Photooxidation of Poly(phenylene oxide) Polymer - American

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22 Photooxidation of Poly(phenylene oxide) Polymer J A M E S E. P I C K E T T General Electric Corporate Research and Development Center, Schenectady, NY 12301

Poly(2,6-dimethyl-l,4-phenylene oxide) has been found to undergo photooxidation not at the benzylic methyl groups but rather across the aromatic ring. The mechanism is an electron-transfer reaction leading to a polymer radical cation (Ar+.) and superoxide (0-.) which combine to give oxidatively degraded polymer. Sensitization, quenching, radical trapping and flash photolysis experiments support this mechanism. 2

The photodegradation of poly(2,6-dimethyl-l,4-phenylene oxide), 1, has received considerable attention both in industrial and in academic laboratories. Workers have observed that when poly(phenylene oxide) films are exposed to light of wavelengths greater than 300 nm in the presence of oxygen, considerable discoloration and crosslinking occur accompanied by the appearance of carbonyl and hydroxyl bands in the infrared spectrum (2-5). Most workers in the field have ascribed these results to a hydroperoxide-mediated free radical oxidation of the benzylic methyl groups (Scheme I). In this mechanism, an initiator species abstracts a hydrogen atom to give a benzylic radical, 2, which reacts with oxygen to yield a hydroperoxy radical, 3. This radical would abstract a hydrogen atom from another repeating unit to give the hydroperoxide, 4. The hydroperoxide then could undergo photolysis to give more initiating species or decompose to aldehydes and esters. These schemes have been described in detail elsewhere (6) . We have recently found that this free radical oxidation of the methyl groups is in fact not a major pathway in the photooxidation of poly(phenylene oxide). Instead, the oxidation apparently occurs through an electron-transfer mechanism on the backbone of the polymer not chemically involving the methyl groups at a l l . In this paper, we present evidence inconsistent with the free radical mechanism and supporting this novel pathway for polymer photooxidation. 0097-6156/85/0280-0313$06.00/0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, Peter P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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POLYMER STABILIZATION AND DEGRADATION


R e s u l t s and D i s c u s s i o n E v i d e n c e A g a i n s t The F r e e R a d i c a l Mechanism. An e s s e n t i a l f e a t u r e o f t h e f r e e r a d i c a l mechanism i s t h a t t h e m e t h y l groups a r e t h e reactive sites. As t h e p h o t o o x i d a t i o n p r o c e e d s , t h e m e t h y l groups s h o u l d d i s a p p e a r . T a b l e I shows t h e l o s s o f m e t h y l groups as d e t e r mined by p r o t o n NMR o f p o l y ( p h e n y l e n e o x i d e ) exposed t o P y r e x f i l t e r e d mercury lamps i n 2% benzene s o l u t i o n o r as a 1 m i l c a s t film. Oxygen consumption was measured by a gas b u r e t a t 1 atm. When 0.5 eq. o f O2 p e r r e p e a t i n g u n i t has been consumed, a 25% l o s s o f t h e m e t h y l groups i s e x p e c t e d i f a l l o f t h e o x i d a t i o n o c c u r s there. I n f a c t , t h e m e t h y l group l o s s was found t o be o n l y one t e n t h t h a t amount. Most o f t h e o x i d a t i o n t h e r e f o r e must o c c u r on the aromatic r i n g . S i m i l a r r e s u l t s were o b t a i n e d when t h e s u r f a c e o f a p h o t o o x i d i z e d f i l m was examined by a t t e n u a t e d t o t a l r e f l e c t a n c e i n f r a r e d s p e c t r o s c o p y ( F i g u r e 1 ) . The band a t 960 cm" i s due s o l e l y t o t h e a r o m a t i c r i n g s whereas t h e band a t 1380 cm"" i s due to t h e m e t h y l groups. Comparison o f t h e a r e a s under t h e s e peaks shows t h a t w h i l e t h e m e t h y l band i s broadened and d e c r e a s e s s l i g h t l y a f t e r i r r a d i a t i o n , t h e a r o m a t i c band i s reduced by 60%. These r e s u l t s a r e c o n s i s t e n t w i t h t h e work o f D i l k s and C l a r k (7, 8) who examined p o l y ( p h e n y l e n e o x i d e ) f i l m s by ESCA. C o n t r a r y t o t h e p r e d i c t i o n s o f t h e f r e e r a d i c a l mechanism, t h e a r o m a t i c r i n g s o f p o l y (phenylene o x i d e ) undergo p h o t o o x i d a t i o n much f a s t e r t h a n the m e t h y l groups. 1

1

Table I.

