Radiation Curing of Polymeric Materials - ACS Publications

Chapter 8

Novel Photoinitiator for Modern Technology

Downloaded by DICLE UNIV on November 9, 2014 | http://pubs.acs.org Publication Date: December 28, 1990 | doi: 10.1021/bk-1990-0417.ch008

V. Desobry, K. Dietliker, R. Hüsler, L. Misev, M. Rembold, G. Rist, and W. Rutsch Additive Research, Ciba-Geigy Ltd., CH-1701 Fribourg, Switzerland

2-Benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1 (BDMB) has been synthesized and shown to be an efficient photoinitiator for UV curing applications. Photochemical and CIDNP investigations suggest that photochemical decomposition occurs mainly via α-clea vage. In comparison with other photoinitiators, BDMB provided superior results in pigmented systems and ima ging applications.

Advances i n t h e f i e l d o f U V - c u r i n g n e c e s s a r i l y engender t h e d e v e l o p ment o f s p e c i a l i z e d p h o t o i n i t i a t o r s t h a t meet t h e s p e c i f i c needs o f new a p p l i c a t i o n s . Whereas t h e p r e p o l y m e r d e t e r m i n e s many p h y s i c a l c h a r a c t e r i s t i c s o f t h e c u r e d f i l m such as g l o s s , h a r d n e s s , s o l v e n t and s c r a t c h r e s i s t a n c e , e t c . , t h e p h o t o i n i t i a t o r must e n s u r e t h e p r o per c u r i n g o f t h e f i l m . The s e l e c t i o n o f t h e p h o t o i n i t i a t o r i s e s p e c i a l l y i m p o r t a n t when a b s o r b i n g s p e c i e s such as pigments o r s t a b i l i z e r s a r e added t o t h e p r e p o l y m e r . Our r e s e a r c h i n t h i s a r e a has f o c u s e d upon t h e t a i l o r i n g o f t h e p h o t o i n i t i a t o r not only t o the formulation but also t o the c u r r e n t l y a v a i l a b l e l i g h t s o u r c e s . I n a p r e c e e d i n g p u b l i c a t i o n [ 1_], we d i s c u s sed t h e s p e c i f i c a p p l i c a t i o n s o f t h r e e s t r u c t u r a l l y d i s t i n c t p h o t o i n i t i a t o r s . We now p r e s e n t a new a l p h a - c l e a v a g e p h o t o i n i t i a t o r 2 - b e n z y l - 2 - d i m e t h y l a m i n o - 1 - ( 4 - m o r p h o l i n o p h e n y l ) - b u t a n o n e - 1 (BDMB) which p r o m i s e s g r e a t u t i l i t y i n v a r i o u s b r a n c h e s o f t h e g r a p h i c a r t s and p r i n t i n g t e c h n o l o g y .

Synthesis

and P r o p e r t i e s o f BDMB

Synthesis. The s y n t h e s i s o f BDMB and i t s a n a l o g s r e q u i r e s an e f f i c i e n t strategy f o r the construction o f the amino-substituted quater-

