2 Photoenolization
in
Polymers
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J. C. SCAIANO , J. P. BAYS, and M. V. ENCINAS Division of Chemistry, National Research Council, Ottawa, Canada K1A 0R6 and Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556
A carbonyl chromophore in a macromolecule can participate in a variety of photochemical processes that can have as end result the degradation of the polymer via processes like the Norrish Type I or Type II reaction, the triggering of a chain reaction leading to peroxidation, the transfer of energy to another chromophore or, it can also behave as an energy sink if a suitable, non-degradative path, is available to the triplet state. The triplet state of carbonyl chromophores frequently shows a high reactivity in hydrogen abstraction reactions (1). These processes can take place intermolecularly (photoreduction) (1) or intramolecularly, for example in the Norrish Type II process, reaction 1 (2,3).
Frequently Β will also undergo a back hydrogen transfer which regenerates the parent ketone, as well as cyclization (in most cases a minor reaction); as a result of this competition the quantum yields of fragmentation are typically in the 0.1-0.5 range in non-polar media. When the Norrish Type II process takes place in a polymer it can result in the cleavage of the polymer backbone. Poly (phenyl vinyl ketone) has frequently been used as a model polymer in which this reaction is resonsible for its photo degradation, reaction 2.
1Current Address: National Research Council of Canada 0097-615 6/ 81 / 0151 -0019$05.00/ 0 © 1981 American Chemical Society Pappas and Winslow; Photodegradation and Photostabilization of Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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PHOTODEGRADATION
CO
CO
hi/
Ph Ph
AND
PHOTOSTABILIZATION
OF
•CjOH CO
Ph
COATINGS
(2)
Ph
Other r e a c t i o n s which e x c i t e d carbonyl groups i n polymers are l i k e l y to be i n v o l v e d i n i n c l u d e the N o r r i s h Type I r e a c t i o n l e a d i n g to the formation o f f r e e r a d i c a l s , s e n s i t i z e d decomposit i o n o f peroxides or hydroperoxides and generation o f s i n g l e t oxygen, v i a d i r e c t quenching by oxygen, or i n d i r e c t l y i f the carbonyl t r i p l e t i s quenched by a molecule which i n t u r n generates a long l i v e d t r i p l e t . Another common hydrogen t r a n s f e r r e a c t i o n o f carbonyl t r i p l e t s i s the p h o t o e n o l i z a t i o n o f the 0-methylbenzoyl chromophore, i l l u s t r a t e d i n r e a c t i o n 3 f o r the syn conformer o f o-methylacetophenone (k). Reaction 3 can act as a very e f f i c i e n t energy sink, and a number o f p r o p e r t i e s o f t h i s group l e d us to b e l i e v e that t h i s process c o u l d be used to reduce photodegradation; i . e . the e x c e l l e n t absorption c h a r a c t e r i s t i c s o f the chromophore, the short t r i p l e t l i f e t i m e and the f a c t that the disappearance o f the carbonyl t r i p l e t does not take p l a c e at the expense o f the formation o f another e x c i t e d s t a t e .
(3)
This paper reports a study o f the photochemistry o f polymers and copolymers c o n t a i n i n g 0 - t o l y l v i n y l ketone u n i t s . Results and D i s c u s s i o n The r e s u l t s presented i n t h i s r e p o r t correspond to systems where r e a c t i o n s 2 and 3 account f o r the decay o f carbonyl t r i p l e t s . A s e r i e s o f copolymers o f phenyl v i n y l ketone and 0 - t o l y l v i n y l
Pappas and Winslow; Photodegradation and Photostabilization of Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
2.
SCAIANO
ET
AL.
Photoenolization
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ketone were examined. Deaerated s o l u t i o n s were i r r a d i a t e d with 366nm l i g h t and the progress o f the photodegradation was monitored f o l l o w i n g the change o f v i s c o s i t y with time. P o l y ( p h e n y l v i n y l ketone), PPVK, photodegrades with a quantum y i e l d o f 0.2U, w h i l e p o l y ( t f - t o l y l v i n y l ketone), PTVK, i s photostable ($£0.00l) even i f both pathways ( i . e . N o r r i s h Type I I and e n o l i z a t i o n ) are i n p r i n c i p l e a v a i l a b l e t o every 0-methylbenzoyl chromophore, r e a c t i o n k.
Perhaps the most i n t e r e s t i n g feature i s e x h i b i t e d by the copolymers, r a t h e r than the homopolymers. We f i n d that the reduction i n the quantum y i e l d o f photodegradation always exceeds the abundance o f 0-methylbenzoyl chromophores (5.). Table I illustrates this effect. TABLE I E f f e c t o f