L o s s o f M e t h y l Groups Upon P h o t o o x i d a t i o n

Extent of O x i d a t i o n ( E q u i v . 02/mer)

0

% Lost (by nmr)

0

% Loss P r e d i c t e d f o r Methyl Oxidation

-

0.12

(solution)

0

0.52

(solution)

2.3

26

0.48

(film)

3.4

24

6

An i m p o r t a n t measure o f polymer d e g r a d a t i o n i s c h a i n s c i s s i o n . However, s t u d y o f t h e p h o t o o x i d a t i v e c h a i n s c i s s i o n i n s o l i d polymer i s c o m p l i c a t e d by accompanying c r o s s l i n k i n g . T h i s problem can be a v o i d e d by c a r r y i n g out t h e p h o t o o x i d a t i o n i n d i l u t e s o l u t i o n where crosslinking i s negligible. The e f f e c t o f p h o t o o x i d a t i o n (2% s o l u t i o n In benzene, P y r e x - f i l t e r e d Hg lamp) on t h e number a v e r a g e and weight average m o l e c u l a r w e i g h t s as d e t e r m i n e d by g e l p e r m e a t i o n chromatography i s shown i n F i g u r e 2. B o t h t h e weight a v e r a g e and number a v e r a g e m o l e c u l a r w e i g h t s d e c r e a s e r a p i d l y upon photooxidation. A p l o t o f t h e i n t r i n s i c v i s c o s i t y ( F i g u r e 3) a l s o shows t h e r a p i d d e c r e a s e i n m o l e c u l a r weight as t h e o x i d a t i o n p r o c e e d s . The d e c r e a s i n g m o l e c u l a r weight cannot be e x p l a i n e d by t h e o x i d a t i o n o f m e t h y l groups a l o n e s i n c e t h i s does not r e s u l t i n c h a i n s c i s s i o n . Some o t h e r mechanism must a c c o u n t f o r t h e c h a i n s c i s s i o n .

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

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1

315

Photooxidation of Poly(phenylene oxide) Polymer

2

3

4

aldehydes acids esters initiation Scheme I. H y d r o p e r o x i d e - m e d i a t e d f r e e r a d i c a l b e n z y l i c methyl groups.

o x i d a t i o n of the

Areas

I

I

I

I

I

2000

1800

1600

1400

I 1200

I 1000

1 800

1 600

Wavenumbers

Figure 1. Attenuated t o t a l reflectance IR spectra of poly(phenylene oxide) f i l m before and after P y r e x - f i l t e r e d Hg lamp irradiation.




316

F i g u r e 3 . L o s s o f i n t r i n s i c v i s c o s i t y upon p h o t o o x i d a t i o n i n benzene s o l u t i o n .


22.

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Photooxidation of Polyfphenylene oxide) Polymer

317


In the absence of oxygen, the molecular weights are not reduced and i n fact increase by approximately 10% upon i r r a d i a t i o n with a Pyrex-filtered Hg lamp as shown i n Table I I . This phenomenon occurs even i n the presence of a hydrogen donor such as isopropyl alcohol thus r u l i n g out the p o s s i b i l i t y of d i r e c t photolytic cleavage as outlined i n Scheme I I . This type of cleavage apparently occurs upon 254 nm i r r a d i a t i o n (9) but i s not important i n real-world polymer degradation where there i s no l i g h t of wavelengths shorter than ca. 290 nm. Since d i r e c t photolytic cleavage does not occur, and methyl group oxidation cannot account for chain s c i s s i o n , another mechanism such as oxidative cleavage of the aromatic rings appears to be the major pathway for poly(phenylene oxide) photodegradation (8) . Table I I .

Changes i n Molecular Weight Upon Photolysis I.V.

