Photocrosslinkable Polymers

nitrene f s reactivity toward insertion into carbon-hydrogen bonds and generation of radicals. ... 22. UV LIGHT INDUCED REACTIONS IN POLYMERS polymer ...
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3 Photocrosslinkable Polymers G E O R G E B. B U T L E R and W I L L I A M I. F E R R E E , JR.

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Center for Macromolecular Science and Department of Chemistry, University of Florida, Gainesville, Fla. 32611

Summary This paper describes some polymeric systems which have recently been shown to undergo photocrosslinking. Photocrosslink­ ing of poly-(m-vinyloxyethoxy)-styrene can be carried out with several different acceptors, although several others are ineffec­ t i v e for reasons unknown. The reaction is not quenched apprecia­ bly by H O , oxygen, or γ-collidine, but is quenched by t r i e t h y l ­ amine, and cannot be i n i t i a t e d by azoisobutyronitrile, thus sup­ porting the supposition that the mechanism i s cationic but not protonic. Quenching by hydroquinone may also be consistent with t h i s conclusion. The reaction is favored by low temperature and by tetrabutylammonium perchlorate, also supporting a cationic mechanism. Small concentrations of H O , added before and after i r r a d i a ­ t i o n , induce a postcrosslinking process that proceeds to comple­ tion within a short time and appears to be proton-initiated. The photocrosslinking is completely quenched by 0.005 M thio­ cyanate ion, and subsequent addition of 0.5 M H O causes the formation of postcrosslinked polymer containing an unexplained IR absorption band in the carbonyl region. Effects of varying light intensity, wavelength, acceptor concentration, and of rotating the sample tube suggest that the tetrachlorophthalic anhydride-catalyzed reaction is biphotonic. The chloranil-catalyzed reaction shows no intensity dependence but unexplainably w i l l not proceed with 334 nm l i g h t , whereas 366 nm light causes very rapid reaction. 2

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Introduction The increasing u t i l i t y of photocrosslinkable polymers in microelectronics, p r i n t i n g , and UV-curable lacquers and inks is providing an incentive for development of new v a r i e t i e s of photopolymers. After a brief overview of some of the commonly used materials, some of the interesting recent developments in t h i s 19

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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U V L I G H T INDUCED REACTIONS IN P O L Y M E R S

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f i e l d w i l l be d i s c u s s e d . Some r e c e n t e x p e r i m e n t a l r e s u l t s o b t a i n e d i n o u r l a b o r a t o r y on photoionîc c r o s s ! i n k i n g o f c e r t a i n p o l y m e r s w i l l a l s o be p r e s e n t e d . P h o t o p o l y m e r s i n Common U s e . I n c r e a s i n g i n d u s t r i a l u s e i s b e i n g made o f U V - c u r a b l e u n s a t u r a t e d p o l y e s t e r f o r m u l a t i o n s . The p o l y e s t e r component i s t y p i c a l l y p r e p a r e d f r o m m a l e i c a n h y d r i d e , an a r o m a t i c a n h y d r i d e , and a d i o l . T h i s i s combined w i t h s t y r e n e , a s e n s i t i z e r , and m i s c e l l a n e o u s a d d i t i v e s . The u n s a t u r a t e d s i t e s associated with t h e maleate e s t e r linkages i n t h e p o l y e s t e r chain undergo a r a d i c a l c o p o l y m e r i z a t i o n w i t h s t y r e n e t o e f f e c t c r o s s I i n k i n g . The s e n s i t i z e r s e r v e s b o t h t o a b s o r b l i g h t and t o generate r a d i c a l s . P a r a f i n , f a t t y e s t e r s , o r s i m i l a r agents a r e o f t e n i n c l u d e d t o r e d u c e o x y g e n i n h i b i t i o n and t h u s improve surface hardening. The p r i n c i p l e u s e f o r t h e s e f o r m u l a t i o n s i s as U V - c u r a b l e l a c q u e r s i n t h e f u r n i t u r e i n d u s t r y . In a t y p i c a l c o m p o s i t i o n , an u n s a t u r a t e d p o l y e s t e r i s p r e p a r e d f r o m a 1:2:3 m i x t u r e o f p h t h a l l i c a n h y d r i d e , m a l e i c a n h y d r i d e , and 1,2-propaned i o l , and a 66% m i x t u r e o f t h e p o l y e s t e r i n s t y r e n e c o n t a i n i n g 0.)% p a r a f f i n , \% [ ( C H 3 ) C 0 C S 2 ] 2 ^ and 0.5% s e n s i t i z e r c o n s t i t u t e s t h e U V - c u r a b l e p o l y e s t e r . T y p i c a l s e n s i t i z e r s a r e b e n z o i n methyl e t h e r , b e n z i l , x a n t h o n e , and a c e t o n a p h t h o n e A s i m i l a r mix­ t u r e has been used t o i m p r e g n a t e f i b e r g l a s s f a b r i c f o r U V - h a r d e n ed o r t h o p e d i c c a s t s (2). The e f f i c i e n t l i g h t - i n i t i a t e d d e c o m p o s i t i o n o f a z i d e s has been t h e b a s i s f o r c o m m e r c i a l l y i m p o r t a n t p h o t o r e s i s t f o r m u l a ­ t i o n s f o r t h e s e m i c o n d u c t o r i n d u s t r y . A common a p p r o a c h i s t o mix a d i a z i d e , s u c h a s d i a z a d i b e n z y l i d e n e c y c I o h e x a n o n e ( I ) , with an u n s a t u r a t e d h y d r o c a r b o n p o l y m e r . E x c i t a t i o n o f t h e d i f u n c t i o n ­ al s e n s i t i z e r produces h i g h l y r e a c t i v e n i t r e n e s which c r o s s l i n k t h e p o l y m e r by a v a r i e t y o f p a t h s i n c l u d i n g i n s e r t i o n i n t o b o t h c a r b o n - c a r b o n d o u b l e bonds and c a r b o n - h y d r o g e n b o n d s , and by generation of radicals. The p o l y m e r component i n t h e most w i d e l y used r e s i s t s i s p o l y i s o p r e n e w h i c h has been p a r t i a l l y c y c l i z e d by r e a c t i o n w i t h p - t o l u e n e s u I f o n i c a c i d (5). O t h e r p o l y m e r s used i n c l u d e p o l y c y c l o p e n t a d i e n e and t h e c o p o l y m e r o f c y c l o p e n t a d i e n e and α-methylstyrene (A). A s i m p l e , p h o t o s e n s i t i v e p o l y m e r t h a t has f o u n d w i d e u s e a s a p h o t o r e s i s t and i n p r i n t i n g p l a t e s i s p o l y ( v i n y l a l c o h o l ) ( I I ) (5). B e c a u s e r a d i c a l p o l y m e r i z a t i o n o f t h e monomer p r e f e r s t h e c y c l o p o l y m e r i z a t i o n p a t h , II i s p r e p a r e d by r e a c t i n g p o l y ( v i n y l a l c o h o l ) w i t h cinnamoyl c h l o r i d e . The mechanism o f p h o t o c r o s s I i n k i n g o f II has been a m a t t e r o f c o n t r o v e r s e y , w h e t h e r p r o c e e d ­ i n g by a r a d i c a l p r o c e s s , by photodîmerization o f c i n n a m a t e g r o u p s t o c y c l o b u t a n e s , o r b o t h i n c o m p e t i t i o n (6). The i n i t i ­ a t i n g species i s d e f i n i t e l y t h e t r i p l e t , consequently t h e c r o s s l i n k i n g e f f i c i e n c y and a b s o r p t i o n b a n d w i d t h a r e g r e a t l y i n c r e a s e d i n t h e p r e s e n c e o f t r i p l e t s e n s i t i z e r s (6). 2

D e v e l o p m e n t s E m p l o y i n g P o t e n t i a l l y Photodimerîzable G r o u p s .

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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Photocrosslinkable Polymers