0097-6156/90/0417-0092$06.00/0 ο 1990 American Chemical Society In Radiation Curing of Polymeric Materials; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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n a r y c e n t e r , οό ,οό - D i s u b s t i t u t e d amine-ketones c a n n o t be p r e p a r e d by the s i m p l e r e a c t i o n o f amines w i t h t h e c o r r e s p o n d i n g oL -bromoketones. While db , cC-dimethyl d e r i v a t i v e s a r e e a s i l y o b t a i n e d by t h e a d d i t i o n o f m o r p h o l i n e t o i n t e r m e d i a t e e p o x y e t h e r [,2] [3], t h i s t r a n s f o r m a t i o n f a i l s when t h e e p o x y e t h e r i s s u b s t i t u t e d w i t h s t e r i c a l l y demanding a l k y l g r o u p s . To c i r c u m v e n t t h i s problem, we employed t h e i n t r a m o l e c u l a r Stevens r e a r r a n g e m e n t [4^ [5] t o c r e a t e t h e q u a t e r n a r y c e n t e r , thereby g a i n i n g access t o a g r e a t v a r i e t y o f h i g h l y s u b s t i t u t e d aC -aminoketones (see F i g u r e 1) [ 6 ^ . Absorption C h a r a c t e r i s t i c s . C l e a r l y , o n l y compounds h a v i n g s t r o n g a b s o r p t i o n s i n t h e e m i s s i o n r a n g e o f t h e l i g h t s o u r c e c a n s e r v e as e f f i c i e n t p h o t o i n i t i a t o r s . T h i s allows the d i r e c t e x c i t a t i o n o f t h e p h o t o i n i t i a t o r t o an e x c i t e d s t a t e where i t s e f f i c i e n t c o n v e r s i o n t o r e a c t i v e s p e c i e s ( r a d i c a l s i n t h e case o f a l p h a - c l e a v a g e type p h o t o i n i t i a t o r s ) i s e s s e n t i a l . The a b s o r p t i o n s p e c t r a o f two commercial p h o t o i n i t i a t o r s BDK (I) and MMMP ( I I ) [J] as w e l l as BDMB a r e r e p r o duced i n F i g u r e 2. I t i s a p p a r e n t t h a t BDMB, which e x h i b i t s a s t r o n g a b s o r p t i o n a t 322 nm., 16 nanometers h i g h e r than t h e s t r u c t u r a l l y r e l a t e d I I , i s t h e p h o t o i n i t i a t o r which b e s t matches t h e e m i s s i o n l i n e s o f t h e medium p r e s s u r e mercury lamp. These a b s o r p t i o n c h a r a c t e r i s t i c s a l s o a l l o w i t s a p p l i c a t i o n i n t h e c u r i n g o f pigmented systems as w e l l as i n r e s i s t f o r m u l a t i o n s and f l e x o g r a p h i c p r i n t i n g p l a t e s .

Photochemi s t r y The p h o t o c h e m i s t r y o f οί> -amino acetophenone d e r i v a t i v e s has been shown by v a r i o u s groups t o be s t r o n g l y dependent upon t h e oC - c a r b o n and n i t r o g e n s u b s t i t u e n t s . U n s u b s t i t u t e d oC - ( d i a l k y l a m i n o ) - a c e t o p h e nones ( - C O - C H ^ - N i a l k y l ) ) undergo an e f f i c i e n t e l i m i n a t i o n r e a c t i o n upon i r r a d i a t i o n ( F i g u r e 3a) t o a f f o r d acetophenone and i m i n e s as t h e s o l e p r o d u c t s [8] [9] [103. However, when t h e l o n e p a i r on t h e amine i s a p a r t o f a 7T-system (N-Acyl [8] [1J_] [ 1_2], N - T o s y l [V2] o r N-Phenyl [1_3] M 4 ] " a z e t i d i n o l s a r e o b t a i n e d , v i a c y c l i z a t i o n o f an i n t e r m e d i a t e 1 , 4 - d i r a d i c a l , u n l e s s s t e r i c f a c t o r s p r e v e n t t h e f o r m a t i o n o f t h e four-membered r i n g ( F i g u r e 3b). These r e s u l t s , and t h e o b s e r v a t i o n t h a t t h e p h o t o e l i m i n a t i o n o f où-(dialkylamino)acetophenones i s n o t s u p p r e s s e d by t h e s t a n d a r d t r i p l e t quenchers c a n be e x p l a i n e d by a mechanism i n v o l v i n g e l e c t r o n t r a n s f e r from t h e amine t o t h e c a r b o n y l group [8] ( F i g u r e 3 a ) . Lower i n g t h e i o n i z a t i o n p o t e n t i a l o f t h e amine, i . e . by a c e t y l a t i o n , s u l f o n y l a t i o n , e t c . , d i s f a v o r s t h e e l e c t r o n t r a n s f e r pathway and r e s u l t s iny-hydrogen abstraction. I r r a d i a t i o n o f N-phenylacetophenones ( F i g u r e 3c) f u r n i s h e s p r o d u c t s r e s u l t i n g from d i r e c t ^ - c l e a v a g e [j_3] [J_4]. G e m i n a l l y d i s u b s t i t u t e d d e r i v a t i v e s (R* = CH^) undergo p h o t o l y t i c d e c o m p o s i t i o n v i a b o t h oC - and β - c l e a v a g e pathways [V3]. Predominant où - c l e a v a g e i s o b s e r v e d upon p h o t o l y s i s o f I I [_3] [ 1 5 ] . T h i s t r e n d i n r e a c t i v i t y c a n be e x p l a i n e d by t h e i n t e r p l a y o f two f a c t o r s : 2

) 3

In Radiation Curing of Polymeric Materials; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


RADIATION CURING O F P O L Y M E R I C MATERIALS

Figure 1. Synthesis of BDMB.