Mn

No I r r a d i a t i o n

0.523

25,000

53,000

5 Days*, N ,

0.618

27,000

63,000

0.33

13,000

30,000

2

5 Days*, 0

2

iPrOH

* Pyrex f i l t e r e d Mercury lamp. F i n a l l y , free r a d i c a l oxidation of the methyl groups would be expected to be i n i t i a t e d by peroxides. The polymer has been found to contain approximately 1 micromole of active oxygen species per gram (4), and t h i s has been invoked to explain the i n i t i a t i o n of photooxidation i n poly(phenylene oxide). I f t h i s were so, one would expect that higher concentrations of peroxide would cause more rapid photooxidation. To test t h i s , a benzene solution of 2.4 g of polymer (20 ramol of repeating units) and 200 micromoles of cumene hydroperoxide was exposed to a P y r e x - f i l t e r e d mercury lamp. The rate of oxygen uptake, a f t e r correcting for that due to the cumene impurity, was only 1.1 times the rate of a peroxide-free polymer solution. S i m i l a r l y , a cast f i l m containing 100 micromoles of cumene hydroperoxide per gram of polymer showed nearly the same rate of carbonyl formation and yellowing as a control f i l m when exposed to a Pyrexf i l t e r e d mercury lamp (Figure 4). Thus, i n contrast to the pred i c t i o n of the methyl group oxidation theory, added peroxide has no s i g n i f i c a n t effect on the photooxidation of poly(phenylene oxide). Model Compounds. Compounds 5 and 6 were prepared according to standard Ullmann procedures. When these compounds were subjected to photooxidation conditions (10 mmol i n 100 mL of benzene, Pyrexf i l t e r e d mercury lamp) both consumed oxygen at a rate only s l i g h t l y less than the equivalent amount of polymer (Figure 5). That the rate was slower f o r the model compounds suggests that some sensit i z i n g impurities are present i n the polymer. However, the simple backbone oxidation alone i s s u f f i c i e n t to account f o r approximately 2/3 of the oxidation rate, and the presence or absence of methyl groups has l i t t l e effect on the rate. Interestingly, the rate of disappearance of starting material was only about 1/6 the rate of




Figure 4 .

Carbonyl formation i n peroxide-doped

film.



22.


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319

oxygen u p t a k e . T h i s i n d i c a t e s t h a t the p r i m a r y o x i d a t i o n p r o d u c t s a r e much more s u s c e p t i b l e to p h o t o o x i d a t i o n than t h e s t a r t i n g m a t e rial. Once a r i n g s u f f e r s o x i d a t i o n , i t becomes t h e s i t e o f s e v e r a l more o x i d a t i o n s u n t i l o n l y s m a l l fragments r e m a i n . L i q u i d chromat o g r a p h y showed more than f o r t y r e a c t i o n p r o d u c t s a l l i n minor amounts. None as y e t has been i s o l a t e d and i d e n t i f i e d . Attempts to i s o l a t e the p r i m a r y p r o d u c t s t h r o u g h l o w - t e m p e r a t u r e , c o n t r o l l e d o x i d a t i o n c o n d i t i o n s have a l s o been u n s u c c e s s f u l . The p h o t o o x i d a t i o n o f t h e s e compounds can be c o n t r a s t e d to t h e t h e r m a l o x i d a t i o n o f 5 a t 175°C ( T a b l e I I I ) . T h r e e major p r o d u c t s were i s o l a t e d , a l l a r i s i n g from m e t h y l group o x i d a t i o n ( 1 0 ) . One m o l e c u l e o f s t a r t i n g m a t e r i a l d i s a p p e a r s per mole o f oxygen consumed and compound 6, h a v i n g no m e t h y l g r o u p s , i s s t a b l e a t 1 7 5 ° C . It is t h e r e f o r e c l e a r t h a t t h e mechanisms o f t h e r m a l and p h o t o o x i d a t i o n s are d i f f e r e n t .

Table I I I .