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Most r e s e a r c h i n t o new p h o t o p o l y m e r s has d e v e l o p e d a l o n g t h e l i n e s of a t t a c h i n g r e a c t i v e groups t o a polymer backbone. Several polymers c o n t a i n i n g pendant cinnamate groups a r e found t o improve on t h e p h o t o s e n s i t i v i t y o f I I . R e a c t i o n o f poly(β-hydroxyethyI aery I ate) w i t h cinnamoyl c h l o r i d e y i e l d s a polymer ( I I I ) t h a t when s e n s i t i z e d by 5 - n i t r o a c e n a p h t h e n e i s more t h a n t w i c e a s r e a c t i v e a s II (7). P o l y ( g l y c i d y I c i n n a m a t e ) ( I V ) , p r e p a r e d by r e a c t i n g p o l y ( e p i c h l o r o h y d r i n ) with potassium cinnamate, i s a l i q u i d a t room t e m p e r a t u r e and p h o t o c r o s s I i n k s w i t h a n e f f i c i e n c y e i g h t t i m e s t h a t o f II ( 8 ) . A s e r i e s o f s i m i l a r p o l y m e r s (V) has been p r e p a r e d d i r e c t l y f r o m monomers by c a t i o n i c p o l y m e r i z a t i o n o f t h e v i n y l e t h e r g r o u p o f t h e monomer, t h e c i n n a m o y l g r o u p b e i n g u n r e a c t i v e u n d e r t h e s e c o n d i t i o n s (9_). P o l y C v i n y l c h a l c o n e ) ( V I ) r e s e n b l e s II i n s t r u c t u r e , and t h e mechanism o f i t s c r o s s l i n k i n g has s i m i l a r l y been a r g u e d i n f a v o r of r a d i c a l o r d i m e r i z a t i o n pathways ( 1 0 , 1 1 ) . One o f t h e more i n t e r e s t i n g and s u c c e s s f u l o f r e c e n t p h o t o polymers i s p o l y ( v i n y l 2 - f u r y l a c r y l a t e ) ( V I I I ) . Its photosensi­ t i v i t y i s r e p o r t e d t o be more t h a n a n o r d e r o f m a g n i t u d e g r e a t e r than t h a t o f I I , whether s e n s i t i z e d o r u n s e n s i t i z e d samples a r e compared. The mechanism o f c r o s s l i n k i n g i s p r o p o s e d t o i n v o l v e photodîmerization o f p e n d a n t g r o u p s t o c y c l o b u t a n e s ( 1 2 ) . Recently, r e s u l t s o f i n c o r p o r a t i n g i n polymers e f f i c i e n t l y photodîmerîzable g r o u p s f a m i l i a r t o modern p h o t o c h e m i s t s have been r e p o r t e d . Reaction o f poly(vinyl alcohol) with 1,2-diphenylc y c l o p r o l e n o y I c h l o r i d e y i e l d s a very p h o t o s e n s i t i v e polymer ( V I I I ) t h a t c r o s s l i n k s by a m i x t u r e o f c y c l o b u t a n e r i n g f o r m a t i o n and s i n g l e bond f o r m a t i o n between two d i p h e n y l eye I o p r o p e n e g r o u p s a f t e r h y d r o g e n - a t o m t r a n s f e r ( 1 3 ) , mechanisms w e l l c h a r a c t e r i z e d i n t h e p h o t o c h e m i s t r y o f t h e s e m o i e t i e s (_T4). Hyde, K r i c k a , a n d L e d w i t h r e p o r t t h a t N-acryloyldîbenz(b,f)azepîne ( I X ) c a n be c o p o l y m e r i z e d w i t h s t y r e n e , methyl methacryI a t e , N - v i n y I c a r b a z o l e , o r m a l e i c a n h y d r i d e under r a d i c a l c o n d i t i o n s t o p r o v i d e polymers t h a t p h o t o c r o s s I i n k by c y c l o b u t a n e r i n g f o r m a t i o n ( V 5 ) . Two more p o l y m e r s d e s i g n e d t o c r o s s l i n k by t h e p h o t o d i m e r i z a t i o n mechanism a r e X and XI (]6). The methoxy s u b s t i t u e n t s a r e r e s p o n s i b l e f o r t h e e f f i c i e n c y o f c r o s s l i n k i n g , p r e s u m a b l y , i n p a r t , by i n c r e a s i n g the l i f e t i m e of t h e r e a c t i v e s i n g l e t e x c i t e d s t a t e . Recent Azido Photopolymers. A c l a s s o f u s e f u l photopolymers t h a t c r o s s l i n k s by non-dîmerizable g r o u p s i s t h a t c o n t a i n i n g a z i d o g r o u p s . P o l y ( v i n y l - p - a z t d o B e n z o a t e ) CXI I ) , p r e p a r e d f r o m p o l y ( v i n y l a l c o h o l ) , i s one e x a m p l e ( 1 7 ) . S i n c e no u n s a t u r a t i o n i s p r e s e n t , c r o s s l i n k i n g e f f i c i e n c y depends on t h e p h o t o - g e n e r a t e d n i t r e n e s r e a c t i v i t y t o w a r d i n s e r t i o n i n t o c a r b o n - h y d r o g e n bonds and g e n e r a t i o n o f r a d i c a l s . Various t r i p l e t s e n s i t i z e r s increase t h e e f f i c i e n c y by one t o two o r d e r s o f m a g n i t u d e . R e a c t i o n o f p o l y ( v i n y l c h l o r i d e ) w i t h s o d i u m a z i d e i n dîmethyl formamide i n c o r p o r a t e s a z i d e g r o u p s t o y i e l d a p h o t o c r o s s l i n k a b l e f

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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L I G H T INDUCED REACTIONS IN P O L Y M E R S

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polymer ( 1 8 ) . PhotocrossIinking of poly(vinyI-p-azidocinnamate) (XIII) i s reported t o i n v o l v e c o m p e t i t i v e p h o t o d i m e r i z a t i o n o f cinnamoyl groups t o c y c l o b u t e n e s a l o n g w i t h t h e dominant n i t r e n e r e a c t i o n s o f i n s e r t i o n and r a d i c a l a b s t r a c t i o n . The r e a c t i o n c a n be triplet-sensitized (19). R e c e n t P h o t o p o l y m e r s C r o s s l i n k e d by R a d i c a l P r o c e s s e s . P o l y m e r s ( X I V ) c o n t a i n i n g t h e p h o t o r e a c t i v e a e r y IoyI and m e t h a c r y o y l g r o u p s have been p r e p a r e d by a c a t i o n i c s y n t h e s i s s i m i l a r t o t h a t m e n t i o n e d e a r l i e r (2(D). P h o t o d i m e r i z a t i o n i s n o t e x p e c t e d to play a r o l e i n c r o s s l i n k i n g i n these cases. A p o l y m e r c o n t a i n i n g p e n d a n t benzophenone g r o u p s (XV) i s f o u n d t o p h o t o c r o s s I i n k . The mechanism i s i n t e r m o l e c u l a r c o u p l i n g o f r a d i c a l s formed v i a p h o t o r e d u c t i o n i n t h e t r i p l e t s t a t e . In s o l u t i o n , c r o s s l i n k i n g i s c o n c e n t r a t i o n dependent as i n t r a m o l e c u ­ l a r eye Iîzation competes ( 2 1 ) . D i a l l y I p h t h a l a t e , w h i c h has been c o n v e r t e d under r a d i c a l c o n d i t i o n s t o a low p o l y m e r , p h o t o c r o s s I i n k s i n t h e p r e s e n c e o f N - p h e n y I m a l e i m i d e and M i c h l e r ' s k e t o n e s e n s i t i z e r (22_). P o s s i b l y the c r o s s l i n k i n g i n v o l v e s c o p o l y m e r i z a t i o n of unreacted a l l y l groups with t h e maleimide. A l a r g e number o f e x a m p l e s o f p o l y m e r s c o n t a i n i n g t e r t i a r y amines a r e r e p o r t e d t o p h o t o c r o s s I i n k i n t h e presence o f a l k y l halides. One c o m b i n a t i o n w o u l d be p o l y ( p - d i m e t h y I a m i n o s t y r e n e ) and e t h y l i o d i d e ( 2 3 ) . The mechanism i s n o t r e p o r t e d , b u t may i n v o l v e c h a r g e - t r a n s f e r e x c i t a t i o n t o form r a d i c a l s . The o x i d a t i v e c o u p l i n g o f monomers s u c h a s t h e d i p r o p a r g y l e t h e r o f bîs-phenol A ( X V I ) r e s u l t s i n p h o t o c r o s s l i n k a b l e m a t e r i ­ a l ( 2 4 ) . The mechanism i s unknown, b u t t h e r a t e i s g r e a t e s t i n examples i n which carbonyl groups a r e a t t a c h e d t o t h e a r o m a t i c rings. A r e c e n t s t u d y (23) r e p o r t e d on t h e r a t e s o f p h o t o c r o s s I i n k i n g and t h e p h o t o s e n s i t i v i t i e s o f p o l y ( v i n y l α-cyanocinnamate) and p o l y ( v i n y l α-cyanocinnamoxyacetate). The a u t h o r s concluded t h a t photocrossIînking o f t h e s e p o l y m e r s p r o c e e d e d m a i n l y t h r o u g h r a d i c a l a d d i t i o n , and t h a t t h e s e p o l y m e r s showed h i g h e r p h o t o ­ s e n s i t i v i t i e s t h a n p o l y ( v i n y l c i n n a m a t e ) and p o l y ( v i n y l c i n n a m o x y ­ a c e t a t e ) i n s p i t e o f t h e lower r a t e s o f p h o t o c r o s s I i n k i n g o f t h e α-cyano-substituted p o l y m e r s . The p h o t o s e n s i t i v i t i e s and t h e r a t e s o f photocrossIînking o f t h e s e α-cyano-substituted p o l y m e r s were a l s o compared w i t h t h a t o f p o l y ( v i n y l β-styryI a e r y l o x y acetate). A r e c e n t r e v i e w (2(5) c o v e r e d f o u r - c e n t e r t y p e photopolymeriz a t i o n i n t h e c r y s t a l l i n e s t a t e . Although these p h o t o i n i t i a t e d p o l y m e r i z a t i o n s by t h e f o u r - c e n t e r mechanism may n o t be c o n s i d e r e d to involve a photocrosslinkable step, the extensive discussion of mechanism by t h e s e a u t h o r s may be o f v a l u e t o t h o s e i n t e r e s t e d i n p h o t o c r o s s I i n k i n g a s t h e r e a r e some marked s i m i l a r i t i e s between t h e s e two c o n c e p t s .

