Synthesis of Biodegradable Polyethylene - ACS Symposium Series

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Synthesis of Biodegradable Polyethylene W I L L I A M J . BAILEY and BENJAMIN G A P U D Department of Chemistry, University of Maryland, College Park, M D 20742

Although polyethylene is essentially nonbiodegradable, it was found that copolymers of ethylene and 2-methylene-l,3-dioxepane prepared by a high pressure free radical solution polymerization were indeed biodegradable. Most synthetic polymers are not biodegradable because they have not been on the earth long enough to have microorganisms evolve to utilize them as food. One exception to this rule is the low melting polyesters since poly(β-hydroxybutyrate) is a naturally occurring material which many bacteria and fungi use to store energy in the same manner that animals use fat. Since the free radical ring-opening polymerization of cyclic ketene acetals, such as 2-methylene-l,3-dioxepane, made possible the introduction of an ester group into the backbone of an addition polymer, this procedure appeared to be an attractive method for the preparation of biodegradable polymers. Copolymers of ethylene containing 10% ester-containing units were shown to be highly biodegradable while copolymers containing 2% ester-containing units were only slowly biodegradable. For many applications in the medical, agricultural, and ecological fields, it is desirable to have a biodegradable polymer that can be easily fabricated by injection molding, melt spinning, and melt extrusion into films. Of course, all naturally occurring polymers, such as starch, cellulose, proteins, nucleic acids, and lignin, are biodegradable, since they have been existing on earth for a length of time sufficient for microorganisms to evolve that can utilize these materials as food. Unfortunately, almost all of these materials contain very polar groups and, therefore, decompose on heating before they melt. This means that these materials can be fabricated only by solution processes, but not by the relatively inexpensive commercial processes mentioned above. Furthermore, i f

0097-6156/85/0280-0423$06.00/0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

424

POLYMER STABILIZATION AND DEGRADATION

one attempts to modify these polymers i n o r d e r to lower the m e l t i n g p o i n t , such as the c o n v e r s i o n of c e l l u l o s e i n t o c e l l u l o s e b u t y r a t e a c e t a t e , the m a t e r i a l s become n o n b i o d e g r a d a b l e .

Downloaded by UNIV OF MINNESOTA on July 24, 2013 | http://pubs.acs.org Publication Date: June 14, 1985 | doi: 10.1021/bk-1985-0280.ch029

Biodegradable

Polyesters

On the o t h e r hand, s y n t h e t i c polymers have n o t , i n g e n e r a l , been i n e x i s t e n c e l o n g enough to have m i c r o o r g a n i s m s e v o l v e t h a t can u t i l i z e them as f o o d . T h u s , t h e r e are very few b i o d e g r a d a b l e s y n t h e t i c polymers c u r r e n t l y known. P o t t s , e t a l . , (1) found t h a t o n l y low m e l t i n g and low m o l e c u l a r w e i g h t p o l y e s t e r s showed any r e a s o n a b l e degree o f b i o d e g r a d a t i o n . I t i s r e a l l y not s u r p r i s i n g t h a t some s y n t h e t i c p o l y e s t e r s are b i o d e g r a d a b l e s i n c e p o l y ( 3 - h y d r o x y b u t y r a t e ) i s a n a t u r a l l y o c c u r r i n g m a t e r i a l t h a t many b a c t e r i a and f u n g i use f o r energy s t o r a g e i n the same way t h a t a n i m a l s use f a t (2). For example, a p o l y t e t r a m e t h y l e n e s u c c i n a t e w i t h a reduced v i s c o s i t y o f 0.08 was b i o d e g r a d a b l e , but a sample o f the same polymer w i t h a reduced v i s c o s i t y of 0.59 was e s s e n t i a l l y not d e g r a d e d . A t y p i c a l use of a b i o d e g r a d a b l e or b i o a b s o r b a b l e p o l y e s t e r i n the medical f i e l d i s the new p o l y g l y c o l a t e a b s o r b a b l e s u t u r e . H i s t o r i c a l l y the surgeon used " c a t g u t , " which i s a b i o d e g r a d a b l e protein. However, many people are a l l e r g i c to t h i s p r o t e i n a c e o u s material. For t h i s reason the surgeon o f t e n s h i f t e d to a n y l o n s u t u r e , which, although nonbiodegradable, i s e s s e n t i a l l y n o n a l l e r g e nic. American C y a n a m i d ' s Dexon p o l y g l y c o l a t e has the b e s t o f both w o r l d s ; i t i s b i o a b s o r b a b l e i n the body and t h e r e f o r e does not have t o be p h y s i c a l l y removed, and a t the same time i t i s n o n a l l e r g e n i c (3). An example of an a g r i c u l t u r a l a p p l i c a t i o n of a b i o d e g r a d a b l e p o l y e s t e r i s Union C a r b i d e ' s use of p o l y c a p r o l a c t o n e (mp 6 0 ° C ) f o r r e f o r e s t a t i o n by i n j e c t i o n molding o f a b u l l e t - s h a p e d c o n t a i n e r i n which a seed i s p l a n t e d . The p l a s t i c c o n t a i n e r , which takes about one y e a r to b i o d e g r a d e i n the s o i l , p r o t e c t s the s e e d l i n g from a t t a c k during i t s f i r s t year of l i f e . T h i s procedure makes p o s s i b l e the m e c h a n i z a t i o n of s e e d l i n g p l a n t i n g and g i v e s a h i g h e r y i e l d o f mature t r e e s (1).