O x i d a t i o n of

Thermal O x i d a t i o n 175°C Moles 0 / Moles Consumed 2

Products S i t e of

^

1

3 major Oxidation

Absence o f M e t h y l s (Compound 6)

M e t h y l Groups No r e a c t i o n

Photooxidat ion (X > 300 nm) ^

6

> 40 Aromatic

Ring

L i t t l e effect on r a t e

E l e c t r o n - T r a n s f e r Mechanism. A mechanism c o n s i s t e n t w i t h our r e s u l t s i s shown i n Scheme I I I . I n t h i s mechanism an e x c i t e d p o l y mer r e p e a t i n g u n i t undergoes e l e c t r o n t r a n s f e r w i t h a n o t h e r u n i t to g e n e r a t e a r a d i c a l c a t i o n and r a d i c a l a n i o n p a i r (7 and 8 ) . Oxygen r e a c t s r a p i d l y w i t h t h e r a d i c a l a n i o n to g e n e r a t e s u p e r o x i d e , 0 . The s u p e r o x i d e and r a d i c a l c a t i o n then combine to form an u n s t a b l e , presumably e n d o p e r o x i d i c p r o d u c t t h a t undergoes f u r t h e r r e a c t i o n s to g i v e t h e degraded p r o d u c t s . T h i s mechanism can be c o n s i d e r e d t o be a s e l f - s e n s i t i z e d v e r s i o n o f the c y a n o a n t h r a c e n e s e n s i t i z e d p h o t o o x i d a t i o n s r e c e n t l y d e s c r i b e d by F o o t e , et. al. (11, 12, 13) and o t h e r s (14, 15, JL6) (Scheme I V ) . The f i r s t e v i d e n c e f o r t h e e l e c t r o n - t r a n s f e r mechanism i s t h a t known e l e c t r o n - t r a n s f e r s e n s i t i z e r s promote the p h o t o o x i d a t i o n o f poly(phenylene o x i d e ) . Thus, 9-cyanoanthracene, 9,10-dicyanoa n t h r a c e n e (12) and methylene b l u e (13) cause a g r e a t i n c r e a s e i n t h e r a t e o f polymer p h o t o o x i d a t i o n b o t h i n s o l u t i o n and i n c a s t films. P o l y s t y r e n e - b o u n d Rose B e n g a l (.17) , w h i c h i s an e f f i c i e n t g e n e r a t o r o f s i n g l e t oxygen but a poor e l e c t r o n - t r a n s f e r s e n s i t i z e r , does n o t s e n s i t i z e t h e r e a c t i o n . S i n g l e t oxygen t h e r e f o r e may be r u l e d out ( 2 0 ) . The s e n s i t i z e r s a c c e l e r a t e t h e r a t e o f oxygen 2

T



•10h

krel 1

PPO

.08


.06

.04

.02

Hours

F i g u r e 5. Oxygen u p t a k e o f p o l y ( p h e n y l e n e o x i d e ) and model compounds. P y r e x - f i l t e r e d Hg lamp, 1-M benzene s o l u t i o n .

hv

Ar-

-> A r *

+ Ar*

7

8

°2

ArO

final

+

1

n

hv,

Scheme I I I .

7

0

2

products

Electron-transfer

mechanism.


22.