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

Downloaded by UNIV OF ALBERTA on November 9, 2014 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch003

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Photocrosslinkable Polymers

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P h o t o i o n i c C r o s s l i n k i n g o f C e r t a i n P o l y m e r s . C e r t a i n mono­ mers a r e r e a d i l y p r e p a r e d h a v i n g two p o l y m e r i z a b l e f u n c t i o n a l g r o u p s w h i c h p r o p a g a t e e x c l u s i v e l y by d i f f e r e n t mechanism, s e l e c t ­ i n g among r a d i c a l , a n i o n i c , and c a t i o n i c p r o p a g a t i o n . These monomers may be p o l y m e r i z e d t h r o u g h one f u n c t i o n a l g r o u p and c r o s s l i n k e d i n a s e p a r a t e s t e p by a d i f f e r e n t c l a s s o f i n i t i a t o r . The f a c t t h a t p h o t o e x c i t a t i o n o f c h a r g e - t r a n s f e r c o m p l e x e s c a n i n i t i a t e i o n i c p o l y m e r i z a t i o n s o f c e r t a i n monomers s u g g e s t e d t h e f e a s i b i l i t y o f using photo-generated ions r a t h e r than photogenerated r a d i c a l s . The two p o s s i b i l i t i e s a v a i l a b l e f o r t h i s p u r p o s e a r e t h e p h o t o c a t i o n i c c r o s s l i n k i n g o f a polymer c o n t a i n i n g donor o l e f i n i c g r o u p s i n t h e p r e s e n c e o f a n a c c e p t o r compound, and t h e p h o t o a n i o n i c c r o s s l i n k i n g o f a polymer c o n t a i n i n g acceptor o l e f i n i c g r o u p s i n t h e p r e s e n c e o f a d o n o r compound. The p o l y m e r c o n t a i n ­ ing donor o l e f i n i c groups, p o l y - ( m - v i n y l o x y e t h o x y ) s t y r e n e ( X V I I I ) was p r e p a r e d i n o u r l a b o r a t o r i e s by Mr. Shaw Chu ( 2 7 ) , and has a m o l e c u l a r w e i g h t o f 110,000. The p o l y m e r c o n t a i n i n g acceptor o l e f i n i c groups, p o l y - [ v i n y I ( 2 - n i t r o s t y r y l o x y e t h y I ) e t h e r ] , (XIX) was s y n t h e s i z e d a c c o r d i n g t o t h e p r o c e d u r e o f J.W. Schwîetert ( 3 0 ) and has a low m o l e c u l a r w e i g h t ( p r o b a b l y 5000 o r l e s s ) . R e s u l t s and D i s c u s s i o n I n i t i a l experiments involved t h e i r r a d i a t i o n o f XVIII i n t h e p r e s e n c e o f c h l o r a n i l i n m e t h y l e n e c h l o r i d e and t h e i r r a d i a t i o n o f X I X i n t h e p r e s e n c e o f Ν,Ν-dîmethylaniIine i n m e t h y l e n e chloride. Samples were p r e p a r e d on t h e vacuum l i n e w i t h p u r e d r y m a t e r i a l s and w i t h c a r e f u l removal and e x c l u s i o n o f m o i s t u r e and o x y g e n . However, no t r a c e o f photocrossIînking was o b s e r v a b l e in e i t h e r system. In a c c o r d a n c e w i t h t h e e x p e c t a t i o n t h a t i n i t i a t i o n by a c h a r g e - t r a n s f e r e x c i t e d s t a t e must be p r e c e d e d by s e p a r a t i o n o f t h e r a d i c a l c a t i o n - r a d i c a l a n i o n p a i r by s o l v e n t , i n o r d e r t o compete w i t h r e c o n v e r s i o n t o t h e g r o u n d s t a t e w i t h i n t h e s o l v e n t c a g e , more p o l a r s o l v e n t s were employed t o f a c i l i t a t e t h i s s t e p . D + A

(D,A)

h V ( t

7

> (Ο+,Α ) }

^

>

D* + A

T

Thus, i r r a d i a t i o n o f XVIII i n t h e presence o f c h l o r a n i l o r t e t r a c h l o r o p h t h a l I i c a n h y d r i d e (TCPA) i n a c e t o n i t r i l e u n d e r d r y , oxygen-free conditions yielded rapid p r e c i p i t a t i o n o f c r o s s l i n k e d p o l y m e r . The i r r a d i a t i o n o f X I X i n t h e p r e s e n c e o f N,N-dimethyI a n i l i n e i n d i m e t h y I f o r m a m i d e u n d e r e x c l u s i o n o f a i r and m o i s t u r e produced a very s l i g h t i n d i c a t i o n o f c r o s s l i n k i n g o c c u r r i n g . U n t i l the present time, the f a c t that the photo-cat ionic c r o s s l i n k i n g o f XVIII i s s u c c e s s f u l while t h e photo-anionic crossl i n k i n g o f X I X i s much l e s s s o has prompted much f u r t h e r work o n

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

UV

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24

LIGHT INDUCED REACTIONS

IN

POLYMERS

t h e f i r s t and none on t h e s e c o n d . Other reasons a l s o e x i s t f o r t h e l a c k o f f u r t h e r work on X I X . The p h o t o - a n i o n i c c r o s s l i n k i n g o f X I X i s p r o b a b l y quenched by t r a c e s o f w a t e r , and i t may be e x t r e m e l y d i f f i c u l t t o p r e p a r e s o l u t i o n s o f t h e p o l y m e r i n an absolutely dry state. A l s o , t h e polymer p a r t i a l l y c r o s s l i n k s s p o n t a n e o u s l y by a i r o x i d a t i o n d u r i n g s t o r a g e , i s p r e p a r e d i n low y i e l d , h a s a low m o l e c u l a r w e i g h t , a n d i s v e r y d i f f i c u l t t o d i s ­ s o l v e , making i t l e s s d e s i r a b l e f o r t h e purpose i n t e n d e d . Never­ t h e l e s s , f u r t h e r a t t e m p t s t o c r o s s l i n k X I X by means o f o t h e r d o n o r s , s u c h a s a n t h r a c e n e , w o u l d be o f i n t e r e s t . The p r o c e s s a p p a r e n t l y b e i n g o b s e r v e d i n t h e p h o t o - c a t i o n i c c r o s s l i n k i n g o f X V I I I i s d e p i c t e d b e l o w , where R0CH=CH2 r e p r e s e n t s XVI I I :

ROCH=CH

2

+ A

hv CH CN

+ R O C H - C H . + A*

3

CH:

I

ROCH=CH. ->

+

2

ROCH-CH -CHOR 2

CH; ROCH -f CH -CH -KX 2 , η OR 0

The t e r m i n a t i o n s t e p o f a p r o p a g a t i n g c h a i n c o u l d i n v o l v e p r o t o n t r a n s f e r t o monomer, c o m b i n a t i o n w i t h t h e a c c e p t o r r a d i c a l a n i o n , o r e l e c t r o n t r a n s f e r from t h e a c c e p t o r r a d i c a l a n i o n t o t h e carbonium i o n t o y i e l d a t e r m i n a l r a d i c a l , a s below. CH;

Ruin

pu · R0CH=CH j 2 ^ R O C H -f C H - C H ->- CH=CH + ROCHCH, 2 η j -> OR O

-f CH -CH * CH -CH 2 j η 2 ι OR 0

0

0

CH; I ROCH -f C H - C H 2 2

0

CH;

η

CH -CH-A ζ ι OR 9

ROCH « - C H - C H + C H - C H 2 η 2 ι OR OR 0

ι

0

P r e s u m a b l y t h e r a d i c a l s i t e s g e n e r a t e d do n o t p r o p a g a t e b u t a r e d e s t r o y e d i n some c h a i n - t r a n s f e r r e a c t i o n o r r a d i c a l - r a d i c a l combination. The c h a r g e - t r a n s f e r c o m p l e x h a s n o t been i n c l u d e d i n t h e i n i t i a t i o n mechanism f o r r e a s o n s t o be d i s c u s s e d . The e f f e c t s o f d i f f e r e n t a c c e p t o r s , o x y g e n , w a t e r , quenchers,

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

3.

BUTLER A N D FERRÉE

Photocrosslinkable Polymers

25

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r a d i c a l i n i t i a t o r s , and o t h e r a d d i t i v e s , t e m p e r a t u r e , g e o m e t r y o f i r r a d i a t i o n v e s s e l , a c c e p t o r c o n c e n t r a t i o n l i g h t i n t e n s i t y , wave­ l e n g t h o f l i g h t , and s p o n t a n e o u s c r o s s l i n k i n g w i l l be d i s c u s s e d . Effect of Different Acceptors. Although both o f t h e e a r l i e s t a c c e p t o r s t e s t e d w o r k e d , most o f t h e o t h e r s l a t e r examined d i d n o t e f f e c t p h o t o c r o s s I i n k i n g f o r unknown r e a s o n s . Table I l i s t s the a c c e p t o r s t e s t e d , a l o n g w i t h two m e a s u r e s o f a c c e p t o r s t r e n g t h and approximate e f f i c i e n c i e s o f p r e c i p i t a t i o n o f c r o s s l i n k e d polymer. The c r o s s l i n k e d p o l y m e r formed w i t h c h l o r a n i l i s p r e d o m i n a n t l y s u s p e n d e d i n t h e l i q u i d w h i l e t h a t formed on t h e t u b e w a l l where t h e l i g h t beam i m p i n g e s . The p r o d u c t o f c r o s s l i n k i n g X V I I I u s i n g c h l o r a n i l o r TCPA c o n t a i n s no d e t e c t a b l e IR bands due t o t h e a c c e p t o r s . The IR s p e c t r u m o f X V I I I c r o s s l i n k e d by i r r a d i a t i o n i n t h e p r e s e n c e o f TCPA i s i d e n t i c a l t o t h a t o f X V I I I c r o s s l i n k e d by B F by Shaw Chu. Spontaneous C r o s s l i n k i n g . In t h e a b s e n c e o f TCPA, e v a p o r a t e d s o l u t i o n s o f X V I I I c a n be c o m p l e t e l y r e d i s s o l v e d i n f r e s h s o l v e n t i n a few m i n u t e s . And s o l u t i o n s o f 0.05 M X V I I I c o n t a i n i n g 0.007 M TCPA i n a c e t o n i t r i l e a r e s t a b l e i n d e f i n i t e l y ( a t l e a s t t h r e e weeks) a t room t e m p e r a t u r e . However, on e v a p o r a t i o n o f t h e s o l v e n t under vacuum, t h e m i x t u r e f o r m s a v e r y t o u g h f i l m t h a t i s i n s o l u b l e i n a c e t o n i t r i l e o r h o t DMF, does n o t m e l t up t o 300°, and i s r e s i s t a n t t o a t t a c k by c h r o m i c a c i d . T h e IR s p e c t r u m i s i d e n t i c a l t o t h a t o f X V I I I c r o s s l i n k e d B F 3 o r by i r r a d i a t i o n i n t h e p r e s e n c e o f TCPA. A p p a r e n t l y a s t h e s o l u t i o n becomes h i g h l y c o n c e n t r a t e d , c r o s s l i n k i n g o c c u r s by t h e r m a l e x c i t a t i o n o f c h a r g e t r a n s f e r c o m p l e x e s o f TCPA and X V I I I . The IR e v i d e n c e and t h e i n s o l u b i l i t y b e h a v i o r a p p e a r t o d i s c o u n t an a l t e r n a t i v e p o s s i b i l ­ i t y o f s i m p l e a s s o c i a t i o n o f p o l y m e r c h a i n s f a c i l i t a t e d by t h e p r e s e n c e o f t h e a c c e p t o r compound. The s p o n t a n e o u s c r o s s l i n k i n g i s a l s o o b s e r v e d w i t h t h e a c c e p t o r s c h l o r a n i l and 1 , 3 , 5 - t r i n i t r o b e n z e n e , and p r e s u m a b l y w o u l d o c c u r w i t h o t h e r s . W h i l e t h e s p o n t a n e o u s c r o s s l i n k i n g i s i n t e r e s t i n g and c e r t a i n l y worthy o f f u r t h e r i n v e s t i g a t i o n , i t p l a c e s l i m i t a t i o n s on t h e methods o f s a m p l e p r e p a r a t i o n f o r t h e p h o t o c r o s s I i n k i n g when d r y c o n d i t i o n s a r e r e q u i r e d . T h a t i s , one c a n n o t d r y t h e a c c e p t o r and X V I I I i n t h e same s t e p by e v a p o r a t i o n o f a s t o c k s o l u t i o n c o n t a i n i n g t h e two t o d r y n e s s on t h e vacuum l i n e . The method t h a t was used t o a v o i d t h e s p o n t a n e o u s c r o s s l i n k i n g was t o prepare a stock s o l u t i o n o f XVIII i n dry a c e t o n i t r i l e , evaporate t h e s o l v e n t on t h e vacuum l i n e , h e a t t h e p o l y m e r f i l m t o 80° b r i e f l y , and t h e n a l l o w i t t o d r y u n d e r vacuum f o r one h o u r , t h e t u b e removed f r o m t h e l i n e and a c c e p t o r w e i g h e d i n , t h e n d r y p o l y m e r and a c c e p t o r d r i e d u n d e r vacuum f o r one h o u r . E x c e s s d r y a c e t o n i t r i l e was c o n d e n s e d i n t o t h e l i q u i d - n i t r o g e n - c o o l e d t u b e d i r e c t l y f r o m P2®5> m e l t e d t h e e x c e s s e v a p o r a t e d by h i g h vacuum, and t h e t u b e s e a l e d w i t h a f l a m e . I f t h e d r y i n g u n d e r vacuum o f t h e combined d r y m i x t u r e o f X V I I I and TCPA i s c a r r i e d o u t f o r a long p e r i o d o f t i m e , p a r t i a l spontaneous c r o s s l i n k i n g i s observed 3

