Polyethers

1 Polyethers CHARLES C. PRICE

Downloaded by 80.82.77.83 on September 8, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0006.ch001

Department of Chemistry, University of Pennsylvania, Philadelphia,Penn.19174

A.

Introduction

Several inherent c h a r a c t e r i s t i c s of the ether l i n k a g e have stimulated extensive research and development o f l i n e a r polymers with ether l i n k s i n the polymer backbone (1,2). The ether l i n k age has low p o l a r i t y and low vander Waals i n t e r a c t i o n character istics (ΔΗ = 87.5 cal/g f o r pentane vs. 83.9 cal/g f o r d i e t h y l e t h e r ) . The carbon-oxygen bond has a lower b a r r i e r to r o t a t i o n than the carbon-carbon bond (2.7 kc/m f o r dimethyl ether vs. 3.3 f o r propane) and thus provides a lower b a r r i e r to coiling and u n c o i l i n g of chains. The ether oxygen has an even lower ex cluded volume than a methylene group (vander Waals radii of 1.4 Å v s . 2.1 Å) and thus, o f all backbone u n i t s , has the s m a l l e s t "excluded volume". This a l s o is a f a c t o r which permits greater chain flexibility. The carbon-oxygen bond has as great a bond energy (85 kc/m) as a carbon-carbon bond (82 kc/m) and much g r e a t e r h y d r o l y t i c r e s i s t a n c e than e s t e r , a c e t a l or amide l i n k s . vap

These u s e f u l c h a r a c t e r i s t i c s have, in the past two decades, led to extensive commercial development and use of a v a r i e t y of p o l y e t h e r s . Poly(propylene oxide) has become the b a s i s of the l a r g e s c a l e , world-wide development of "one-shot" polyurethan foam rubber, f o r mattresses, f u r n i t u r e , cushions, padding, e t c . P o l y ( t e t r a h y d r o f u r a n ) has been an important component of Corfam s y n t h e t i c l e a t h e r s u b s t i t u t e and of elastic f i b r e s . Linear poly(2,6-xylenol) i s made on a large s c a l e as an engineering plastic with an important combination of p r o p e r t i e s , such as high g l a s s t r a n s i t i o n temperature, good thermal stability, good electrical p r o p e r t i e s , e x c e l l e n t adhesion and ready s o l u b i l i t y in common organic s o l v e n t s . In each o f these p o l y e t h e r s , the ether l i n k is p a r t of the "backbone" of the polymer chain. In each, the ether linkage makes an important c o n t r i b u t i o n to the p h y s i c a l p r o p e r t i e s and chemical stability on which the utility i s based. Since the chemistry of the generation of polyethers has been d i f f e r e n t from that of the classical v i n y l p o l y m e r i z a t i o n 1

Vandenberg; Polyethers ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

POLYETHERS

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processes, many academic i n v e s t i g a t o r s have a l s o been i n v o l v e d i n studying the mechanism, stereochemistry and other aspects o f polyether formation. I t i s our purpose here to present some of the r e s u l t s o f these s t u d i e s f o r those polyether systems i n which we have been p a r t i c u l a r l y i n t e r e s t e d , the polyepoxides, the poly(£-phenylene ethers) and the a l t e r n a t i n g poly(£-phenylenemethylene e t h e r s ) .


B.

Polyepoxides

Epoxide p o l y m e r i z a t i o n can be described i n terms of three d i f f e r e n t mechanisms; (1) a n i o n i c (base-catalyzed), (2) c a t i o n i c ( a c i d - c a t a l y z e d ) , and (3) coordinate. The t h i r d a c t u a l l y com bines features of the f i r s t two extremes, since i t involves c o o r d i n a t i o n o f the monomer oxygen a t a Lewis a c i d c a t a l y s t s i t e ( L ) , followed by attack on the thus a c t i v a t e d monomer by an a l k o x i d e already bound to the s i t e .