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Figure 2. Absorption spectra of benzildimethylketal (BDK), 4-methyl-thiophenyl-2morpholino-2-methyl-propanone-l (MMMP) and BDMB (concentration: 0,001 % in methanol).


DESOBRY ET AL.

Novel Photoinitiatorfor Modem Technology

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i n c r e a s i n g s u b s t i t u t i o n a t t h e cC - c a r b o n f a v o r s $6 - c l e a v a g e due t o t h e weakening o f t h e acyl-οί-carbon bond [_^6] ; and 2. e l e c t r o n d o n a t i n g groups ( a l k o x y , amino, e t c . ) i n t h e ^ - p o s i t i o n c a n s t a b i l i z e t h e t r a n s i t i o n s t a t e o f où - c l e a v a g e by i n t e r a c t i o n o f a lone p a i r o f electrons with the breaking c a r b o n - c a r b o n bond [ J_7 ] . In view o f t h e continuum o f r e a c t i v i t y e x h i b i t e d by t h i s c l a s s o f compounds, t h e p h o t o c h e m i s t r y o f BDMB was i n v e s t i g a t e d . CIDNP Spectrum o f BDMB. The CIDNP t e c h n i q u e c a n p r o v i d e much u s e f u l i n f o r m a t i o n on p r o c e s s e s i n which r a d i c a l s p e c i e s a r e formed and h a s been u s e d t o d e t e r m i n e t h e s p e c i e s r e s p o n s i b l e f o r t h e i n i t i a t o r a c t i v i t y o f compounds s u c h a s I and I I [2] [3] [Ji_8] [ J _ 9 ] . The NMR s p e c trum t a k e n d u r i n g p h o t o l y s i s o f BDMB was o b t a i n e d under s i m i l a r c o n d i t i o n s as employed f o r t h e s t u d y o f I and I I [ ^ 0 ] [ 2 1 _ ] . The enhanced s i n g l e t a b s o r p t i o n a t 9 , 6 3 ppm ( F i g u r e 4 ) i s a s s i g n e d t o 4-morphol i n o b e n z a l d e h y d e I I I which must r e s u l t from an i n i t i a l oi - c l e a v a g e o f BDMB t o engender a b e n z o y l and a m i n o a l k y l r a d i c a l p a i r ( F i g u r e 5 ) . The a l d e h y d e p o l a r i z a t i o n , a c c o r d i n g t o K a p t e i n ' s r u l e s [ 2 ! 2 ] , and t h e ESR p a r a m e t e r s f o r t h e s e two r a d i c a l s a r e i n f u l l agreement w i t h a t r i p l e t s t a t e p r e c u r s o r . By comparing t h e s p e c t r a o b t a i n e d i n d i f f e r e n t s o l v e n t s , i t was d e t e r m i n e d t h a t t h e r a d i c a l p a i r i s formed e s s e n t i a l l y v i a an u n i m o l e c u l a r p r o c e s s , t h u s r u l i n g o u t t h e p h o t o l y t i c d e c o m p o s i t i o n o f BDMB v i a i n t e r m o l e c u l a r e l e c t r o n t r a n s f e r o r o t h e r b i m o l e c u l a r r e a c t i o n s . The two q u a r t e t s a t 4 . 4 6 and 4 . 