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976. -1.31

PhthalIic

0.56

0.56

(est.

0.70

0.77

1.37

E.A.,

0.4)

eV

300

275,

334

300, 254,

236

275 236

334 ( 3 3 4 , 313,

334 ( 3 3 4 , 313,

334

λ(+6.4),

366 )

. 366)

nm

D

d

e

0.75

o.o '

0.75

0.0

0.0

0.0

2.3

d

Eff iciency

9

e

c

C o n c n . XVI1 I was 0.050 M ( i n monomer u n i t s o f M.W. 190) i n a c e t o n i t r i l e , c o n c n . a c c e p t o r was 0.007 M. A c c e p t o r s t r e n g t h d a t a f r o m book by F o r s t e r . ^Samples c o n t a i n i n g 0.50 M H2O; a l l o t h e r s p r e p a r e d u n d e r r i g o r o u s l y d r y c o n d i t i o n s . A l l s a m p l e s s e a l e d u n d e r vacuum g r e a t e r t h a n 10-4 t o r r . E x p r e s s e d i n t e r m s o f m o l e s o f monomer u n i t s o f p o l y m e r p r e c i p i t a t e d p e r e i n s t e i n of protons absorbed. ^ I r r a d i a t e d i n q u a r t z t u b e s a t room temp, o f 25°; a l l o t h e r s were P y r e x t u b e s i r r a d i a t e d w h i l e immersed i n an i c e b a t h a t 3±1°. A f i n e p r e c i p i t a t e formed t h a t q u i c k l y r e d i s s o l v e d .

Anhydride

-0.86

TCPA

a

-0.86

TCPA

( e s t . -0.8).

-0.60

1,3,5-Trinitrobenzene

1,3,5-Tricyanobenzene

-0.51

/ v

Benzoqu i none

1/2 +0.01

L

Chlorani1

Acceptor

in the Photo-cat i o n i c C r o s s l i n k i n g of X V I I I

Acceptor Strength

Tests of V a r i o u s Acceptors

Table I

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3.

BUTLER AND FERRÉE

Photocrosslinkable Polymers

27

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by i n s o l u b i l i t y i n f r e s h s o l v e n t , p a r t i c u l a r l y where t h e TCPA p a r t i c l e s d i r e c t l y c o n t a c t e d t h e polymer, but t h e r e a l s o appears t o be some c r o s s l i n k i n g c a u s e d by TCPA v a p o r . T h i s p r o c e s s h a s a l s o been o b s e r v e d w i t h c h l o r a n i l , w h i c h i s more v o l a t i l e t h a n TCPA. E f f e c t o f Oxygen. A l t h o u g h a c a t i o n i c p r o p a g a t i o n i s g e n e r ­ a l l y n o t a f f e c t e d by o x y g e n , t h e e x c i t e d s t a t e g i v i n g r i s e t o t h e p h o t o - c a t i o n i c c r o s s l i n k i n g m i g h t be e x p e c t e d t o be s u s c e p t i b l e t o oxygen quenching, p a r t i c u l a r l y i f t h e t r i p l e t e x c i t e d s t a t e i s i n v o l v e d . To t e s t t h i s , d r y r e a c t i o n s a m p l e s were p r e p a r e d on t h e vacuum l i n e a s a b o v e , t h e n a i r a d m i t t e d t h r o u g h a d r y i n g t u b e and t h e sample t u b e s s e a l e d o f f and i r r a d i a t e d . W i t h TCPA a s t h e a c c e p t o r , t h e photocrossIînking o c c u r r e d w i t h c o m p a r a b l e e f f i c i e n ­ c y t o t h a t o f t u b e s k e p t s e a l e d u n d e r vacuum ( T a b l e I I ) . Thus b o t h a t r i p l e t e x c i t e d s t a t e i n i t i a t o r and a r a d i c a l p r o p a g a t i o n mechanism a r e d i s f a v o r e d by t h i s e v i d e n c e when TCPA i s u s e d . However, w i t h c h l o r a n i l a s t h e a c c e p t o r , p h o t o c r o s s I i n k i n g i s c o m p l e t e l y quenched by a i r . S i n c e s e v e r a l r e a s o n s t o be d i s c u s s e d below d i s f a v o r a r a d i c a l p r o p a g a t i o n mechanism f o r t h e p h o t o c r o s s I i n k i n g , i t i s b e l i e v e d t h a t t h e oxygen quenching b e h a v i o r i n d i ­ c a t e s t h a t t h e c r o s s l i n k i n g i s i n i t i a t e d by t r i p l e t e x c i t e d chloranil. Q u e n c h i n g o f a s i n g l e t e x c i t e d i n i t i a t o r would have been o n l y p a r t i a l . I t would be o f i n t e r e s t t o t e s t t h e h y p o t h e s i s o f t r i p l e t i n i t i a t o r by e x a m i n i n g t h e e f f e c t o f s m a l l c o n c e n t r a ­ t i o n s (0.02 M) o f c y c l o o c t a t e t r a e n e , a q u e n c h e r o f low t r i p l e t e n e r g y (39 K c a l . ) , i n t h e c a s e o f c h l o r a n i l . E f f e c t o f W a t e r . A s a m p l e was p r e p a r e d a s d e s c r i b e d a b o v e e x c e p t t h a t t h e s o l v e n t condensed i n t o t h e tube c o n t a i n i n g dry X V I I I and TCPA was a measured a l i q u o t o f a s o l u t i o n o f 0.010 M H 0 in dry a c e t o n i t r i l e . I r r a d i a t i o n produced p h o t o c r o s s I i n k i n g i n a p p a r e n t l y c o m p a r a b l e amounts t o t h a t f o r a d r y s a m p l e . However, a f t e r o p e n i n g t h e t u b e and s t o p p e r i n g , t h e n a l l o w i n g t o s t a n d o v e r n i g h t a t room t e m p e r a t u r e , a d d i t i o n a l c r o s s l i n k i n g o c c u r r e d , a s a p r e c i p i t a t e t h a t s e t t l e d t o t h e bottom o f t h e t u b e , i n a d d i ­ t i o n t o t h e p a t c h e s a d h e r i n g t o t h e a r e a s where t h e l i g h t beam had impinged. T h e t o t a l c o n v e r s i o n i s o l a t e d was 93% o f t h e 18.1 mg X V I I I s t a r t e d w i t h . On r e p e a t i n g t h e e x p e r i m e n t w i t h 0.50 Μ H 0 , t h e p h o t o c r o s s I i n k i n g o c c u r r e d w i t h t y p i c a l e f f i c i e n c y , but a f t e r i r r a d i a t i o n , a s t h e t u b e was b e i n g a l l o w e d t o warm t o room t e m p e r ­ a t u r e , t h e c r o s s l i n k i n g resumed s p o n t a n e o u s l y t o f i l l t h e s o l u t i o n w i t h heavy p r e c i p i t a t e w i t h i n a few m i n u t e s . T o t a l c o n v e r s i o n was 95% t o t h e 18.1 mg X V I I I s t a r t e d w i t h . The s e a l e d s a m p l e had p r e v i o u s t o i r r a d i a t i o n s t o o d a t room t e m p e r a t u r e o v e r n i g h t w i t h no c h a n g e . The i n d i c a t i o n o f two s e p a r a t e p r o c e s s e s , a p h o t o c r o s s l i n k i n g and a w a t e r - i n d u c e d p o s t c r o s s Iînking, h e r e a f t e r s h o r t e n e d t o p o s t l i n k i n g , was v e r i f i e d d r a m a t i c a l l y by t h e f o l l o w ­ ing experiment. A c o m p l e t e l y d r y s a m p l e was p r e p a r e d and i r r a d i ­ a t e d , t h e n t h e t u b e opened and enough H 0 (180 mg/2.0 ml a c e t o 2

2

2

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

UV

28

Table

L I G H T INDUCED REACTIONS IN

POLYMERS

I I

E f f e c t s o f V a r y i n g C o n d i t i o n s and o f A d d i t i v e s on t h e P h o t o c r o s s I i n k i n g o f X V I I I i n t h e P r e s e n c e o f TCPA o r C h l o r a n i l . Additive or Variation

Acceptor TCPA

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TCPA

mg pcpt. 7.3

E f f ί­ ο ciency 0.82

334

Time Irrad. 60 m i n .