ο

θ

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C ^ ι ,ο

C

^ C ». ^ 0

φ \

(1)

0

G

0 ^ L

~>

L '

-

0

G

>

L

(3)

°

C

G

0

The ring-opening step i n each case i s a n u c l e o p h i l i c sub s t i t u t i o n and, i n every case where the stereochemistry has been e s t a b l i s h e d , has been shown to occur with i n v e r s i o n of c o n f i g u r a t i o n at the carbon atom undergoing n u c l e o p h i l i c attack (.3,4). The e t h e r - l i k e oxygen i n the s t r a i n e d three-membered r i n g thus behaves l i k e a good " l e a v i n g group. In the monomer i t s e l f , a strong n u c l e o p h i l e such as alkoxide i o n i s r e q u i r e d f o r S^2 11

a t t a c k . However, when the epoxide i s converted to an oxonium s t a t e , as i n the c a t i o n i c process, i t becomes such a good " l e a v i n g " group that even the weakly n u c l e o p h i l i c oxygen of the monomer i s able to attack e f f e c t i v e l y . The c h a r a c t e r i z a t i o n o f the epoxide oxygen as a good " l e a v i n g " group i s supported by the f a c t that i t not only can undergo S 2 displacement but E e l i m i n a t i o n . For example, t e t r a M

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methylethylene oxide, on treatment with a c a t a l y t i c amount of a base such as potassium t^-butoxide, undergoes " e l i m i n a t i o n " almost q u a n t i t a t i v e l y (5^.

Θ 0 t-BuO

+

H6H C •

0 ν

C

2

CH

t-BuOH C H

3

(

I

+

3

CH =Ç - Ç — CH CH

CH

2

3

)

OH II

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0

ZZ C —

I


2

CH

I I

C —CH CH

3

0

3 3

(98%) B ( l ) . A n i o n i c . Because of the i n t e r v e n t i o n of the E2 process, only ethylene oxide i s r e a d i l y converted to highpolymer by b a s e - c a t a l y s i s . The p o l y m e r i z a t i o n can be c a r r i e d out so that one polymer molecule forms f o r each c a t a l y s t molec u l e added (5). With propylene oxide, however, the E2 r e a c t i o n intervenes and serves, i n e f f e c t , as a c h a i n - t r a n s f e r process. The a l k y l o x i d e i o n formed by e l i m i n a t i o n i s , of course, capable of i n i t i a t i n g a new chain, now capped at one-end by an a l l y l ether group (6).

CH 0CH CH0© 3

2

******* Q0

OH +CH =CHCH 0© 2

2

The maximum molecular weight of base-catalyzed propylene o x i d e polymer i s l i m i t e d by the r a t i o of the r a t e s of propagation (k ) to t r a n s f e r ( k ) ; s i n c e at operable temperatures k ^ / k ^ i t r

100,

t h i s i s 6000 awu. I t was t h i s molecular-weight l i m i t i n g chain t r a n s f e r which led us to seek s y n t h e t i c approaches to b u i l d i n g a c r o s s l i n k e d rubber s t r u c t u r e from poly(propylene oxide) chains. T h i s was s u c c e s s f u l l y accomplished i n 1949 by b u i l d i n g branched chain


POLYETHERS

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l i q u i d polymer, followed by conversion i n t o a rubber network s t r u c t u r e by r e a c t i o n with d i i s o c y a n a t e s (7).

CH CH-CH V 0 3

2

+

Base (CH OH) - = 2 »

C

2

4

C[CH 0(CH CHO)„H] CH 2

f

4

3

+ HO(CH^HO)„H CH,


The a l l y l ether end group i s preserved only under m i l d c o n d i t i o n s of p o l y m e r i z a t i o n . Studies on i t s disappearance under more vigorous c o n d i t i o n s l e d to discovery of the basec a t a l y z e d rearrangement of a l l y l to c i s - p r o p e n y l ethers (8,9,10, 11) and p o s t u l a t i o n of a t r a n s i t i o n complex i n v o l v i n g oxygenpotassium c o o r d i n a t i o n to e x p l a i n the c i s - s t e r e o s p e c i f i c i t y .

R0 ROCH CH=CH 2

2

*

CH

»»

3

CH=CH

This rearrangement was a l s o found to extend to a l l y l although without c i s - s t e r e o s p e c i f i c i t y (12).

