Photocrosslinkable Polymers

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.

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

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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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 .



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BUTLER AND FERRÉE

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


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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 .



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BUTLER AND FERRÉE


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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


UV


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,


3.

BUTLER A N D FERRÉE


25


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. -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


3.

BUTLER AND FERRÉE


27


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


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


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


3.

BUTLER AND FERRÉE

29


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


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


UV

30


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.)


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



3.

BUTLER AND FERRÉE

PhotocwssUnkable Polymers

31

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


3. BUTLER AND FERRÉE


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

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



3.

BUTLER AND FERRÉE

Photocwsslinkable Polymers

35

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

Cited

1. Hartmann, H.; Krauch, CH.; and Marx, M.; Ger. Of fen. (Jul.9, 1970), 1,813,011. Chem. Abst. 74, 78650 (1970). 2. Beightol, L.E.; B r i t . 1,245,937, Sep. 15, 1971. Chem. Abst. 76, 26114 (1972). 3. Agnihotri, R.K.; Falcon, D.L.; Hood, F.P.; Lesoine, L.G.; Needham, C.D.; and Offenbach, J.Α.; Tech. Pap., Reg. Tech. Conf., Soc. Plast. Eng., Mid-Hudson Sect. (1970, Oct. 15-16), 33 4. Kolbe, G.; Schmidt, Α.; Schmitz-Josten, R; and Wolff, E.; Ger. Offen. 2,019,598, Nov. 11, 1971. Chem. Abst. 76, 73294 (1972). 5. Minsk, L.M.; Van Deusen, W.P.; and Robertson, E.M.; U.S. Pat. 2,610,120 (1952). 6. Delzenne, G.A.; J . Mac. S c i . , Rev. Polym. Technol. (1971), 185.


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7. 8. 9. 10.


11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.

UV L I G H T INDUCED REACTIONS IN

POLYMERS

Nishikubo, T.; Watauchi, T.; Maki, Κ.; and Takaoka, T.; Nippon Kagaku Kaishi; (1972); 9, 1626. Taditomi, Ν.; Ichijyo, T.; and Takaoka, T.; Nippon Kagaku Kaishi; (1973), 10, 35. Kato, M.; Ichijo, T.; I s h i i , K.; and Hasegawa, M.; J . Polym. Sci.(1971); 9; (A-1); 2109. Lyalikov, K.S.; Gaeva, G.L.; and Evlasheva, N.A.; Tr. Leningrad Inst. Kinoinzh. (1970). No. 16, 42. Oster, G.; Encyclopedia of Polymer S c i . & Tech.; (1969); 10; 145. Tsuda, M.; J . Polym. S c i . ; (1969); 7; 259. Deboer, C.D.; J . Polym. S c i . (1973); 11; (B); 25. Deboer, C.D.; Wadsworth, D.H.; and Perkins, W.C.; J . Amer. Chem. Soc.; (1973); 95, 861. Hyde, P.; Kricka, L.J.; and Ledwith, Α.; J . Polym. S c i . ; (1973); 11 (B); 415. Stuber, F.A.; U l r i c h , H.; Rao, D.V.; and Sayigh, A.A.R; J . Appl. Polym. S c i . (1969(; 13; 2247. Tsunoda, T.; Yameoka, T.; Nagamatsu, G.; and Hirohashi, M.; Tech. Pap., Reg. Tech. Conf., Soc. Ρlast. Eng., Mid-Hudson Sect.; (1970); 25. Takeishi, M. and Okawara, M., J . Polym. S c i . , (1969), 7, (b), 201. Yabe, Α.; Tsuda, M.; Honda, K.; Tanaka, H.; J . Polym. S c i . ; (1972); 10; (A-1); 2379. Ichijo, T.; Nishikubo, T.; and Maki, Κ.; Japan Kokai; (Oct. 7, 1972); 72; 22,490. Chem. Abst. 78, 30778 (1973). David, C.; Demarteau, W.; and Geuskins, G.; Polymer (1969); 10; 21. Kleeburg, W.; Rubner, R.; and Kuehn, E.; Ger. Offen. 130,904; Dec. 28, 1972. Chem. Abst. 78, 91045 (1973). Hrabak, F.; Bezdek, M.; Hynkova, V.; and Bouchal, Κ.; Ger. Offen. 2,150,076; Apr. 13, 1972. Chem. Abst. 77., 49485 (1972). Hay, A.S.; Bolon, D.A.; Leimer, K.R.; and Clark, R.F.; J . Polym. S c i . ; (1970); 8; (B); 97. Nishikubo, T.; Ichiiyo, T.; and Takaoka, T.; J . Applied Polymer S c i . ; (1974); 18; 2009. Hasegawa, M.; Susuki, Y.; Nakanishi, H.; and Nakanishi, F.; "Progress in Polymer Science, Kodansha-Tokyo," Vol. 5; pp. 143-210; John Wiley & Sons; New York; 1973. Chu, Shaw, Ph.D. Dissertation, University of Florida, 1974. "Photochemistry,: Calvert & P i t t s . Sadtler Standard Infrared Spectra No. 4291. Schwietert, J.W.; Ph.D. Dissertation; University of Florida; 1971. Hatchard, Parker, Proc. Roy. Soc. (London) A235, 518 (1956).


Photocrosslinkable Polymers

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