334

60

6.0

0.68 0.96

TCPA

Air

334

60

8.5

TCPA

0.0010 M TCPA

334

30

0.0

0.00

TCPA

Narrow S1 i t s

334

120

1.3

0.073

TCPA

n-Bu.NCIO. 0.10 Ή

334

20

4.1

1.4

TCPA

0.0050 M HQ

334

20

0.0

0.00

TCPA

0.010 M

334

20

0.0

0.00

TCPA

Air, H 0 0.0005 M E t N

334

10

0.0

0.00

0.010 M γ-CoI I i d i ne

334

10

>0.0

0.50 M γ-Col1îdine

334

10

0.0

0.00

TCPA

0.005 M KSCN

334

10

0.0

0.00

TCPA

0.10 M AIBN

366

10

0.0

0.00

366

4

8.3

2.6

4

Et N 3

?

3

TCPA TCPA

Chlorani1 Chlorani1

Narrow s I i t s

366

12

6.2

2.0

Chlorani1

25°

366

8

5.0

0.80

Chlorani1

Air

366

8

0.0

0.00

334

10

0.0

0.00

C h l o r a n î1 a

C o n c . X V I I I was 0.050 M i n A c e t o n i t r i l e ; c o n e , a c c e p t o r was 0.0070 M e x c e p t i n t h e e x c e p t i o n n o t e d ; i r r a d i a t i o n s p e r f o r m e d on t u b e s immersed i n an i c e b a t h e x c e p t where n o t e d . ^ D i s p e r s i o n o f monochromator was 6.4 nm, e x c e p t where narrow s l i t s were e m p l o y e d , r e d u c i n g d i s p e r s i o n t o 3.2 nm. Intensities f o r t h e two s l i t c o n d i t i o n s w e r e , f o r 334, 4.7 and 1.64 χ 1 0 ^ p h o t o n s / m i n . , and f o r 366 nm, 2.5 and 0.83 χ 1 0 photons/min. E x p r e s s e d i n t e r m s o f m o l e s o f monomer u n i t s o f p o l y m e r p r e c i p i t a t e d p e r e i n s t e i n o f photons absorbed. 1 8

c

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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29

Photocrosslinkable Polymers

nîtrile) t o make t h e s o l u t i o n 0.5 M i n H 0 was added and t h e s o l u ­ t i o n shaken g e n t l y t o m i x . No c h a n g e o c c u r r e d i m m e d i a t e l y , b u t on r e e x a m i n a t i o n a f t e r one h o u r i n t h e d a r k t h e w h o l e sample had crosslinked. F i l t r a t i o n p r o v i d e d 99% c o n v e r s i o n . Dry s a m p l e s t h a t a r e i r r a d i a t e d and k e p t s e a l e d have n e v e r been o b s e r v e d t o p o s t l i n k even a f t e r p e r i o d s o f a few d a y s . Addi­ t i o n o f 0.5 M H 0 t o an u n i r r a d i a t e d s a m p l e c a u s e d no p r e c i p i t a ­ t i o n f o r s e v e r a l days i n t h e dark. A f t e r s i x days a m i l k y c a s t a p p e a r e d , a n d f o r m a t i o n o f a t h i n p r e c i p i t a t e a p p e a r e d t o have reached c o m p l e t i o n i n about e l e v e n days. 2

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2

E f f e c t o f Quenchers. The p h o t o c r o s s I i n k i n g o f X V I I I i n t h e p r e s e n c e o f 0.007 M TCPA i s quenched c o m p l e t e l y by 0.010 M t r i e t h y l a m i n e o r even by 0.005 M t r i e t h y l a m i n e ( a i r and a d v e n t i t i o u s H 0 were p r e s e n t i n t h e l a t t e r r u n ) . T h i s i s b e l i e v e d t o be good evidence t h a t t h e propagating s p e c i e s i s c a t i o n i c r a t h e r than radical. T h e s e s a m p l e s a l s o do n o t p o s t l i n k when 0.5 M H 0 i s added. The h i n d e r e d amine γ-colIidîne (2,4,6-trîmethyIpyridine) was t h e n e x a m i n e d a s a q u e n c h e r s i n c e i t w o u l d s e l e c t i v e l y bond t o p r o t o n s b u t n o t t o l a r g e r c a t i o n s . I t was o b s e r v e d t h e 0.010 M γ-collidine does n o t quench t h e p h o t o c r o s s I i n k i n g i n t h e p r e s e n c e o f TCPA c o m p l e t e l y , b u t p o s t l i n k i n g a f t e r a d d i t i o n o f 0.5 M H 0 was c o m p l e t e l y i n h i b i t e d u n t i l two weeks l a t e r . T h i s experiment s h o u l d be r e p e a t e d on a l a r g e r s c a l e t o o b t a i n q u a n t i t a t i v e measurements o f t h e i n h i b i t i o n . The e x p e r i m e n t a p p e a r s t o p r o v e t h a t t h e photocrossIînking i s c a t i o n i c b u t t h e i n i t i a t i n g s p e c i e s a r e n o t p r o t o n s , w h i l e t h e p o s t l i n k i n g i s i n i t i a t e d by p r o t o n s . A t a c o n c e n t r a t i o n o f γ-collîdîne o f 0.50 M t h e p h o t o c r o s s I i n k i n g i s c o m p l e t e l y i n h i b i t e d , w h i c h c o u l d be due t o c o m p e t i t i v e c h a r g e t r a n s f e r c o m p l e x a t i o n o f g r o u n d s t a t e o f e x c i t e d s t a t e TCPA r a t h e r than i n t e r f e r i n g with c a t i o n i c propagation. ( F o r s t e r ' s book on c h a r g e - t r a n s f e r c o m p l e x e s shows t h a t γ-collîdîne c a n f o r m compI e x e s . ) H y d r o q u i n o n e a t a c o n c e n t r a t i o n o f 0.0050 M q u e n c h e s p h o t o c r o s s l i n k i n g i n t h e p r e s e n c e o f TCPA c o m p l e t e l y . Possibly the b a s i c i t y o f hydroquinone i s r e s p o n s i b l e f o r t h e i n h i b i t i o n , s i n c e t h e r a d i c a l mechanism i s d i s f a v o r e d compared t o t h e c a t i o n i c mechanism by s e v e r a l o t h e r p i e c e s o f e v i d e n c e . The i d e a l q u e n c h e r w h i c h was on o r d e r , i s d i p h e n y I p i c r y I h y d r a z y I , w h i c h a f f e c t s r a d i ­ cal but not c a t i o n i c processes. 2

2

2

E f f e c t o f A z o i s o b u t y r o n i t r i l e (AIBN). AI t h o u g h XVI I I i s formed f r o m t h e monomer u n d e r r a d i c a l c o n d i t i o n s w i t h o u t r a d i c a l propagation o c c u r r i n g along t h e v i n y l ether groups, t h e p o s s i b i l ­ i t y t h a t i n t h e p r e s e n c e o f an a c c e p t o r s u c h a s TCPA o r c h l o r a n i l t h e c o m p l e x e d v i n y l e t h e r g r o u p s w o u l d be s u s c e p t i b l e t o r a d i c a l p r o p a g a t i o n seemed p l a u s i b l e enough t o t e s t t h i s p o s s i b i l i t y . Thus a s a m p l e o f 0.050 M X V I I I and 0.007 M TCPA c o n t a i n i n g 0.10 M AIBN i n d r y a c e t o n i t r i l e was i r r a d i a t e d a t 366 ± 6.4 nm, where

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

UV

30

L I G H T INDUCED REACTIONS IN

POLYMERS

AIBN a b s o r b s and decomposes t o r a d i c a l s w i t h a quantum y i e l d o f 0.43 (0.86 r a d i c a l s p e r p r o t o n ) ( 2 8 K No p h o t o c r o s s I i n k i n g occurred. T h i s seemed t o show t h a t t h e c r o s s l i n k i n g i n t h i s s y s ­ tem i s n o t r a d i c a l - i n i t i a t e d . ( I t m i g h t be m e n t i o n e d t h a t t h e o n l y p o s s i b i l i t y l e f t open by t h i s e x p e r i m e n t f o r a r a d i c a l p r o ­ c e s s i s t h a t d i r e c t p h o t o - i n i t i a t i o n p r o d u c e s a r a d i c a l o f much g r e a t e r r e a c t i v i t y than (CH3) C-CN which i s a b l e t o i n i t i a t e c r o s s l i n k i n g in the presence of the acceptor.)