R NCH,CH=CH, 2

-

amines,

R NCH=CHCH, 2

t>Butylethylene oxide was s t u d i e d i n the hope that a s t r u c t u r e not p e r m i t t i n g E2 e l i m i n a t i o n to an a l l y l oxide would lead to high molecular weight polymer (5). However, the S 2 d i s placement i s also much slower i n t h i s monomer, presumably due to s t e r i c hindrance. Under the more vigorous c o n d i t i o n s necessary f o r p o l y m e r i z a t i o n , another c h a i n t r a n s f e r process must occur, although i t s exact nature has not yet been e s t a b l i s h e d . In any event, the l i m i t i n g molecular weight f o r poly (t.-butyl ethylene oxide) i s about 2000.

t-Bu 0CH CH0© ?

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H

3

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3

3

Θ + 0 Η (S)

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CH

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I CH

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(S)


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oxonium intermediate d e r i v e d from S-VIII, as i l l u s t r a t e d , with the two three-membered r i n g s i n a f a v o r a b l e r i g h t angled o r i e n t a t i o n , an H i n V I I I confronts a methyl i n the oxonium r i n g and v i c e v e r s a . I f the approaching monomer were R-VIII, then there would be two h i g h l y unfavorable methyl-methyl c o n f r o n t a t i o n s . Thus the stereochemistry of the growing chain end d i c t a t e s the stereochemistry of the incoming monomer. Β(3). Coordination P o l y m e r i z a t i o n . S t e r e o s e l e c t i v e polym e r i z a t i o n s of epoxides have played an important r o l e i n d e v e l oping the mechanistic concepts f o r t h i s major development i n polymer chemistry. The f i r s t r e p o r t by Natta o f i s o t a c t i c p o l y propylene (24) appeared a few months before the P r u i t t and Baggett patent i s s u e d on the p r e p a r a t i o n of i s o t a c t i c p o l y propylene oxide (25). Undoubtedly, the work of P r u i t t and Baggett antedated that o f Natta, although i t was a year a f t e r N a t t a s f i r s t paper before our work appeared showing that the P r u i t t Baggett polymer was indeed i s o t a c t i c (26). The c h i r a l i t y (asymmetry) of the methine carbon i n propylene oxide and i n i t s polymer has provided many e x t r a experimental parameters to use i n s t u d i e s o f the mechanism o f s t e r e o s e l e c t i v e p o l y m e r i z a t i o n by c o o r d i n a t i o n c a t a l y s t s . The f i r s t proposal of the c o o r d i n a t i o n p o l y m e r i z a t i o n scheme t o e x p l a i n s t e r e o s e l e c t i v e o/-olefin and o l e f i n oxide p o l y m e r i z a t i o n arose from the propylene oxide s t u d i e s (26,27). 1

There are many e f f e c t i v e c a t a l y s t systems, such as FeCl^propylene oxide, d i e t h y l z i n c - w a t e r and trialkylaluminum-water acetylacetone r e a c t i o n products. For none i s the exact s t r u c t u r e of the c a t a l y s t s i t e e s t a b l i s h e d . For a l l , the number of polymer molecules formed i s small compared to the metal atoms used to make the c a t a l y s t . T h i s suggests that the f r a c t i o n o f metal atoms at c a t a l y s t s i t e s must be at l e a s t as s m a l l . While the d e t a i l e d s t r u c t u r e of a c a t a l y s t s i t e remains to be e l u c i d a t e d , s e v e r a l important f e a t u r e s of these c a t a l y s t s are now known. Perhaps the most important feature was e s t a b l i s h e d by Tsuruta (28) who proved that the s t e r e o s e l e c t i v i t y was not a f e a t u r e of the c h i r a l i t y o f the growing end but was b u i l t i n t o the c a t a l y s t s i t e i t s e l f ( " c a t a l y s t - s i t e " c o n t r o l ) . The normal c a t a l y s t p r e p a r a t i o n gives an equal number of R- and S - c h i r a l c a t a l y s t s i t e s which s e l e c t i v e l y coordinate with R- and S-monomer, r e s p e c t i v e l y . This was demonstrated by s t a r t i n g with 75% Rand 25% S-monomer. A f t e r extensive p o l y m e r i z a t i o n , the recovered monomer was o f unchanged o p t i c a l p u r i t y . I f the c h i r a l i t y o f the c a t a l y s t s i t e were due to the c h i r a l i t y of the growing chain ("chain end" c o n t r o l ) one would expect that the growing chains would assume a 75 to 25 r a t i o . The p r e f e r e n t i a l r e a c t i v i t y f o r R-monomer would then be 9 to 1 ( r a t h e r than the experimentally observed 3 to 1) and the recovered monomer should tend toward a 50:50 mixture with i n c r e a s i n g conversion. The c h i r a l i t y of the c a t a l y s t s i t e s can be i n f l u e n c e d i n