Biodegradable

Polyamides

In an e f f o r t to f i n d a n o t h e r c l a s s o f polymers which would be b i o d e g r a d a b l e , i t appeared t h a t the polyamides had many a t t r a c t i v e features. In our l a b o r a t o r i e s , a new s y n t h e s i s of p o l y p e p t i d e s through the p o l y m e r i z a t i o n of the amino a c i d a z i d e hydrobromides had been d e v e l o p e d . T h u s , i t was f e a s i b l e to extend t h i s procedure to the s y n t h e s i s of a dimer of g l y c i n e and e - a m i n o c a p r o i c a c i d f o l l o w e d by the p o l y m e r i z a t i o n of t h i s dimer through the amino a c i d a z i d e hydrobromide to the d e s i r e d r e g u l a r l y a l t e r n a t i n g copolyamide o f nylon 2/nylon 6 (4).

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

29.

BAILEY AND GAPUD

Synthesis of Biodegradable Polyethylene

0

425

0

HBr-NH2-CH2-C-NMCH )5-C-N3

E t ~ N , DMF

2

78%, -15°C 0

0

NH-CH2-C~NMCH )5-C 2


x The r e g u l a r l y a l t e r n a t i n g copolyamide o f nylon 2 / n y l o n 6, mp 268-270° C and [r,] ( m - c r e s o l ) a t 25° C o f 0.46 d l / g , was h i g h l y c r y s t a l l i n e as e v i d e n c e d by an x - r a y d e t e r m i n a t i o n . The m e l t i n g p o i n t i s i n t e r m e d i a t e between t h a t o f n y l o n 6, 220° C, and p o l y g l y c i n e , which decomposes a t about 400° C. It is hydrophilic, a b s o r b i n g about twice as much water as nylon 6, s i n c e i t has the same average c o n c e n t r a t i o n o f amide groups as nylon 4, b u t i s much more t h e r m a l l y s t a b l e , l e s s flammable, and lower m e l t i n g . S i n c e the copolyamide c o n t a i n e d two d i f f e r e n t amide bonds, both o f which were s i m i l a r , b u t not i d e n t i c a l to a p e p t i d e bond, i t was hoped t h a t the e x t r a c e l l u l a r enzymes o f the b a c t e r i a or f u n g i would c l e a v e the polymer to small fragments t h a t c o u l d be u t i l i z e d by the microoganisms as a f o o d . In a f a s t s c r e e n i n g t e c h n i q u e , a c u l t u r e of b a c t e r i a a n d / o r f u n g i was f e d a s t a n d a r d amount o f h y d r o l y z e d c a s e i n and the amount and r a t e o f carbon d i o x i d e l i b e r a t e d was monitored. In o r d e r to determine whether a m a t e r i a l was b i o d e g r a d a b l e on the time s c a l e u s e d , the m a t e r i a l was added to a s i m i l a r f l a s k and any i n c r e a s e i n the r a t e or amount o f carbon d i o x i d e over the c o n t r o l was noted (J5,6h I t was n o t s u r p r i s i n g t h a t g l y c i n e and e - a m i n o c a p r o i c a c i d were r e a d l y consumed by the microorganisms as f o o d , w h i l e nylon 6 was i n e r t as was p o l y g l y c i n e . When the r e g u l a r l y a l t e r n a t i n g c o p o lyamide (nylon 2 / n y l o n 6) was t e s t e d , i t was degraded by both f u n g i and b a c t e r i a . The fungus A s p e r g i l l u s n i g e r c o m p l e t e l y degraded the copolymer i n about 3 weeks. E x a m i n a t i o n o f the photomicrographs o f the polymer d u r i n g d e g r a d a t i o n i n d i c a t e d t h a t c e r t a i n p o r t i o n s , p r e sumably the amorphous r e g i o n s t h a t were s w o l l e n i n w a t e r , were a t t a c k e d f i r s t and the o t h e r r e g i o n s , presumably the more c r y s t a l l i n e p a r t s , were a t t a c k e d more s l o w l y (7). B i o d e g r a d a b l e A d d i t i o n Polymers A d d i t i o n polymers have been g e n e r a l l y q u i t e r e s i s t a n t to b i o d e g r a d a t i o n s i n c e the c a r b o n - t o - c a r b o n bonds i n the backbone a r e n o t very s u s c e p t i b l e to b i o l o g i c a l c l e a v a g e . As a consequence, many h y d r o p h i l i c a d d i t i o n polymers have been banned from m e d i c i n a l use because the high m o l e c u l a r weight polymers a r e not degraded or e l i minated from the body i n a r e a s o n a b l e l e n g t h of t i m e . We reasoned t h a t i f an e a s i l y h y d r o l y z a b l e group c o u l d be i n t r o d u c e d i n t o an a d d i t i o n copolymer by a f r e e r a d i c a l p r o c e s s , a wide v a r i e t y o f b i o d e g r a d a b l e polymers c o u l d be p r e p a r e d .


