l s i l a n e as an I n t e r n a l r e f e r e n c e . We cannot work at e l e v a t e d temperatures to reduce the broadening of the bands because p o l y a c r o l e i n s are very s e n s i t i v e to heat. Results K i n e t i c Results The k i n e t i c s of the a n i o n i c p o l y m e r i z a t i o n of a c r o l e i n has been s t u d i e d by d i l a t o m e t r y . The k i n e t i c orders are shown to be u n i t y f o r monomer and a l s o f o r i n i t i a t o r : The l i v i n g ends are not a s s o c i a t e d at the s t u d i e d concentrations of i n i t i a t o r s {I } in THF with L i and N a as counter-ions (1.4.10" < {I } < 2.10" mole.dm f o r L i and 2.2.10" < {I } < 1.4.10" mole.dm"" f o r Na+). The p o l y m e r i z a t i o n rate is g r e a t l y modified by the c a t i o n and increases such as : L i < N a < K . (The p o l y m e r i z a t i o n l a s t s s e v e r a l days with L I , a few hours with N a and only a few minutes with K ) . Moreover, it is not a f f e c t e d by whether the i n i t i a t o r s is a carbanion or a oxanion. Terminations of growing chains do not occur. However, num erous t r a n s f e r r e a c t i o n s to monomer and to polymer do take place during t h i s polymerization. During our e f f o r t s to synthesize block polymers, these t r a n s f e r r e a c t i o n s have been e a s i l y proved (Figure 1). The i n i t i a t o r is an α-ω diNa polyisoprene with a high molecular weight (Mn • 90,000) t o which a c r o l e i n is added. A f t e r t h i s a d d i t i o n , the r e f r a c t i v e index response (RI) shows chains having molecular weights lower than those of the i n i t i a tor. And, simultaneously, no s h i f t of the maximum peak of the curve (Ve -34 counts) corresponding to the polyisoprene i n i t i a t o r is observed. On the other hand, only the p o l y a c r o l e i n weight d i s t r i b u t i o n s can be seen on the UV responses because the i n i t i a t o r has no a b s o r p t i o n at the wavelength λ 254my. On these UV chromatograms the maximum peak is s i t u a t e d widely down from Mn 90,000. Numerous chains of homopolyacroleins are synthesized by t r a n s f e r r e a c t i o n s to monomer. T r a n s f e r r e a c t i o n s to polymer are e a s i l y observed. Indeed, f o r the very high y i e l d s of the a c r o l e i n polymerization, i n s o l u ble gels are obtained. With K as counter i o n , the polymeriza t i o n being very f a s t , t h i s phenomenon cannot be avoided. On the other hand, with L i and N a , no g e l is obtained if the y i e l d s remain lower than 70%. These t r a n s f e r r e a c t i o n s have no e f f e c t on the polymeriza t i o n rate, but, nevertheless, g r e a t l y modify the molecular weight 0
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In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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o f the p o l y a c r o l e i n s . The m e a s u r e d Mn is much l o w e r t h a n the t h e o r e t i c a l Mn. M o r e o v e r , v e r y o f t e n , 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 s o f the p o l y a c r o l e i n s a r e b i m o d a l a s shown on the F i g u r e 2. I n t h e s e c o n d i t i o n s , the p o l y m e r i z a t i o n rate c a n be g i v e n by the f o l l o w i n g e q u a t i o n ( 1 0 ) : -d{A} V
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dt w h e r e {A} is the monomer c o n c e n t r a t i o n , {I } the i n i t i a l c o n c e n t r a t i o n o f the i n i t i a t o r , k the p r o p a g a t i o n rate c o n s t a n t , h the t r a n s f e r rate c o n s t a n t ( t r a n s f e r t o the monomer only), 0
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W i t h o u t d e t a i l i n g the c a l c u l a t i o n s a g a i n , the l a s t e q u a t i o n f o r kp is o b t a i n e d a f t e r the s t e a d y - s t a t e a p p r o x i m a t i o n f o r the l i v i n g ends A"", h a s b e e n i n t r o d u c e d . A new h y p o t h e s i s is now presented: {A~} is