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2

E f f e c t o f M i s c e l l a n e o u s A d d i t i v e s . The s o u r c e o f t h e p r o t o n s r e s p o n s i b l e f o r p o s t l i n k i n g i s not y e t c l e a r . The r e s u l t s o b t a i n ­ ed r e q u i r e t h a t i r r a d i a t i o n p r o d u c e s e i t h e r i n t h e c r o s s l i n k e d p o l y m e r o r t h e s o l u t i o n an a c i d i c s p e c i e s t h a t does n o t c o n t i n u e t o p r o p a g a t e c a t i o n i c c r o s s l i n k i n g when i r r a d i a t i o n i s s t o p p e d , b u t w h i c h can r e a c t w i t h H 0 t o r e l e a s e p r o t o n s . One p o s s i b i l i t y t h a t has been u n d e r c o n s i d e r a t i o n i s t h a t d u r i n g p h o t o c r o s s I i n k i n g some o f t h e p r o p a g a t i n g c a t i o n i c c e n t e r s become t r a p p e d i n c r o s s l i n k e d polymer, such t h a t molecules of H 0 c o u l d d i f f u s e in and bond t o t h e c a r b o n i u m i o n and t h e n r e l e a s e a p r o t o n . Thus H 0 would a c t as a c h a i n t r a n s f e r a g e n t . If t h i s t h e o r y i s c o r r e c t , other small molecules could s i m i l a r l y i n t e r c e p t the trapped c a r ­ bonium i o n s . One a p p r o a c h t h a t was t o be a t t e m p t e d i s t o add v i a s e a l e d ampoule t e c h n i q u e s β-chloroethyI v i n y l e t h e r t o an i r r a d i ­ a t e d m i x t u r e o f X V I I I and a c c e p t o r i n a c e t o n i t r i l e and a n a l y z e f o r i n c o r p o r a t i o n of c h l o r i n e in t h e c r o s s l i n k e d polymer, assuming the β-chloroethyI v i n y l e t h e r i s a b l e t o d i f f u s e t o t h e c a r b o n i u m c e n ­ t e r s and f o r m g r a f t e d c h a i n s by c a t i o n i c p r o p a g a t i o n . This exper­ iment i s p r e f e r a b l y t o be c a r r i e d o u t w i t h an a c c e p t o r t h a t ab­ s o r b s a b o v e t h e P y r e x c u t o f f o f 310 nm and does n o t c o n t a i n chlorine. The r e c e n t l y r e c e i v e d p y r o m e l l i t i c d i a n h y d r i d e was t o be i n v e s t i g a t e d f o r t h i s p u r p o s e . A n o t h e r a g e n t i n v e s t i g a t e d a s an i n t e r c e p t o r o f t h e t r a p p e d c a r b o n i u m i o n s i s p o t a s s i u m t h i o c y a n a t e , KSCN. T h i s i s t h e o n l y common s a l t t e s t e d t h a t i s s i g n i f i c a n t l y s o l u b l e i n a c e t o n i t r i l e . The t h i o c y a n a t e a n i o n has a m o l e c u l a r s i z e c o m p a r a b l e t o t h a t o f H 0 and i t s i n c o r p o r a t i o n i n t o t h e p o l y m e r s h o u l d be e a s i l y de­ t e c t e d by t h e s t r o n g , s h a r p i n f r a r e d a b s o r p t i o n n e a r 2100 cm-1. A c c o r d i n g l y , a s a m p l e o f X V I I I and TCPA i n d r y a c e t o n i t r i l e was i r r a d i a t e d f o r 20 min. t o p r o d u c e c r o s s l i n k i n g , t h e n t h e t u b e opened and an e q u a l volume o f 0.10 M KSCN i n d r y a c e t o n i t r i l e a d d ­ ed. A f t e r s t a n d i n g i n i c e - w a t e r f o r 2 h o u r s ( i t was assumed t h a t c o m b i n a t i o n o f t h i o c y a n a t e and c a r b o n i u m c e n t e r s w o u l d be r a p i d ) , t h e s o l u t i o n was made 0.5 M i n H 0 t o o b s e r v e w h e t h e r t h e p o s t l i n k i n g would o c c u r . A f t e r 3 h o u r s a t room t e m p e r a t u r e , no p o s t l i n k i n g had o c c u r r e d , w h e r e a s p o s t l i n k i n g was c o m p l e t e i n an i d e n ­ t i c a l i r r a d i a t e d sample not t r e a t e d w i t h t h i o c y a n a t e i o n . But a f t e r t h r e e d a y s n e a r l y c o m p l e t e p o s t l i n k i n g (93%) had o c c u r r e d . The i n f r a r e d s p e c t r u m o f t h e f i l t e r e d p o l y m e r showed no d e t e c t a b l e band i n t h e 2100 cm"! r e g i o n n o r any o t h e r d i f f e r e n c e f r o m s i m i l a r s a m p l e s n o t t r e a t e d w i t h t h i o c y a n a t e i o n . However, d u r i n g t h e 2

2

2

2

2

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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same s e t o f r u n s a s a m p l e o f X V I I I a n d TCPA i n d r y acetonitrîle c o n t a i n i n g 0.005 M KSCN was p r e p a r e d a n d i r r a d i a t e d , b u t no p h o t o crosslinking occurred, ( A l t h o u g h TCPA and KSCN f o r m a y e l l o w c h a r g e - t r a n s f e r c o m p l e x i n acetonitrîle, a b s o r p t i o n o f t h e c o m p l e x i s i n s i g n i f i c a n t a t t h e c o n c e n t r a t i o n s used h e r e . I t might a l s o be n o t e d t h a t t h e s t r o n g e r a c c e p t o r c h l o r a n i l f o r m s a r e d c o m p l e x w i t h KSCN t h a t s p o n t a n e o u s l y r e a c t s t o f o r m a brown p r e c i p i t a t e t h a t p r o b a b l y r e s u l t s from d i s p l a c e m e n t o f c h l o r i n e s from t h e 2 and 5 p o s i t i o n s by t h i o c y a n a t e i o n . ) A d d i t i o n o f a n 0.5 M c o n c e n ­ t r a t i o n o f H2O p r o d u c e d no p o s t l i n k i n g i n t h r e e h o u r s b u t c o m p l e t e p r e c i p i t a t i o n (measured 107% based o n s t a r t i n g p o l y m e r a l o n e ) i n t h r e e days. H e r e we seem t o have an e x a m p l e o f a more r a p i d p o s t l i n k i n g i n an i r r a d i a t e d s a m p l e t h a n i n an u n i r r a d i a t e d s a m p l e e v e n t h o u g h no v i s i b l e p r e c i p i t a t i o n due t o p h o t o c r o s s I i n k i n g occurred. F u r t h e r , a l t h o u g h t h e p r o d u c t d o e s n o t c o n t a i n a band n e a r 2 1 0 0 c n r H , t h e r e i s a m o d e r a t e l y s t r o n g and s l i g h t l y b r o a d band a t 1737 c r r H , w i t h no o t h e r o b s e r v a b l e d i f f e r e n c e i n t h e spectrum from t h a t t y p i c a l o f c r o s s l i n k e d X V I I I . Evidently the s p e c i e s p r o d u c i n g t h e 1737 c r r H band must have i n c o r p o r a t e d i n t h e polymer d u r i n g t h e i r r a d i a t i o n p e r i o d , o t h e r w i s e i t would a l s o be p r e s e n t i n t h e p r o d u c t o f t h e e x p e r i m e n t d e s c r i b e d j u s t p r e ­ viously. One p o s s i b i l i t y i s t h a t t h e t h i o c y a n a t e i o n quenched p h o t o c r o s s I i n k i n g by c o m b i n a t i o n w i t h t h e i n i t i a t i n g r a d i c a l c a t i o n o r t h e p r o p a g a t i n g c a t i o n a n d t h e n was h y d r o l y z e d t o a t h i o l c a r b a m a t e o n a d d i t i o n o f H 2 O . However, t h e band maximum d o e s n o t a g r e e w i t h t h o s e r e p o r t e d f o r N - p h e n y l - and N - p r o p y l t h i o I carbamates ( 2 9 ) , R-S(C0)NH2, which g i v e sharp d o u b l e t s i n t h e 1600-1650 cm-1 r e g i o n a n d a n o t h e r band n e a r 1300 α τ Η . The p o s i t i o n o f t h e band i s more t y p i c a l o f an e s t e r o r k e t o n e , b u t i t s e x a c t o r i g i n i s a s y e t unknown. The e f f e c t o f t e t r a b u t y I ammonium p e r c h I o r a t e was e x a m i n e d , s i n c e i t c o u l d i n c r e a s e the e f f i c i e n c y o f t h e r e a c t i o n i n two ways. T h e p e r c h I o r a t e i o n w o u l d c o m p e t e e f f e c t i v e l y w i t h t h e r a d i c a l a n i o n o f TCPA a s a c o u n t e r i o n t o t h e i n i t i a t i n g r a d i c a l c a t i o n and t o t h e p r o p a g a t i n g c a t i o n , t h u s p o s s i b l y d e c r e a s i n g both r a d i c a l a n i o n - r a d i c a l c a t i o n r e c o m b i n a t i o n and whatever t e r m i n a t i o n s t e p s t h e r a d i c a l a n i o n o f TCPA i s i n v o l v e d i n . I t was f o u n d t h a t a s a m p l e c o n t a i n i n g 0.10 M t e t r a b u t y I ammonium p e r c h l o r a t e i n c r e a s e d t h e p h o t o c r o s s I i n k i n g y i e l d t o about double t h a t o b s e r v e d i n t h e a b s e n c e o f t h e e l e c t r o l y t e ( T a b l e I I ) . The a p p e a r a n c e o f t h e p r o d u c t was a l s o b e t t e r i n t h a t i t was w h i t e r and more c o a g u l a t e d . A p o s s i b l e e x p e r i m e n t o f i n t e r e s t w o u l d be t o i r r a d i a t e a s i m i l a r sample i n t h e s o l v e n t methylene c h l o r i d e , where t h e p r e s e n c e o f t h e e l e c t r o l y t e m i g h t a l l o w t h e p h o t o c r o s s I i n k i n g t o p r o c e e d and t a k e a d v a n t a g e o f t h e improved p r o p a g a t i o n of t h e c a t i o n i c process i n methylene c h l o r i d e than i n a c e t o nitrîle. E f f e c t o f Temperature. In t h e p r e s e n c e o f c h l o r a n i l , t h e e f f i c i e n c y o f p h o t o c r o s s I i n k i n g a t room t e m p e r a t u r e i s d e c r e a s e d

