9 Irregular Structures in Polyepichlorohydrin Κ. E. STELLER
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Hercules Inc., Wilmington, Del. 19899
Introduction Information concerning the m i c r o s t r u c t u r e o f a polymer is e s s e n t i a l f o r e l u c i d a t i o n o f the p o l y m e r i z a t i o n mechanism. The p o l y m e r i z a t i o n o f s u b s t i t u t e d epoxides such as propylene oxide is p a r t i c u l a r l y i n t e r e s t i n g . This monomer is unsymmetrical, and p o l y m e r i z a t i o n can proceed by attack a t e i t h e r the head o r the tail end. I f both types of ring openings occur in the propagation step, h e a d - t o - t a i l , head-to-head, and tail-to-tail linkages appear i n the polymer c h a i n [eq.(1)].
CH. U-0-
•0-C-CH -0-i 2
HEAD
TAIL
CH
2
HEAD-TO-TAIL UNIT ISOTACTIC (dd a n d i i )
CH
POLYMERIZATION KCH ) 3
:-CH,-0H
L
CH (H)
HEAD-TO-HEAD UNIT (di and i d o r dd a n d U t )
3
CH.3
H(CH-) I •TAIL UNIT i d o r dd and ML )
Hercules Research Center C o n t r i b u t i o n Number 1646
136
Vandenberg; Polyethers ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
(1)
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9.
137
Polyepichlorohydrin
STELLER
In a d d i t i o n t o t h i s s t r u c t u r a l problem, there is a l s o a stereochemical problem. U n l i k e v i n y l monomers o r α-olefins, propylene oxide has an asymmetric carbon atom before polymeriza t i o n . Thus, it is p o s s i b l e to o b t a i n o p t i c a l l y a c t i v e polymers if the p o l y m e r i z a t i o n proceeds with e i t h e r complete r e t e n t i o n or complete i n v e r s i o n a t the asymmetric center. Four d i f f e r e n t dimer u n i t s in the main chain are t h e r e f o r e p o s s i b l e [eq.(1)]. It is common knowledge that the m i c r o s t r u c t u r e o f a polymer has a profound i n f l u e n c e on many o f its p r o p e r t i e s . In f a c t , there have been s e v e r a l such e f f e c t s observed with poly (propylene oxide). In 1959, Madorsky and Straus reported that i s o t a c t i c poly(propylene oxide) is somewhat more s t a b l e than the a t a c t i c polymer (1). Aggarwal and coworkers showed that the melting p o i n t o f crystalline poly(propylene oxide) could be r e l a t e d t o the sequence length o f isotactic u n i t s in the poly mer (2). Furthermore, Oguni and coworkers showed that the e x i s tence o f tail-to-tail linkages in poly(propylene oxide) d i s t u r b e d crystallization o f the polymer and thereby depressed its melting point (3). In polyether research, however, Dr. Charles C. P r i c e was perhaps the first t o recognize the i n f l u e n c e o f polymer micros t r u c t u r e on the r e s u l t a n t p h y s i c a l p r o p e r t i e s . In 1956, he reported the formation o f crystalline, low molecular weight, o p t i c a l l y - a c t i v e polymer from Jt-propylene oxide by potassium hydroxide-catalyzed p o l y m e r i z a t i o n [eq.(2)].
CHo CH
/ CH
2
K0H_
^O^ A
C R Y S T A L L I N E , OPTICALLY A C T I V E POLYMER ( l o w M.W.) CHo CH~ CH., CH„ I I I I -CH -CH-0~CH -CH-0-CH -CH-0-CH.,-CH-03
9
3
3
o
2
3
9
2
%
2
2
JZ JL ISOTACTIC
/ο\
i
( 2 )
This product was i n sharp c o n t r a s t t o the l i q u i d polymer o f the same molecular weight produced from racemic monomer under the same c o n d i t i o n s [eq.(3)].
/
CH \
H
3
KOH
LIQUID POLYMER (SAME M.W. AS EQ. 2) CHo CH I I -CH -CH-0-CH2-CH-0-CH -CH-0-CH -CH-03
3
d,Jt
2
2
CH A
d
3
d
2
CH
3
X
ATACTIC
Vandenberg; Polyethers ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
POLYETHERS
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138
He concluded that t h i s remarkable d i f f e r e n c e i n p h y s i c a l proper t i e s r e s u l t e d from i d e n t i c a l c o n f i g u r a t i o n o f a l l the asymmetric centers i n the c r y s t a l l i n e polymer. The l i q u i d polymer from racemic monomer, on the other hand, was e v i d e n t l y a stereorandom a t a c t i c polymer (4). Both polymers were l a t e r shown t o be formed almost e x c l u s i v e l y by h e a d - t o - t a i l p o l y m e r i z a t i o n . Vandenberg, P r i c e , and others l a t e r showed that a d d i t i o n o f each epoxide u n i t to the polymer chain occurs with i n v e r s i o n o f c o n f i g u r a t i o n a t the carbon atom where r i n g opening occurs. The asymmetry i n t h i s case i s not d i s t u r b e d , however, because the bond between the oxygen and the asymmetric carbon i s never broken. Dr. P r i c e a l s o showed t h a t f e r r i c c h l o r i d e - c a t a l y z e d polym e r i z a t i o n o f e i t h e r %- o r d£-propylene oxide gave polymeric m a t e r i a l which could be separated i n t o amorphous, intermediate molecular weight and c r y s t a l l i n e , high molecular weight f r a c t i o n s [eqs. (4) and (5)]. CH, * / CH
CHo
^CH CH
C R Y S T A L L I N E , OPTICALLY (HIGH M.W.)
kB.
AMORPHOUS, LESS OPTICALLY (INTERMEDIATE M.W.)
- PO
^ 0 £
2
A.
r
FeCl,
{
rC
3
FeCU
-
SAME AS A BUT OPTICALLY