426

P O L Y M E R STABILIZATION A N D D E G R A D A T I O N

A l t h o u g h t h e r e are m i c r o o r g a n i s m s which w i l l degrade l i n e a r h y d r o c a r b o n s , the d e g r a d a t i o n o f p o l y e t h y l e n e i s very slow ( 8 ) . R e c e n t l y , C o r b i n [9) m o n i t o r e d the b i o d e g r a d a t i o n of C-labelled p o l y e t h y l e n e i n soTl by means o f the carbon d i o x i d e e v o l v e d . Since he observed a minimum c o n v e r s i o n r a t e of 2% per y e a r , he c o n c l u d e d t h a t p o l y e t h y l e n e f i l m d i d not degrade to any s i g n i f i c a n t e x t e n t the b i o a c t i v e system s t u d i e d . T h i s slow d e g r a d a t i o n r e s u l t s because the mechanism o f d e g r a d a t i o n of l i n e a r hydrocarbons i n v o l v e s the o x i d a t i o n of the t e r m i n a l methyl group to a c a r b o x y l i c a c i d group and t h e n , d e g r a d a t i o n of the r e s u l t i n g f a t t y a c i d by s t e p - w i s e 3o x i d a t i o n , two carbon u n i t s a t a t i m e . In l i n e a r p o l y e t h y l e n e o f high m o l e c u l a r w e i g h t , there are very few methyl groups and even they are l o c a t e d i n the bulk of a h y d r o p h o b i c medium n o t r e a d i l y a c c e s s i b l e to the m i c r o o r g a n i s m . I t was r e a s o n e d , t h e r e f o r e , i f e s t e r groups c o u l d be i n t r o d u c e d i n t o the backbone o f p o l y e t h y l e n e , i t would become b i o d e g r a d a b l e . In a s e a r c h to f i n d monomers which would undergo f r e e r a d i c a l


1 4

ring-opening

polymerization,

several

unsaturated

heterocyclic

monomers were i n v e s t i g a t e d ( 1 0 ) . One of the monomers t h a t we s t u d i e d was 2 - m e t h y l e n e - l , 3 - " d T o x e p a n e (I). I t was reasoned t h a t s i n c e the r e s u l t i n g e s t e r c a r b o n y l group was t h e r m o d y n a m i c a l l y more s t a b l e by a t l e a s t 40 kcal than the c a r b o n - t o - c a r b o n double bond i n the s t a r t i n g m a t e r i a l , t h e r e would be a d r i v i n g f o r c e f o r the r i n g opening p o l y m e r i z a t i o n . Treatment of t h i s monomer w i t h benzoyl p e r o x i d e gave a high m o l e c u l a r weight polymer by a f r e e r a d i c a l r i n g - o p e n i n g p o l y m e r i z a t i o n w i t h 100% r i n g opening even a t room temperature ( 1 0 ) . T h i s procedure r e p r e s e n t e d the f i r s t s y n t h e s i s o f a p o l y e s t e r by a f r e e r a d i c a l mechanism as w e l l as the f i r s t g e n e r a l method to i n t r o d u c e an e s t e r group i n t o the backbone of an a d d i t i o n a l polymer. 0 y