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t o a b o u t o n e - t h i r d o f t h a t a t an i c e - b a t h t e m p e r a t u r e ( T a b l e I I ) . T h i s i n d i c a t i o n of a negative temperature c o e f f i c i e n t i s s t r o n g e v i d e n c e t h a t a c a t i o n i c mechanism i s i n v o l v e d i n t h e p r o p a g a t i o n of the c r o s s l i n k i n g . The e f f e c t o f t e m p e r a t u r e s as low as -40°C w o u l d be o f i n t e r e s t . In t h e p r e s e n c e o f TCPA, t h e e f f i c i e n c y o f p h o t o c r o s s I i n k i n g a l s o a p p e a r e d t o be r e d u c e d a t room t e m p e r a t u r e but because of f i l t r a t i o n d i f f i c u l t i e s a r e l i a b l e q u a n t i t a t i v e measurement was n o t o b t a i n e d i n t h e e x p e r i m e n t r u n . E f f e c t o f R o t a t i n g Sample Tube D u r i n g I r r a d i a t i o n . A l l of t h e e a r l y e x p e r i m e n t s were p e r f o r m e d on 2.0 ml s a m p l e s c o n t a i n i n g 0.05 M X V I I I (19 mg) and u s u a l l y 0.007 M a c c e p t o r . The s a m p l e s were i r r a d i a t e d w h i l e s t a t i o n a r y , u s u a l l y i n an i c e b a t h , and t h e t u b e was t u r n e d t o a f r e s h s u r f a c e a t c o n s t a n t f r e q u e n t i n t e r v a l s . In t h e f i r s t a t t e m p t a t o b t a i n i n g q u a n t i t a t i v e quantum y i e l d m e a s u r e m e n t s , a s e r i e s o f 10 ml s a m p l e s c o n t a i n i n g 0.05 M X V I I I (95 mg e a c h ) and TCPA c o n c e n t r a t i o n s o f 0.001, 0.003, and 0.007 M were p r e p a r e d . T h e s e t u b e s were i r r a d i a t e d w h i l e t u r n e d by an e l e c t r i c m o t o r a t a b o u t 1 r . p . s . and w h i l e immersed i n an i c e b a t h , i n t e n d i n g t o form a u n i f o r m c o a t i n g of c r o s s l i n k e d polymer on t h e t u b e w a l l r a t h e r t h a n t h e s e r i e s o f p a t c h e s o b t a i n e d i n t h e previous runs. However, u n d e r t h e s e c o n d i t i o n s no s i g n i f i c a n t s o l i d formed on t h e t u b e w a l l s b u t i n s t e a d o n l y s l i g h t l y m i l k y s o l u t i o n s were p r o d u c e d , d e c r e a s i n g i n c l o u d i n e s s w i t h d e c r e a s i n g TCPA c o n c e n t r a t i o n . Even t h e p r e c i p i t a t e formed i n t h e 0.007 M TCPA s a m p l e was n o t f i l t e r a b l e . However, t h e s e i r r a d i a t e d sam­ p l e s , a f t e r u n s e a l i n g , s t o p p e r i n g , and b e i n g l e f t s t a n d i n g f o r a week, formed g e l s o c c u p y i n g t h e w h o l e l i q u i d volume o f t h e s a m p l e . The g e l s were no more opaque t h a n t h e r e s p e c t i v e o r i g i n a l s o l u ­ t i o n s a f t e r i r r a d i a t i o n , t h e 0.001 M s a m p l e h a v i n g l i t t l e o p a c i t y . A p p a r e n t l y a d v e n t i t i o u s H2O c a u s e d a s l o w p r o t o n - i n i t i a t e d c r o s s Ii nking. E f f e c t of Acceptor C o n c e n t r a t i o n . With the f a i l u r e of the r o t a t i n g - s a m p l e - t u b e e x p e r i m e n t s , a l l t h e quantum e f f i c i e n c y measurements were p e r f o r m e d w i t h 10 ml s a m p l e s i n s t a t i o n a r y t u b e s turned at constant, frequent i n t e r v a l s t o a f r e s h surface. The c o n c e n t r a t i o n o f TCPA used i n most o f t h e r u n s was 0.007 M, w h i c h i s near the s o l u b i l i t y l i m i t in a c e t o n i t r i l e or methylene chloride. The a b s o r b a n c e a t 334 nm a t t h i s c o n c e n t r a t i o n f o r t h e 0.9 cm p a t h l e n g t h o f t h e t u b e s i s 20, a b o u t t e n t i m e s t h a t n e c e s ­ sary t o absorb a l l the l i g h t . When a s a m p l e c o n t a i n i n g o n l y 0.0008 M TCPA ( a b s o r b a n c e 2.3 f o r t h e 0.9 cm p a t h l e n g t h , t h u s s t i l l a b s o r b i n g o v e r 99% o f t h e l i g h t ) was p r e p a r e d and i r r a d i ­ a t e d , no p h o t o c r o s s I i n k i n g was o b s e r v a b l e w i t h i n 10 min. A second s a m p l e c o n t a i n i n g 0.0010 M TCPA p r o d u c e d no p h o t o c r o s s I i n k i n g a f t e r 30 min. o f i r r a d i a t i o n ( T a b l e I I ) . T h i s o b s e r v a t i o n c o r r e l a t e s with the q u a l i t a t i v e observations in the rotating-tube run. The o b s e r v a t i o n o f a c o n c e n t r a t i o n dependence o f c r o s s l i n k ­ i n g e f f i c i e n c y i s v e r y s u r p r i s i n g , and a f u r t h e r s t u d y o f

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c o n c e n t r a t i o n e f f e c t s w o u l d be o f i n t e r e s t . E f f e c t o f L i g h t I n t e n s i t y . As n o t e d , t h e f a i l u r e o f t h e r o t a t e d TCPA s a m p l e s t o r e s e m b l e t h e b e h a v i o r o f s t a t i o n a r y sam­ p l e s d u r i n g i r r a d i a t i o n i s v e r y u n e x p e c t e d . However, t h e d e c r e a s e o f c r o s s l i n k i n g e f f i c i e n c y when e i t h e r t h e s a m p l e t u b e i s r o t a t e d o r t h e TCPA c o n c e n t r a t i o n i s r e d u c e d p o t e n t i a l l y h a s a common e x p l a n a t i o n , w h i c h i s t h a t t h e r e a c t i o n e f f i c i e n c y depends s t r o n g ­ l y on t h e c o n c e n t r a t i o n o f e x c i t e d s p e c i e s . In a s t a t i o n a r y t u b e c o n t a i n i n g 0.007 M TCPA, t h e r e i s a h i g h c o n c e n t r a t i o n o f l i g h t e x c i t e d s p e c i e s j u s t i n s i d e t h e t u b e w a l l where t h e l i g h t beam impinges. Rotating t h e tube o r decreasing the concentration o f TCPA d e c r e a s e s l o c a l c o n c e n t r a t i o n s o f e x c i t e d s t a t e s w i t h o u t changing t h e i r t o t a l c o n c e n t r a t i o n . (Whether r o t a t i o n o f t h e tube could s i g n i f i c a n t l y a f f e c t local concentrations o f e x c i t e d s t a t e s i s by no means c e r t a i n b u t i s a p o s s i b i l i t y . ) These con­ s i d e r a t i o n s made a t e s t o f t h e e f f e c t o f c h a n g i n g l i g h t i n t e n s i t y on t h e r e a c t i o n i m p e r a t i v e , e v e n t h o u g h p h o t o r e a c t i o n s w h i c h a r e more e f f i c i e n t a t h i g h e r l i g h t i n t e n s i t i e s ( b i p h o t o n i c r e a c t i o n s ) a r e g e n e r a l l y v e r y r a r e , a n d t h e assumed mechanism does n o t r e q u i r e a b i p h o t o n i c e x c i t a t i o n scheme. Thus a s a m p l e c o n t a i n i n g 0.0070 M TCPA a c c e p t o r was i r r a d i ­ a t e d w i t h n a r r o w e r s l i t s i n t h e monochromator t o reduce l i g h t i n t e n s i t y by a b o u t t w o - t h i r d s , and t h e i r r a d i a t i o n t i m e i n c r e a s e d proportionately. The o b s e r v e d quantum y i e l d o f p h o t o c r o s s I i n k i n g was i n f a c t r e d u c e d by a f a c t o r o f t e n ( T a b l e I I ) , s u p p o r t i n g t h e supposition t h a t a biphotonic process i s involved i n t h e r e a c t i o n c a t a l y z e d by TCPA. On t h e o t h e r hand, a s i m i l a r e x p e r i m e n t w i t h c h l o r a n i l a s t h e a c c e p t o r f a i l e d t o show a s i g n i f i c a n t e f f e c t o f d e c r e a s i n g l i g h t i n t e n s i t y ( T a b l e I I ) . Thus a b i p h o t o n i c mechanism d o e s n o t a p p e a r t o be i n v o l v e d i n t h e c h l o r a n i l s y s t e m . As a s i d e comment, i t had been o b s e r v e d i n t h e p h o t o p o l y m e r i z a t i o n o f f u m a r o n i t r i l e and d i v i n y l e t h e r i n methanol t h a t t h e r e a c t i o n w o u l d n o t p r o c e e d a s u s u a l when t h e s e a l e d s a m p l e was magnetically s t i r r e d . T h i s may be s i m i l a r i n c a u s e t o t h e e f f e c t of r o t a t i n g t h e tube above, t h a t i s t h a t t h i s r e a c t i o n a l s o depends on t h e l o c a l c o n c e n t r a t i o n o f e x c i t e d s t a t e s . E f f e c t o f Wavelength. In e a r l y t e s t e x p e r i m e n t s w i t h 0.007 M TCPA a c c e p t o r , i t was o b s e r v e d t h a t a t 366 nm, where a b s o r b a n c e o f t h e s o l u t i o n i s l e s s t h a n 0.10, b u t i n w h i c h a n y c h a r g e - t r a n s ­ f e r a b s o r p t i o n s u p e r i m p o s e d on t h e t a i l o f t h e a c c e p t o r a b s o r p ­ t i o n band w o u l d be a t t h e optimum p e r c e n t a g e o f t h e l i g h t a b s o r b ­ ed by t h e s o l u t i o n , no p h o t o c r o s s I i n k i n g was o b s e r v e d . Also, in q u a r t z t u b e s a t 275 nm a n d 236 nm ( i c e - b a t h c o o l i n g c o u l d n o t be used i n t h e s e c a s e s s i n c e a s u i t a b l e q u a r t z v e s s e l was n o t on h a n d ) , no p h o t o c r o s s I i n k i n g was o b s e r v e d . In t h e l i g h t o f s u b s e ­ quent f i n d i n g s t h a t t h e r e a c t i o n e f f i c i e n c y decreases markedly w i t h decreased l i g h t i n t e n s i t y , these r e s u l t s a r e adequately