6 2 ppm which e x h i b i t enhanced a b s o r p t i o n s a r e a s s i g n e d t o t h e o l e f i n i c p r o t o n s o f IVa and b . The s i n g l e t s a t 5 . 2 8 ppm ( e m i s s i o n ) and 5 . 3 3 ppm (enhanced a b s o r p t i o n ) a r e a t t r i b u t e d t o t h e o l e f i n i c p r o t o n s i n Va and b. The p a t t e r n o f t h e s e s i g n a l s i s due t o t h e c o m b i n a t i o n o f cage r e a c t i o n s l e a d i n g t o p r o d u c t s e x h i b i t i n g a b s o r p t i o n p o l a r i z a t i o n and escape r e a c t i o n s f u r n i s h i n g t h e same p r o d u c t s b u t e x h i b i t i n g e m i s s i o n pol a r i z a t i o n . The spectrum i n d e u t e r a t e d c y c l o h e x a n e ( F i g u r e 4 ) and t h e CIDNP e x p e r i m e n t s i n o t h e r s o l v e n t s l e a d t o t h e c o n c l u s i o n t h a t t h e f o r m a t i o n o f Va and b o c c u r s p r e f e r e n t i a l l y v i a e s c a p e r e a c t i o n s whereas f o r t h e p o l a r i z a t i o n s o f I V a and b t h e c o n t r i b u t i o n o f t h e cage r e a c t i o n i s s l i g h t l y l a r g e r . P r e p a r a t i v e Photochemistry. I r r a d i a t i o n o f BDMB on a p r e p a r a t i v e s c a l e [ 2 3 ^ ] l e a d s t o a p r o d u c t m i x t u r e t h a t would be e x p e c t e d b a s e d upon t h e r e s u l t s o f t h e CIDNP e x p e r i m e n t s ( F i g u r e 6 ) . I n benzene, t h e main p h o t o p r o d u c t s a r e 4 - m o r p h o l i n o b e n z a l d e h y d e I I I ( 2 1 %) and 1 - p h e n y l - b u t a n - 2 - o n e V I I ( 3 2 % ) . The l a t t e r compound i s b e l i e v e d t o a r i s e from h y d r o l y s i s o f t h e i n i t i a l l y formed enamines d u r i n g workup. On t h e b a s i s o f o u r e x p e r i m e n t s , however, o t h e r pathways f o r i t s f o r m a t i o n c a n n o t be c o m p l e t e l y r u l e d o u t . The deaminated d e r i v a t i v e V I I I was a l s o i s o l a t e d i n 1 1 % y i e l d i n d i c a t i n g t h a t competing r e a c t i o n s - i . e . N o r r i s h type I I o r d i r e c t -cleavage - a l s o take p l a c e . S u r p r i s i n g l y N,N-dimethyl-4-morpholinobenzamid V I was i s o l a t e d i n 1 9 %. The mechanism o f t h e f o r m a t i o n o f VI w i l l be t h e s u b j e c t o f subsequent i n v e s t i g a t i o n s . I r r a d i a t i o n o f BDMB i n i s o p r o p a n o l a f f o r d s t h e same p r o d u c t s a l b e i t i n s l i g h t l y d i f f e r e n t r a t i o s . D o d e c a n t h i o l IX has been o f t e n u s e d a s a s c a v e n g e r f o r non-cage b e n z o y l r a d i c a l s [ 2 £ ] [ 2 5 J . I r r a d i a t i o n o f BDMB i n benzene w i t h a