CH =C ( 2

0-CM -CHo

AIBN

I 0-CH -CH

50°C

2

2

CH -C-0-(CH ) 2

2

2

I

repeat

0-CH -CH R-CH -C| 0-CH -CH /

4

2

2

R-CH -C 2

2

CH -CH

2

I 0-CH -CH

2

#

2

2

V

2

2

S i n c e the monomer I would c o p o l y m e r i z e w i t h a wide v a r i e t y o f comonomers w i t h the i n t r o d u c t i o n of an e s t e r group i n t o the main c h a i n , t h i s appeared to make p o s s i b l e the p r e p a r a t i o n o f b i o d e g r a dable a d d i t i o n polymers. C o p o l y m e r i z a t i o n of e t h y l e n e and the ketene a c e t a l I a t 120°C produced a s e r i e s of copolymers c o n t a i n i n g e s t e r groups i n the backbone o f the c o p o l y m e r , a g a i n w i t h q u a n t i t a t i v e r i n g opening.


29.


BAILEY AND GAPUD

/

CH =CH 2

+

2

CH =C 2

0-CH -CH | 0-CH -CH 2

di-tert-butyl = 120°C

2

V

2

2

427

peroxide >

I 0

0

CH -C-0-(CH ) -(CH -CH ) -CH -C-0-(CH )4-(CH -CH )


2

2

4

2

2

x

2

2

2

2

y

In o r d e r to ensure t h a t the copolymers were r e a s o n a b l y homogeneous i n view of the f a c t t h a t the 2 - m e t h y l e n e - l , 3 - d i o x e p a n e (I) i s o n l y s l i g h t l y more r e a c t i v e than e t h y l e n e , a l l c o n v e r s i o n s were h e l d to below 2% as i n d i c a t e d i n T a b l e I . A s e r i e s of copolymers c o n t a i n i n g from 2.1 to 10.4 mol-% o f the e s t e r - c o n t a i n i n g u n i t s was p r e p a r e d . A l l o f these copolymers were about 5,000 m o l e c u l a r w e i g h t and had m e l t i n g p o i n t s v a r y i n g from 1 0 0 - 1 0 5 ° C f o r the copolymer c o n t a i n i n g 2.1 mole-% of I to 8 4 - 8 8 ° C f o r the copolymer c o n t a i n i n g 10.4 mole-%. D e t e r m i n a t i o n of

Biodegradability

The r a t e of b i o d e g r a d a b i l i t y was determined by a method based on the d e t e r m i n a t i o n of the carbon d i o x i d e produced by the m i c r o b i a l metabolism of the polymer samples. Our s c r e e n i n g t e s t , which was the method developed by Kramer and E n n i s ( 5 , £ ) , c o n s i s t e d o f f e e d i n g a s t a n d a r d amount of h y d r o l y z e d c a s e i n and the polymer to a c u l t u r e c o n t a i n i n g a mixed m i c r o - f l o r a from s o i l or known m i c r o o r g a n i s m s i n a m o d i f i e d Warburg a p p a r a t u s . The amount of C 0 l i b e r a t e d was m o n i t o r e d by a F i s h e r - H a m i l t o n gas p a r t i t i o n e r . The i n c r e a s e i n the amount of carbon d i o x i d e i n p o l y m e r - c o n t a i n i n g c u l t u r e s over t h a t of the c o n t r o l was used as a measure o f the r a t e o f b i o d e g r a d a t i o n on the time s c a l e u s e d . S t a t i s t i c a l a n a l y s i s i n v o l v e d a n a l y s i s of v a r i a n c e and the s t a t i s t i c a l d i f f e r e n c e was d e t e r m i n e d w i t h a D u n c a n ' s t e s t a t a 5% l e v e l . The r e s u l t s are l i s t e d i n T a b l e II and a r e p l o t t e d i n F i g u r e 1. I t i s o b v i o u s t h a t the copolymers c o n t a i n i n g a t l e a s t 6.7 mol-% of the e s t e r - c o n t a i n i n g u n i t s are h i g h l y d e g r a d a b l e p r o d u c i n g C 0 a t a r a t e 108 to 118% of the h y d r o l y z e d casein c o n t r o l . The copolymers c o n t a i n i n g the lower amount o f the e s t e r groups b i o d e g r a d e a t q u i t e low r a t e s but much g r e a t e r than polyethylene. I n o c u l a t i o n o f a sandy loam to n u t r i e n t b r o t h and i n c u b a t i n g f o r 4 days p r o v i d e d the mixed s o i l m i c r o - f l o r a s o u r c e f o r 1 ml i n o c u l a t i o n o f the a r t i f i c i a l medium. Thus 20 mg o f t e s t m a t e r i a l was added to 20 ml o f the medium i n 50 ml f l a s k s , which c o n t a i n e d 5 g o f a p a n c r e a t i c d i g e s t o f c a s e i n p l u s 1 g o f d e x t r o s e per l i t e r . A f t e r s t e r i l i z a t i o n a t 121° C f o r 15 m i n . , the samples were i n o c u l a t e d w i t h 1 ml o f the s o i l m i c r o - f l o r a s o u r c e and the headspace was f l u s h e d w i t h oxygen. C o n t r o l s c o n s i s t e d o f samples t e s t e d i n d e n t i c a l l y but c o n t a i n i n g no t e s t m a t e r i a l . 2