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321

u p t a k e i n t h e model compounds as w e l l . I t i s clear therefore that Ar- and O2 , i f formed, w i l l r e a c t t o g i v e degraded polymer. D i r e c t e v i d e n c e f o r t h e f o r m a t i o n o f 0 ~ was o b t a i n e d t h r o u g h a n i t r o n e - t r a p p i n g experiment. A r a d i c a l t r a p p i n g agent, 5,5d i m e t h y l - l - p y r r o l i d i n e - N - o x i d e , 9, r e a c t s r a p i d l y w i t h s u p e r o x i d e i o n i n the presence of a proton source to g i v e the r a d i c a l hydroperoxide 10 (Scheme V) (18, 1 9 ) . T h i s compound i s n o t t h e r m a l l y o r p h o t o c h e m i c a l l y s t a b l e and i s c o n v e r t e d t o t h e N-oxy p y r r o l i d o n e 11. The l a t t e r compound has a c a r b o n y l i n t h e i n f r a r e d spectrum a t 1780 cm"". The e l e c t r o n paramagnetic r e s o n a n c e ( e p r ) s p e c t r a o f t h e s e compounds a r e shown i n F i g u r e 6. The r e s o l u t i o n l i m i t s o f t h e i n s t r u m e n t used do n o t p e r m i t t h e o b s e r v a t i o n o f t h e e x p e c t e d f i n e s t r u c t u r e , but t h e f o u r - l i n e spectrum g o i n g t o t h e t h r e e - l i n e spectrum w i t h t h e a t t e n d a n t f o r m a t i o n o f a c a r b o n y l i n t h e i n f r a r e d spectrum s h o u l d be d i a g o n i s t i c f o r s u p e r o x i d e . To guard a g a i n s t the p o s s i b i l i t y o f t r a p p i n g polymer-bound r a d i c a l s , a two phase system was employed. A c a s t f i l m o f t h e polymer was p l a c e d i n a P y r e x NMR tube and c o v e r e d w i t h a methanol s o l u t i o n o f t h e r a d i c a l t r a p 9. T h i s was i r r a d i a t e d a t 0°C f o r s e v e r a l minutes w i t h a mercury lamp. The methanol s o l u t i o n was t h e n d e c a n t e d and t h e epr spectrum t a k e n . The r e s u l t s a r e shown i n F i g u r e 7. The i n i t i a l spectrum showed t h e f o u r - l i n e s i g n a l w i t h t h e t h r e e - l i n e spectrum b e g i n n i n g t o appear. A f t e r standing overn i g h t i n t h e d a r k a t room t e m p e r a t u r e , o n l y t h e t h r e e - l i n e spectrum o f 11 was seen. An i n f r a r e d spectrum showed a weak c a r b o n y l peak a t c a . 1780 cm"" . An epr spectrum o f t h e washed polymer f i l m showed no r a d i c a l s i g n a l s a t a l l . The n i t r o n e a l o n e under t h e s e c o n d i t i o n s produces no d e t e c t a b l e r a d i c a l s . Thus, t h e n i t r o n e i s t r a p p i n g a p h o t o c h e m i c a l l y - g e n e r a t e d , m o b i l e r a d i c a l s p e c i e s t h a t has epr and i r s p e c t r a l c h a r a c t e r i s t i c s consistent with superoxide. 7

2


1

1

C h e m i c a l e v i d e n c e f o r t h e r a d i c a l c a t i o n , 7, can be o b t a i n e d from quenching e x p e r i m e n t s . A known e l e c t r o n t r a n s f e r quencher, 1,2,4-trimethoxybenzene ( 1 2 ) , was found t o r e d u c e t h e r a t e o f oxygen u p t a k e by 50% ( F i g u r e 8 ) . Other compounds, d i a z a b i c y c l o - 2 . 2 . 2 o c t a n e (DABCO) and b i s ( 1 , 1 , 6 , 6 - t e t r a m e t h y l - 4 - p i p e r i d i n y l ) s e b a c a t e , 12, a l s o quenched 50% o f t h e r e a c t i o n . These compounds presumably a c t by d o n a t i n g an e l e c t r o n t o t h e r a d i c a l c a t i o n , 7, and t h e n r e a c t i n g r e v e r s i b l y w i t h the superoxide to r e t u r n a l l the species t o t h e i r ground s t a t e s and d i s p e r s e t h e energy t h e r m a l l y (Scheme VI). Compounds w i t h h i g h e r o x i d a t i o n p o t e n t i a l s (and t h e r e b y l e s s l i k e l y t o t r a n s f e r an e l e c t r o n ) , such as 1,3,5-trimethoxybenzene, 1,4-dimethoxybenzene, and t r a n s - s t i l b e n e , do n o t quench t h e r e a c t i o n . Some quenching was o b s e r v e d i n c a s t f i l m s b u t i t was s i g n i f i c a n t o n l y a t v e r y h i g h c o n c e n t r a t i o n s of quencher. Thus, compounds e x p e c t e d t o d i v e r t t h e p a t h o f o x i d a t i o n by d o n a t i n g an e l e c t r o n t o the r a d i c a l c a t i o n reduce the r a t e of o x i d a t i o n . The q u e s t i o n o f why o n l y 50% o f t h e o x i d a t i o n can be s t o p p e d , even a t v e r y h i g h c o n c e n t r a t i o n s o f quencher, remains open and w i l l be d i s c u s s e d below. S p e c t r o s c o p i c e v i d e n c e f o r t h e r a d i c a l c a t i o n and r a d i c a l a n i o n was o b t a i n e d from f l a s h p h o t o l y s i s e x p e r i m e n t s ( 2 0 ) . T r a n s i e n t s p e c t r a o f a 2% polymer s o l u t i o n i n benzene 10 m i c r o s e c o n d s a f t e r a P y r e x - f i l t e r e d Xenon f l a s h a r e shown i n F i g u r e 9. T h e r e a r e two major t r a n s i e n t s of i n t e r e s t , one c e n t e r e d a t 450 nm and a n o t h e r a t 510-560 nm. Approximate l i f e t i m e s o f t h e s e t r a n s i e n t s , o b t a i n e d by