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a c c o u n t e d f o r , s i n c e t h e 275 nm and 236 nm i n t e n s i t i e s a r e much weaker t h a n t h e 334 nm l i n e , w h i l e t h e 366 nm l i n e i s o n l y a b o u t 5 t o 6 t i m e s a s i n t e n s e a s t h e 334 nm l i n e and o n l y a s m a l l f r a c ­ t i o n i s a b s o r b e d . However, some u n e x p l a i n e d w a v e l e n g t h e f f e c t s a r e d e s c r i b e d below. When t h e measurement o f quantum y i e l d s i n t h e c h l o r a n i l s y s ­ tem was t o be begun, i t was d e c i d e d t o u s e t h e 334 nm l i n e r a t h e r t h a n 366 nm s i n c e w i t h t h e h i g h e f f i c i e n c y o b s e r v e d i n t h e t e s t r u n w i t h c h l o r a n i l a t 366 nm, i t a p p e a r e d d e s i r a b l e t o u s e a l e s s i n t e n s e e x c i t a t i o n band i n o r d e r t o have more c o n t r o l l a b l e r e a c ­ t i o n t i m e s , a s w e l l a s t o u s e t h e same w a v e l e n g t h used i n t h e TCPA s y s t e m f o r c o n v e n i e n c e and d i r e c t c o m p a r i s o n . S u r p r i s i n g l y , no p h o t o c r o s s I i n k i n g w a t s o e v e r c o u l d be i n i t i a t e d a t 334 nm i n t h r e e d i f f e r e n t c h l o r a n i l s a m p l e s p r e p a r e d . However, when t h e e x c i t a ­ t i o n w a v e l e n g t h was changed t o 366 nm, t h e same s a m p l e s p r o d u c e d photocross Iinking very r a p i d l y . This observation i s very puzzling and d e s e r v e s f u r t h e r i n v e s t i g a t i o n . Experimental A c c e p t o r compounds (TCPA, c h l o r a n i l , 1,3,5-triηitrobenzene, b e n z o q u i n o n e , p h t h a l l i c a n h y d r i d e , and 1 , 3 , 5 - t r i c y a n o b e n z e n e ) were r e c r y s t a I I i z e d and s u b l i m e d . Acetonîtrîle (MaI I i n c k r o d t r e a g e n t ) was d i s t i l l e d f r o m P2O5 and r e d i s t i l l e d , r e t a i n i n g m i d d l e c u t s . D i c h l o r o m e t h a n e (MaI I i n c k r o d t r e a g e n t ) was d r i e d o v e r c a l c i u m h y d r i d e . X V I I I was p r e p a r e d by Shaw Chu (27) and has a m o l e c u l a r w e i g h t o f 110,000 by membrane osmometry. X I X was p r e p a r e d a c c o r d ­ i n g t o t h e p r o c e d u r e o f J.W. S c h w i e t e r t ( 3 0 ) . The i r r a d i a t i o n s o u r c e i s a 2500 Watt M e r c u r y - X e n o n lamp ( H a n o v i a t y p e 929B-9U) c o n t a i n e d i n t h e S c h o e f f e l LH 152N/2 Lamp Housing ( s u p p l i e d w i t h 2 i " diameter v a r i a b l e focus double q u a r t z c o n d e n s o r , p a r a b o l i c r e f l e c t o r , c o o l i n g f a n , and f i n n e d h e a t s i n k s f o r t h e a r c l a m p ) . The o u t p u t beam i s d e f l e c t e d t h r o u g h a S c h o e f f e l LHA 165/2 S t r a y L i g h t R e d u c i n g & I l l u m i n a t i o n P r e d i s p e r s i o n P r i s m A s s e m b l y i n t o a S c h o e f f e l GM 250 H i g h I n t e n s i t y Monoc h r o m a t o r ( f o c a l l e n g t h 0.25 M., l i n e a r d i s p e r s i o n 3.2 nm/mm., g r a t i n g b l a z e d a t 300 nm w i t h 1180 grooves/mm., a p e r t u r e r a t i o f / 3 . 9 ) . The power s u p p l y f o r t h e lamp i s a S c h o e f f e l LPS 400 e q u i p p e d w i t h t h e S c h o e f f e l LPS 400S s t a r t e r , w h i c h o p e r a t e t h e lamp a t 50 V and 50 A. A l e n s s u p p l i e d by S c h o e f f e l f o c u s e s t h e e x i t beam o f t h e monochromator i n t o a v e r t i c a l n a r r o w l i n e s u c h t h a t o n l y t h e c e n t e r p o r t i o n o f a sample tube i s i r r a d i a t e d . Sample v e s s e l s were e i t h e r P y r e x t u b e s o f 11MM. o . d . (9mm. i . d . ) o r 199 mm. o . d . (16 mm. I.d.) o r q u a r t z t u b e s o f 12 mm. o . d . (10 mm. i . d . ) s e a l e d t o g r a d e d s e a l s . The p r e p a r a t i o n o f s a m p l e s was a s f o l l o w s . An a l i q u o t o f a s o l u t i o n o f t h e p o l y m e r i n d r y acetonîtrîle was e v a p o r a t e d t o a f i l m i n t h e sample t u b e on a vacuum l i n e , h e a t e d t o a b o u t 80° b r i e f l y , and d r i e d f o r o n e h o u r . The t u b e was removed f r o m t h e vacuum l i n e , weighed a c c e p t o r ( a n d any n o n v o l a t i l e a d d i t i v e ) were a d d e d , t h e t u b e reevâcuated f o r o n e

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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3.

BUTLER AND FERRÉE

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h o u r , and e x c e s s d r y acetonîtrîle t r a n s f e r r e d f r o m P 2 O 5 by c o o l i n g t h e t u b e i n l i q u i d n i t r o g e n on t h e vacuum l i n e a t 10~4 f o r r o r less. The f r o z e n s o l i d i s m e l t e d and e x c e s s s o l v e n t e v a p o r a t e d a t 25° u n t i l t h e marked v o l u m e l e v e l i s r e a c h e d , a n d t h e t u b e sealed o f f w i t h a flame. F o r experiments i n which v o l a t i l e a d d i ­ t i v e s s u c h a s t r i e t h y l a m i n e were u s e d , a measured a l i q u o t o f t h e a d d i t i v e i n d r y acetonîtrîle was d r i e d o v e r c a l c i u m h y d r i d e , d e g a s s e d by f r e e z e - t h a w c y c l e s , and t r a n s f e r r e d i n t o t h e sample t u b e o n t h e vacuum l i n e . S i m i l a r l y , f o rtesting the e f f e c t of H2O, measured a l i q u o t s o f m i x t u r e s o f H2O i n d r y acetonîtrîle were d e g a s s e d and c o n d e n s e d i n t o t h e s a m p l e t u b e s on t h e vacuum l i n e . F o r t e s t i n g t h e e f f e c t o f a i r , t h e s a m p l e was p r e p a r e d a s a b o v e , a i r a d m i t t e d t h r o u g h a d r y i n g t u b e (CaS04) and t h e sample s e a l e d off. The sample t u b e s were u s u a l l y i r r a d i a t e d w h i l e immersed i n an i c e b a t h . The l i g h t beam f o r m s an image 24 mm. by 4 mm. on t h e c e n t e r o f t h e sample s o l u t i o n . F o r experiments i n which polymer formed on t h e i r r a d i a t e d w a l l , t h e sample t u b e was t u r n e d t o a f r e s h s u r f a c e a t c o n s t a n t , f r e q u e n t i n t e r v a l s , u s u a l l y 2.0 m i n . , w h i l e u s e o f i n t e r v a l s o f t w i c e t h i s l o n g were f o u n d t o y i e l d n e a r l y equal c r o s s l i n k i n g e f f i c i e n c i e s . P r e c i p i t a t e was measured by w e i g h t a f t e r f i l t r a t i o n , w a s h i n g w i t h a l a r g e q u a n t i t y o f a c e t o n e , and d r y i n g t w o h o u r s u n d e r vacuum a t 5 5 ° . L i g h t i n t e n s i t i e s were measured by p o t a s s i u m f e r r i o x a l a t e actinometry (31). ( N o t e on f u t u r e e x p e r i m e n t a l : F i l t r a t i o n may be d i f f i c u l t , but i s b e s t performed w i t h c o u r s e s i n t e r e d g l a s s f u n n e l s , which s h o u l d be c l e a n e d by d r a w i n g s o l v e n t b a c k w a r d s t h r o u g h them, n o t using a c i d . Sample t u b e s were f u s e d t o g e t h e r b e f o r e f i n a l c l e a n ­ i n g , s o a k e d o v e r n i g h t w i t h d i c h r o m a t e c l e a n i n g s o l u t i o n , washed w i t h w a t e r , l e t s t a n d s e v e r a l h o u r s c o n t a i n i n g d i l u t e (1-2#) KOH s o l u t i o n , r i n s e d w i t h d i s t i l l e d H 2 O , and d r i e d i n an o v e n . ) Literature

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