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Novel Photoinitiator for Modem Technology

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RADIATION CURING OF POLYMERIC MATERIALS


l a r g e e x c e s s o f IX a f f o r d e d I I I i n 88 % y i e l d ( F i g u r e 7) as w e l l as p r o d u c t s r e s u l t i n g from t h e r e a c t i o n o f t h e b e n z o y l r a d i c a l w i t h 1 - d o d e c a n t h i y l r a d i c a l s (X, 6 % ) . The t r a p p i n g p r o d u c t o f the ^ a m i n o a l k y l r a d i c a l was a l s o o b t a i n e d (XI, 50 %). The d e c o m p o s i t i o n o f v a r i o u s p h o t o i n i t i a t o r s i n t o c a r b o n c e n t e r e d r a d i c a l s has a l s o been i l l u s t r a t e d by Hageman [25] who used TMPO ( 2 , 2 , 6 , 6 - t e t r a m e t h y l p i p e r i d i n o x y l ) as a t r a p p i n g agent. A d d i t i o n o f a t h r e e f o l d e x c e s s o f TMPO t o a s o l u t i o n o f BDMB r e s u l t s i n t h e f o r m a t i o n o f X I I i n h i g h y i e l d (91 %) ( F i g u r e 8). V I I c o u l d a l s o be i s o l a t e d . T h i s p r o d u c t may a r i s e f r o m an i n s t a b l e p r i m a r y a d d i t i o n p r o d u c t ( X I I I ) o f TMPO and t h e où -amino-alky1 r a d i c a l which i s h y d r o l y sed d u r i n g work-up. These r e s u l t s a l s o c o n f i r m t h a t the dominant pathway o f d e c o m p o s i t i o n i s t h a t o f où - c l e a v a g e . T r a p p i n g R e a c t i o n s w i t h 2 - t - B u t y l a c r y l i c A c i d M e t h y l e s t e r XIV. This t r a p p i n g r e a c t i o n , which mimics t h e i n i t i a t i o n s t e p o f t h e p o l y m e r i s a t i o n p r o c e s s , has been used t o o b t a i n i n f o r m a t i o n on the r e a c t i v i t y o f t h e p r i m a r y r a d i c a l s formed upon i r r a d i a t i o n [J_] [_26 ]. P h o t o l y s i s o f BDMB i n t h e p r e s e n c e o f a t h r e e f o l d e x c e s s o f XIV a f f o r d s t h e benz o y l d e r i v a t i v e XV i n 87 % y i e l d ( F i g u r e 9). A g a i n V I I was a l s o i s o l a t e d (64 % ) , whereas no s t a b l e a d d i t i o n p r o d u c t o f cC - a m i n o a l k y l r a d i c a l c o u l d be i d e n t i f i e d . T h i s r e s u l t s u g g e s t s t h a t t h e b e n z o y l r a d i c a l i s m a i n l y r e s p o n s i b l e f o r t h e p o l y m e r i s a t i o n o f v i n y l i c monomers and i s i n agreement w i t h p r e v i o u s s t u d i e s on b e n z i l k e t a l s and b e n z o i n e t h e r s [27]. But, as où - a m i n o a l k y l r a d i c a l s have a l s o been shown t o i n i t i a t e a c r y l a t e p o l y m e r i z a t i o n [28], f u r t h e r i n v e s t i g a t i o n s w i l l be d e v o t e d t o t h e e l u c i d a t i o n o f the r o l e o f t h i s p r i m a r y photoproduct i n the o v e r a l l polymerisation process. A p p l i c a t i o n S t u d i e s w i t h BDMB White Pigmented L a c q u e r . BDMB was i n c o r p o r a t e d i n t o t h e f o r m u l a t i o n a t c o n c e n t r a t i o n l e v e l s up t o 5 %, and t h e r e s u l t i n g low v i s c o s i t y l a c q u e r was a p p l i e d t o an aluminium f o i l t o form a 30 g/m f i l m . C u r i n g was a c h i e v e d by p a s s i n g t h e sample t h r o u g h a c o n v e y o r system e q u i p p e d w i t h two 80 watt/cm medium p r e s s u r e mercury lamps. The r e a c t i v i t y o f t h e U V - c u r a b l e c o a t i n g was q u a n t i f i e d i n terms o f t h e m a x i mum c o n v e y o r b e l t speed y i e l d i n g s u f f i c i e n t c u r e . The l a t t e r i s d e t e r m i n e d by t h e r e s i s t a n c e o f t h e c o a t i n g a g a i n s t r u b b i n g w i t h t i s s u e paper. As d e m o n s t r a t e d i n F i g u r e 10, BDMB m a r k e d l y exceeds i t s s t r u c t u r a l a n a l o g I I i n p h o t o p o l y m e r i z a t i o n r e a c t i v i t y . Even though a d d i t i o n o f an e f f i c i e n t t r i p l e t s e n s i t i z e r , i s o p r o p y l - t h i o x a n t h o n e (ITX) b o o s t s t h e performance o f t h e l a t t e r by a f a c t o r o f s i x , i t s r e a c t i v i t y i s s t i l l i n f e r i o r t o t h a t o f BDMB. T h i s o b s e r v a t i o n c a n be r a t i o n a l i z e d by comparing t h e a b s o r p t i o n s p e c t r a o f BDMB, I I , ITX and t i t a n i u m d i o x i d e . W h i l e I I i s e f f i c i e n t l y s c r e e n e d from i n c i d e n t UV r a d i a t i o n by t i t a n i u m d i o x i d e , BDMB s t i l l a b s o r b s above t h e c u t - o f f w a v e l e n g t h o f t i t a n i u m d i o x i d e . The marked i n c r e a s e i n r e a c t i v i t y o f II i n t h e p r e s e n c e o f ITX r e s u l t s from t r i p l e t s e n s i t i z a t i o n by ITX [26] w h i c h a l s o a b s o r b s a t l o n g e r w a v e l e n g t h t h a n t i t a n i u m d i o x i d e .


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