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some cases. For example, Tsuruta (29) has shown that the c a t a l y s t made from R-borneol and d i e t h y l z i n c i s s t e r e o e l e c t i v e , i . e . , i t w i l l polymerize RS-propylene oxide g i v i n g i s o t a c t i c polymer with an excess of R-monomer u n i t s and l e a v i n g the unreacted monomer r i c h i n the S-isomer. This behavior can be accounted f o r by assuming that R-borneol produces the two c h i r a l c a t a l y s t s i t e s i n unequal numbers. A number of other c h i r a l a l c o h o l s and R-monomer f a i l e d to give s t e r e o e l e c t i v e c a t a l y s t , i . e . , they gave c a t a l y s t which behaved as though i t had equal numbers of R- and S - s i t e s (29,30). One p o s s i b l e explanation which has been o f f e r e d (31) i s that the more hindered bornyl group prevents i t from m i g r a t i n g from the c a t a l y s t s i t e L i n the propagation step ( r e a c t i o n ( 3 ) ) . Other simpler groups are known to migrate, becoming end groups i n the polymer (29,32). Another e f f e c t i v e s t e r e o e l e c t i v e c a t a l y s t f o r both epoxide and e p i s u l f i d e p o l y m e r i z a t i o n i s the r e a c t i o n product of R-(or S-) J:-butylethylene g l y c o l with d i e t h y l z i n c (33). One of the key f e a t u r e s of the s t e r e o s e l e c t i v e c o o r d i n a t i o n c a t a l y s t s f o r propylene oxide p o l y m e r i z a t i o n i s that, while they can give i s o t a c t i c polymer with a very high r a t i o of i s o t a c t i c to s y n d i o t a c t i c sequences (e.g. > 370) (34), i n the same r e a c t i o n mixture a l a r g e p a r t of the polymer i s amorphous. Even when Rpropylene oxide i s used as s t a r t i n g m a t e r i a l , much of the product i s amorphous polymer of low o p t i c a l r o t a t i o n c o n t a i n i n g many Spropylene oxide u n i t s (30). T h i s amorphous polymer has been shown to c o n t a i n head-tohead u n i t s as the imperfections i n s t r u c t u r e . T h i s was shown by degradation and i s o l a t i o n of the dimer g l y c o l s (35,36).

1. -/OCHCH V I ^ CH 3

0

H O

( f

3

=—• 2. LAH

H0CHCH 0CHCH 0H I I CH CH ix 9

3

9

+

C H

2> ° 2

CHL χ J

3

^HOCH CH) O 2

XI

2

GH

3

The diprimary (XI) and disecondary (X) dimer g l y c o l s w i l l a r i s e only from head-to-head u n i t s i n the polymer. F o r t u n a t e l y , the three isomeric g l y c o l s can be separated by g l c . For i s o t a c t i c polymer or amorphous polymer made by base c a t a l y s i s of RS-monomer, l i t t l e i f any X and XI were found. The amorphous polymer separated from i s o t a c t i c polymer prepared f o r a v a r i e t y of c o o r d i n a t i o n c a t a l y s t s gave 25 to 40% of X and XI. F u r t h e r more, the c o r r e l a t i o n of the head-to-head content from such degradative s t u d i e s with the o p t i c a l a c t i v i t y of the amorphous f r a c t i o n u s i n g R-monomer with the same c a t a l y s t shows that f o r every head-to-head u n i t there i s one R-monomer converted to an S-polymer u n i t . T h i s proves that, at the c o o r d i n a t i o n s i t e



