3 Propylene Oxide Polymers of Varying Stereosequence Distribution S. L. A G G A R W A L and L. F. M A R K E R
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Research and Development Division, The General Tire and Rubber Co., Akron, Ohio 44329
Introduction In 1956, C. C. P r i c e (1) demonstrated that when l-isomer of propylene oxide i s polymerized w i t h potassium hydroxide c a t a l y s t , a s t e r e o s p e c i f i c and c r y s t a l l i n e polymer i s produced. Yet, the racemic monomer polymerized w i t h the same c a t a l y s t gave an amorphous product. A ferric c h l o r i d e c a t a l y s t on the other hand produced, from e i t h e r racemic or o p t i c a l l y a c t i v e monomer, a crystalline high molecular weight f r a c t i o n having i d e n t i c a l c r y s t a l s t r u c t u r e . These studies demonstrated that long s t e r e o r e g u l a r sequences having e i t h e r d or l c o n f i g u r a t i o n , necessary f o r crystallization, can be produced by one c a t a l y s t , w h i l e another c a t a l y s t may lead to amorphous polymers, which may have only short s t e r e o r e g u l a r sequences of a p a r t i c u l a r c o n f i g u r a t i o n . Poly(propylene oxide) was unique among s t e r e o s p e c i f i c p o l y mers developed up to that time. The monomer has an o p t i c a l cent e r which, under a p p r o p r i a t e synthesis c o n d i t i o n s , can be preserved i n the polymer so that d or l forms could be i d e n t i f i e d at each asymmetric carbon atom. Examination of the c r y s t a l s t r u c ture of i s o t a c t i c poly(propylene oxide) (2) shows that it crystallizes in a helical form and that the c r y s t a l s can accommodate only the h e l i c e s of one o p t i c a l form and are t h e r e f o r e o p t i c a l l y active. A number of c a t a l y s t systems have been developed (3,4,5) which r e s u l t i n propylene oxide polymers of d i f f e r e n t s t e r e o s e quence d i s t r i b u t i o n . In the f o l l o w i n g , we review some of our work on the c h a r a c t e r i z a t i o n of stereosequence length i n propylene oxide polymers prepared w i t h d i f f e r e n t c a t a l y s t s , and more import a n t l y , studies on the e f f e c t of the d i f f e r e n c e s i n stereosesequence length on the crystallization behavior and mechanical p r o p e r t i e s of the polymers. D e s c r i p t i o n of Stereosequence
Distribution
As i n other polymeric systems, both head-to-head
and
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Vandenberg; Polyethers ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
3. AGGARWAL A N D M A R K E R
Propylene Oxide Polymers
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h e a d - t o - t a i l arrangements of consecutive monomer u n i t s have been i d e n t i f i e d i n poly(propylene oxide (6). These arrangements can be detected by chemical means and have important e f f e c t s i n modi f y i n g the c r y s t a l l i z a t i o n behavior of the polymer (6 7)· Another s t r u c t u r a l feature that may a f f e c t c r y s t a l l i z a t i o n behavior i s that, f o r a given c o n f i g u r a t i o n (d or 1_) of the repeating u n i t i n the i s o t a c t i c s t e r e o r e g u l a r polymer chain, two d i s t i n c t chains s t r u c t u r e s are p o s s i b l e depending upon the r e l a t i v e p o s i t i o n s of the oxygen and the asymmetric carbon. These may be designated as up and down chain s t r u c t u r e f o r each of the ci and _1 c o n f i g u r a t i o n of the i s o t a c t i c polymer c h a i n . They are superimposable by turning the polymer chain end-over-end ( 8 ) . We recognize such complications as above i n the c r y s t a l l i z a t i o n of s t e r e o r e g u l a r poly(propylene o x i d e ) . The main i n t e r e s t of our s t u d i e s , how ever, has been i n the stereosequence d i s t r i b u t i o n i n propylene oxide polymers prepared with d i f f e r e n t c a t a l y s t systems and the e f f e c t of stereosequence d i s t r i b u t i o n on the p r o p e r t i e s of these polymers. The simplest s t a t i s t i c a l model which has s u f f i c i e n t gener a l i t y to represent the sequence d i s t r i b u t i o n i n poly(propylene oxide) i s one i n which the c o n f i g u r a t i o n of the l a s t three mono mer u n i t s i n the growing chain determine the c o n f i g u r a t i o n of the next u n i t coming onto the chain. The c o n f i g u r a t i o n of the t r i a d of successive u n i t s may take on e i g h t p o s s i b l e arrange ments as l i s t e d i n Table I below. Recent studies have jhown that s e v e r a l types of such t r i a d s may be d i s t i n g u i s h e d from H and carbon-13 NMR spectra (9,10,11). We have adopted a shorthand n o t a t i o n of representing by + the case when two successive u n i t s have the same c o n f i g u r a t i o n , and by - when they have opposite configurations. Triads are r e f e r r e d to as i s o t a c t i c , t r i a d s as s y n d i o t a c t i c , and E„ and E^ as h e t e r o t a c t i c . The A and Β t r i a d s i n a polymer made from a racemic monomer cannot be d i s t i n g u i s h e d from each other by any p h y s i c a l method, and equal pop u l a t i o n s , i n any case, of the corresponding A and Β t r i a d s may be assumed. Thus, the number of p o s s i b l e t r i a d s t a t e s of i n t e r est i n a model f o r analyzing stereosequence d i s t r i b u t i o n reduces to f o u r .
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