2



5.00 10.00 15.00 19.00 22.00

2.1 4.8 6.7 9.3 10.4

Amount of I (mol-%) In In Feed Copolymer

Table

0.80 1.6 1.2 1.5 1.7 100-105 95-99 90-95 89-96 84-88

Copolymer (mp, °C)

0.13 0.14 0.122 0.16 0.144

Intrinsic* Viscosity (dl/g)

(I)

83.90 81.90 80.61 79.25 78.46

13.84 13.35 13.04 12.71 12.51

83.78 81.85 80.55 78.98 78.18

14.00 13.62 13.38 13.03 12.80

Analysis Calculated Found

Copolymerization of Ethylene with 2-Methylene-l,3-dioxepane

Conversion (weight-%)

I.



BAILEY AND GAPUD


DAYS Figure

1.

ethylene

Cumulative headspace and

of

C0

2

from copolymer

2-methylene-l,3-dioxepane.


of

430

P O L Y M E R STABILIZATION A N D D E G R A D A T I O N

Table

II.

B i o d e g r a d a t i o n o f Copolymers o f E t h y l e n e and 2-Methylene-l,3-dioxepane


Amount o f I i n Copolymer (mol-%) 2.1 4.8 6.7 9.3 10.4

(I)

Cumulative C 0

2

a

after

7 days

20 days

103 107 116 116 121

98 103 108 113 118

b

b

b

E x p r e s s e d as p e r c e n t o f c o n t r o l . S i g n i f i c a n t l y d i f f e r e n t from the c o n t r o l . b

Thus i t has been demonstrated t h a t the i n t r o d u c t i o n o f e s t e r groups i n t o the backbone of p o l y e t h y l e n e w i l l r e n d e r the copolymer highly biodegradable. The copolymers a r e s u f f i c i e n t l y high m e l t i n g to make them u s e f u l f o r a wide v a r i e t y o f a p p l i c a t i o n s i n the form o f i n j e c t i o n molded p l a s t i c s , e x t r u d e d f i l m s , and melt spun f i b e r s . For example, t h e copolymer c o n t a i n i n g 9.3% e s t e r - c o n t a i n i n g u n i t s and 90.7% e t h y l e n e u n i t s has a m e l t i n g p o i n t o f 8 9 - 9 6 ° C and has many o f t h e c h a r a c t e r i s t i c s o f h i g h p r e s s u r e p o l y e t h y l e n e . At the same t i m e , t h i s copolymer b i o d e g r a d e s a t a r a t h e r f a s t r a t e .