CA

—

C

A

*

— ^

St +

CA

T

o

0• 2


s-o CA =

cyanoanthracene

S

e l e c t r o n - r i c h molecule

=

Scheme IV.

10

+

CA

2

Cyanoanthracene-sensitized

9

2

photooxidations.

11

Scheme V. R e a c t i o n of 5 , 5 - d i m e t h y l - l - p y r r o l i d i n e - N - o x i d e w i t h superoxide i o n i n the presence of a p r o t o n source t o g i v e the r a d i c a l h y d r o p e r o x i d e 10.

F i g u r e 6.

EPR s p e c t r a o f a u t h e n t i c t r a p p e d

0 ~. 2



•

O

1,2,4-Trimethoxybenzene

DABCO _

0

A ^ ^

12

Molar Equivalents of Quencher

Figure 8, uptake.

Effect of electron-transfer

quenchers on oxygen


324


Art

+

Q

>

A

r

+

gt

J

2

.

[Q-0 ] 2

->

Q

+

0

9


Scheme V I . Mechanism o f quenchers.

Wavelength

F i g u r e 9. P o l y ( p h e n y l e n e o x i d e ) t r a n s i e n t Xenon f l a s h i n benzene s o l u t i o n .

s p e c t r a 10 y s e c


after

22.

Photooxidation of Polyfphenylene oxide) Polymer

PICKETT

325

k i n e t i c f l a s h s p e c t r o s c o p y , a r e shown i n T a b l e IV ( 2 1 ) . The 450 nm t r a n s i e n t has been a s s i g n e d t o the r a d i c a l c a t i o n 7. It is relat i v e l y i n s e n s i t i v e t o oxygen and i t s l i f e t i m e i s r e d u c e d by t h e a d d i t i o n o f e l e c t r o n - t r a n s f e r quenchers. The 510-560 nm t r a n s i e n t i s b e l i e v e d t o be t h e r a d i c a l a n i o n . I t s l i f e t i m e i s l e s s t h a n 50 y s e c i n t h e p r e s e n c e of a i r , presumably due t o r a p i d r e a c t i o n w i t h oxygen t o g i v e 0 . I n degassed s o l v e n t , t h e l i f e t i m e o f t h i s t r a n s i e n t i s i n c r e a s e d s l i g h t l y i n the presence of the r a d i c a l c a t i o n quenchers. These r e s u l t s a r e c o n s i s t e n t w i t h t h e b e h a v i o r of r a d i c a l c a t i o n s and r a d i c a l a n i o n s p h o t o l y t i c a l l y produced i n o t h e r e l e c t r o n - t r a n s f e r p h o t o o x i d a t i o n s ( 1 2 ) . Thus, e v i d e n c e has been o b t a i n e d f o r a l l t h r e e o f t h e proposed i n t e r m e d i a t e s o f Scheme I I I . T


2

T a b l e IV. T r a n s i e n t L i f e t i m e s ( M i c r o s e c o n d s ) A f t e r Xenon F l a s h o f 1.6-M P o l y ( P h e n y l e n e Oxide) S o l u t i o n i n Benzene Additive

450

nm

510

nm

None Degassed

220

100

Air

180

Photooxidation of Poly(phenylene oxide) Polymer - American

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