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g i v i n g amorphous polymer, the abnormal r i n g opening which occurs at the secondary carbon, to give the head-to-head u n i t , has done so with i n v e r s i o n of c o n f i g u r a t i o n . A more recent s i m i l a r study of amorphous polymer accompanying i s o t a c t i c polymer i n the polymerization of RS-jt-butylethylene oxide by c o o r d i n a t i o n c a t a l y s t s has shown that i t a l s o contains head-to-head u n i t s (14). In t h i s case, the 1°, 2°dimer g l y c o l , which was only 30 t o 40% of the dimer mixture, could be separated i n t o i t s erythro and threo isomers by g l c . The i s o t a c t i c polymer gave, as expected, almost e x c l u s i v e l y the erythro-isomer, while the amorphous f r a c t i o n s gave only 40-45% e r y t h r o and 55-60% threo. These data combined i n d i c a t e that a t y p i c a l c o o r d i n a t i o n c a t a l y s t , such as Et^Zn-HÔ, contains i s o t a c t i c c a t a l y s t s i t e s and amorphous c a t a l y s t s i t e s . The i s o t a c t i c s i t e s are h i g h l y s e l e c t i v e i n c o o r d i n a t i n g with e i t h e r R-(or S-) monomer (react i o n (3), step 1) and h i g h l y s e l e c t i v e i n propagating by r i n g opening only at the primary carbon of the epoxide r i n g (react i o n (3), step 2). The amorphous c a t a l y s t s i t e can coordinate e q u a l l y w e l l with R- or S-monomer i n step 1, and has very l i t t l e preference f o r attack at the primary o r secondary carbon i n the r i n g opening propagation step. Not much q u a n t i t a t i v e information i s a v a i l a b l e on the r e l a t i v e r e a c t i v i t i e s of epoxides i n copolymerization. With base i n DMSO, phenyl g l y c i d y l ether i s 7 times more r e a c t i v e than p r o p y l ene oxide, presumably due to the i n d u c t i v e electron-withdrawing e f f e c t o f the phenoxy group (37). A £-chloro-substituent enhances the r e a c t i v i t y o f PGE while a £-methoxy group diminishes it. That c o o r d i n a t i o n c a t a l y s t s e x h i b i t p r o p e r t i e s c l o s e r to c a t i o n i c than a n i o n i c c a t a l y s t s i s i n d i c a t e d by the opposite i n f l u e n c e o f £-chloro and £-methoxy groups i n PGE, the l a t t e r now being the more r e a c t i v e (38). Furthermore, propylene oxide i s 50% more r e a c t i v e than phenyl g l y c i d y l ether and e p i c h l o r o h y d r i n 33% l e s s r e a c t i v e u s i n g EtÂt-HÔ as a c a t a l y s t . C.

Polyphenylene Oxides

The thermal and chemical s t a b i l i t y o f diphenyl ether has long been recognized as suggesting i n t e r e s t i n g and u s e f u l prope r t i e s f o r high polymers IbuiÎÊ with t h i s s t r u c t u r a l u n i t (2). The development o f s u c c e s s f u l methods f o r preparing such s t r u c tures has been a challenge to the s y n t h e t i c chemist. The r e markable chemistry o f the processes discovered has a l s o l e d t o s i g n i f i c a n t new f a c t s and hypotheses about the behavior o f phenoxy r a d i c a l s . The i n v e s t i g a t i o n s i n our l a b o r a t o r y were o r i g i n a l l y i n s p i r e d by the work o f Hunter and h i s students (39). They concluded that the amorphous products formed from t r i h a l o p h e n o l s on treatment with v a r i o u s o x i d i z i n g agents were low-molecular weight polymers formed by l o s s o f a halogen atom i n e i t h e r the 2- or 4-