Experimental Copolymerization of Ethylene with 2-Methylene-l,3-dioxepane (I)— T y p i c a l c o p o l y m e r i z a t i o n s were c a r r i e d o u t as f o l l o w s : To a 10.125 X 1 . 5 - i n c h s t e e l p r e s s u r e v e s s e l (300-ml c a p a c i t y ) was added 50 ml o f a s o l u t i o n o f 1.1 g ( 1 . 0 mol-%) o f d i - t e r t - b u t y l p e r o x i d e and 5.0 g (0.043 mol) o f 2 - m e t h y l e n e - l , 3 - d i o x e p a n e , bp 4 9 - 5 0 ° C (20 mm), i n p u r i f i e d c y c l o h e x a n e to g i v e 50 ml o f the r e a c t i o n m i x t u r e . The s e a l e d v e s s e l was f l u s h e d twice w i t h e t h y l e n e gas (99.9% pure) and f i n a l l y f i l l e d w i t h e t h y l e n e to an e q u i l i b r i u m p r e s s u r e o f 1000 psi. I f one n e g l e c t s the amount o f e t h y l e n e t h a t d i s s o l v e s i n the o r g a n i c l a y e r , the amount o f e t h y l e n e gas w i t h a volume of 250 ml a t 1000 p s i was c a l c u l a t e d to be 21 g (0.75 m o l e ) . The c o p o l y m e r i z a t i o n was a l l o w e d to proceed to low c o n v e r s i o n s ( l e s s than 2%) a t 120°C f o r 30 m i n u t e s . A f t e r the r e a c t i o n v e s s e l was q u i c k l y c o o l e d i n Dry I c e , i t was opened and methanol was added to i t to f a c i l i t a t e the removal o f the p r o d u c t as a white p r e c i p i t a t e . A f t e r the s o l i d was c o l l e c t e d by f i l t r a t i o n with s u c t i o n and washed w i t h m e t h a n o l , the polymer was p u r i f i e d f u r t h e r by d i s s o l u t i o n i n h o t c h l o r o f o r m and a d d i t i o n o f the r e s u l t i n g s o l u t i o n i n t o the n o n s o l v e n t m e t h a n o l . The polymer was c o l l e c t e d by f i l t r a t i o n and d r i e d i n vacuo a t 40°C f o r 24 hours to g i v e a white powder.


29.

BAILEY AND GAPUD


431

Acknowledgments The authors are grateful to the Frasch Foundation, the Polymer Progam of the National Science Foundation, and the Goodyear Tire and Rubber Company for partial support of this work. Literature Cited


1.

Potts, J . E . ; Clendinning, R. A.; Ackart, W. B.; Niegisch, W. D. In "Polymers and Ecological Problems"; Guillet, J., Ed.; Plenum Press: New York, 1973, p. 61. 2. Shelton, J . R.; Agostini, D. E . ; Lando, J . B. Am. Chem. Soc., Div. Polymer Chem., Preprints 1971, 12(2), 483. 3. Frazza, E. J.; Schmitt, E. E. J . Biomed. Mater. Res. Symp. 1970, 1, 43. 4. Bailey, W. J.; Okamoto, Y.;Kuo, W. C.; Narita T. In "Proc. Third International Biodegradation Symposium"; Sharpley, J . M.; Kaplan, A. M., Eds.; Applied Science Publishers: Essex, England, 1976, p. 765. 5. Ennis, D.; Kramer, A. Lebensm. -Wiss. u. Technol. 1974, 7(4), 214. 6. Ennis, D.; Kramer, A. J . Food Science 1975, 40, 181. 7. Bailey, W. J . Preprints, IUPAC Post-congress Symposium, Biomedical Materials, Kyoto, Japan, September 13, 1977, p. 10. 8. Albertson, A. - C . ; Ranby, B. In "Proc. Third International Biodegradation Symposium"; Sharpley, J . M.; Kaplan, A. M., Eds. Applied Science Publishers: Essex, England, 1976, p. 743. 9. Corbin, D. G., cited by Henman, T. J . Proc. Third International Conference on Advances in the Stabilization of Polymers, Lucerne, Switzerland, June 1981, p. 116. 10. Bailey, W. J.; Chen, P. Y.; Chiao, W. - B . ; Endo, T.; Sidney, L.; Yamamoto, N.; Yamazaki, N.; Yonezawa, K. In "Contemporary Topics in Polymer Science"; Shen, M., Ed.; Plenum Publishing Company: New York, 1979, Vol. 3, p. 29. 11. Bailey, W. J.; Ni, Z.; Wu, S. -R. J . Polym. S c i . , Polym. Chem. Ed. 1982, 20, 3021. 12. Bailey, W. J.; Gapud, B. Am. Chem. Soc., Div. Polym. Chem., Preprints 1984, 26(1), 58. RECEIVED February 21, 1985


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