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POLYETHERS


p o s i t i o n . We decided to i n v e s t i g a t e t h i s r e a c t i o n f u r t h e r , but w i t h 4~halo-2,6-xylenols as monomers. T h i s proved s u c c e s s f u l when c a t a l y s t s were used which Cook (40,41) found u s e f u l i n converting 2 , 4 , 6 - t r i - t - b u t y l p h e n o l to i t s s t a b l e blue f r e e r a d i c a l . Under these c o n d i t i o n s , 4-bromo-2,6x y l e n o l can be converted to a high-molecular weight polymer i n seconds, l i b e r a t i n g the bromine as bromide i o n . In the absence of oxidant c a t a l y s t , no bromide i o n i s l i b e r a t e d even a f t e r months. The p o l y m e r i z a t i o n i s thus obviously not a n u c l e o p h i l i c displacement. Furthermore, i t has the c h a r a c t e r i s t i c s of a chain r e a c t i o n , s i n c e polymer formed even at only 5% conversion i s of high molecular weight (42). While the polymer has a very low b r i t t l e temperature (-150°), i t s high g l a s s - t r a n s i t i o n tempera ture (> 200°) makes i t very hard to c r y s t a l l i z e . The polymer i s

L

CH

3

J

normally amorphous and s o l u b l e i n the usual organic s o l v e n t s , such as benzene. By h e a t i n g a s o l u t i o n i n α-pinene overnight, i t separates as c r y s t a l l i n e polymer, mp 275° (43). The commercial synthesis of p o l y x y l e n o l (sometimes r e f e r r e d to as PPO, £ply(uhenylene oxide)) i s c a r r i e d out by the oxida t i v e c o u p l i n g of 2,6-xylenol. This remarkable r e a c t i o n was d i s covered by Hay and h i s coworkers (44). One i n t r i g u i n g f e a t u r e of t h i s p o l y m e r i z a t i o n i s that i t i s a stepwise condensation. The product at 50% conversion i s a low-molecular weight o i l . The dimer (XIII) and trimer i n t h i s m a t e r i a l are as e a s i l y polymeri z a b l e as the monomer. The accepted mechanism i n v o l v e s coupling of aryloxy intermediates to give quinone k e t a l s (45,46,47). The Dutch workers i n p a r t i c u l a r s t u d i e d many simple models



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Polyethers

to determine what s t r u c t u r a l f a c t o r s and c o n d i t i o n s favored con v e r s i o n of XIV to XV by d i r e c t rearrangement (a) or through d i s s o c i a t i o n and recombination (b). One of the i n t r i g u i n g features of the o x i d a t i v e coupling i s the question of the features d i c t a t i n g C-0 coupling to give dimer X I I I vs_. C-C coupling to give the diphenoquinone XVI. For

o x i d a t i o n by Ag20 (48) or MnO^ (47), excess oxidant gives polyx y l e n o l , while excess x y l e n o l gives XVI. Even under the Hay con d i t i o n s , replacement of one methyl by t^-butyl or both by _i-Pr l e d to the diphenoquinone as the main product. The r a t e of phenol o x i d a t i o n under the Hay c o n d i t i o n s i s f i r s t order i n c a t a l y s t , f i r s t order i n 0^ pressure and zero order i n phenol, although the magnitude of the r a t e i s q u i t e s e n s i t i v e to s u b s t i t u t i o n s i n the phenol r i n g , being favored by e l e c t r o n - r e l e a s i n g groups (49). We have suggested the scheme on the next page to account f o r these observations. The dimer X I I I could a r i s e e i t h e r by coupling of two f r e e f r e e r a d i c a l s or by attack of a r a d i c a l , l i b e r a t e d i n the transformation from Β to C i n Scheme I to remove ArO* from D. Decreased base c o o r d i n a t i o n on Cu (by l i g a n d concentration, l i g a n d hindrance or ortho hindrance i n the phenol) favors C-G coupling (50). We have proposed the modified Scheme I I to exl a i n t h i s behavior. The e s s e n t i a l f e a t u r e i s that decreased ase c o o r d i n a t i o n at Cu increases the a f f i n i t y of the Cu f o r the

f


14

POLYETHERS

Scheme I

ArO^

y \

(pyrV

\(

,(pyr>2

(Pyr)

2

(pyr)2

gAr

CI

Β + 2ArOH,


-2ArO-

ArO.

yPL Cu°

Polyethers

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