[ACS Symposium Series] Radiation Curing of Polymeric Materials Volume 417 || Novel Photoinitiator for Modern Technology

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  • Chapter 8

    Novel Photoinitiator for Modern Technology

    V. Desobry, K. Dietliker, R. Hsler, 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)-bu-tanone-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 -cleavage. In comparison with other photoinitiators, BDMB provided superior results in pigmented systems and imaging applications.

    Advances i n the f i e l d of UV-curing n e c e s s a r i l y engender the development of 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 that meet the s p e c i f i c needs of new a p p l i c a t i o n s . Whereas the prepolymer determines many p h y s i c a l c h a r a c t e r i s t i c s of the cured f i l m such as g l o s s , hardness, solvent and scratch r e s i s t a n c e , e t c . , the p h o t o i n i t i a t o r must ensure the proper c u r i n g of the f i l m . The s e l e c t i o n of the p h o t o i n i t i a t o r i s espec i a l l y important when absorbing species such as pigments or s t a b i l i zers are added to the prepolymer.

    Our research i n t h i s area has focused upon the t a i l o r i n g of the p h o t o i n i t i a t o r not only to the formulation but a l s o to the c u r r e n t l y a v a i l a b l e l i g h t sources. In a preceeding p u b l i c a t i o n [ 1_], we d i s c u s sed the s p e c i f i c a p p l i c a t i o n s of three s t r u c t u r a l l y d i s t i n c t photo-i n i t i a t o r s . We now present a new alpha-cleavage p h o t o i n i t i a t o r 2-benzyl -2-dimethylamino -1-(4-morpholinophenyl)-butanone -1 (BDMB) which promises great u t i l i t y i n various branches of the graphic a r t s and p r i n t i n g technology.

    Synthesis and Prope r t i e s of BDMB

    Synthesis. The synthesis of BDMB and i t s analogs re q u i r e s an e f f i c i e n t strategy f o r the co n s t r u c t i o n of the amino-substituted quater-

    0097-6156/90/0417-0092$06.00/0 1990 American Chemical Society

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

  • 8. DESOBRY ET AL. Novel Photoinitiator for Modern Technology 93

    nary center, , - D i s u b s t i t u t e d amine-ketones cannot be prepared by the simple r e a c t i o n of amines with the corresponding oL -bromoketones. While db , cC-dimethyl d e r i v a t i v e s are e a s i l y obtained by the a d d i t i o n of morpholine to intermediate epoxyether [,2] [3], t h i s transformation f a i l s when the epoxyether i s s u b s t i t u t e d with s t e r i c a l l y demanding a l k y l groups. To circumvent t h i s problem, we employed the intramolec u l a r Stevens rearrangement [4^ [5] to create the quaternary center, thereby gaining access to a great v a r i e t y of h i g h l y s u b s t i t u t e d aC -aminoketones (see Figure 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 , only compounds having strong absorptions i n the emission range of the l i g h t source can serve 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 . This allows the d i r e c t e x c i t a t i o n of the p h o t o i n i t i a t o r to 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 conversion to r e a c t i v e species ( r a d i c a l s i n the case of alpha-cleavage type photo-i n i t i a t o r s ) i s e s s e n t i a l . The absorption spectra of two commercial p h o t o i n i t i a t o r s BDK (I) and MMMP (II) [J] as wel l as BDMB are reproduced i n Figure 2. I t i s apparent that BDMB, which e x h i b i t s a strong absorption at 322 nm., 16 nanometers higher than the s t r u c t u r a l l y r e l a t e d I I , i s the p h o t o i n i t i a t o r which best matches the emission l i n e s of the medium pressure mercury lamp. These absorption c h a r a c t e r i s t i c s a l s o allow i t s a p p l i c a t i o n i n the curin g of pigmented systems as wel l as i n r e s i s t formulations and flex 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 photochemistry of > -amino acetophenone d e r i v a t i v e s has been shown by various groups to be st r o n g l y dependent upon the oC -carbon and nitrogen s u b s t i t u e n t s . Unsubstituted oC -(dialkylamino)-acetophe-nones (-CO-CH^-Nialkyl) 2) 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 (Figure 3a) to a f f o r d acetophenone and imines as the sole products [8] [9] [103.

    However, when the lone p a i r on the amine i s a par t of a 7T-sy-stem (N-Acyl [8] [1J_] [ 1_2], N-Tosyl [V2] or N-Phenyl [1_3] M 4 ] ) 3 " a z e t i d i n o l s are obtained, v i a c y c l i z a t i o n of an intermediate 1,4-di-r a d i c a l , unless s t e r i c f a c t o r s prevent the formation of the four-mem-bered r i n g (Figure 3b).

    These r e s u l t s , and the observation that the photoelimination of o-(dialkylamino)acetophenones i s not suppressed by the standard t r i -p l e t quenchers can be explained 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 the amine to the carbonyl group [8] (Figure 3a). Lowe-r i n g the i o n i z a t i o n p o t e n t i a l of the 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 the 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 i n y-hydrogen a b s t r a c t i o n .

    I r r a d i a t i o n of N-phenylacetophenones (Figure 3c) furnishes pro-ducts 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]. Geminally disub-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 decomposition v i a both oC - and -cleavage pathways [V3]. Predominant o -cleavage i s observed upon p h o t o l y s i s of II [_3] [15]. This trend i n r e a c t i v i t y can be explained by the i n t e r p l a y of two f a c t o r s :

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

  • RADIATION CURING OF POLYMERIC MATERIALS

    Figure 1. Synthesis of BDMB.

    Abeocbance

    10H

    0 ^

    BDK MMMP

    BDMB

    20 300 So (0.001% in MeOH) (nm)

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

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

  • DESOBRY ET AL. Novel Photoinitiator for Modem Technology

    Ar-C-CH,. -N 2 W

    A r C

    X H H

    /?-Cleavage || > Ar-C-CH 3 + R

    Ar-C-CH-N_ 2 \

    OH Ar-C N -

    Ar-C-C-N R< \ > h e n y l

    ? I ^ Ar-C-CH.

    Phenyl

    R'-H s, Ar-C-CH 2- + -N

    CH. Il ' 3

    Ar-C-4' CH.

    I products

    \ Phenyl

    R

    Phenyl

    products

    Figure 3. Photochemical r e a c t i o n pathways of c -amino aceto-phenones.

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

  • 96 RADIATION CURING OF POLYMERIC MATERIALS

    1 . i n c r e a s i n g s u b s t i t u t i o n at the cC -carbon favors $6 -cleavage due to the weakening of the acyl--carbon bond [_^6] ; and

    2 . e l e c t r o n donating groups (alkoxy, amino, etc.) i n the ^ - p o s i t i o n can s t a b i l i z e the t r a n s i t i o n s t a t e of o -cleavage by i n -t e r a c t i o n of a lone p a i r of ele c t r o n s with the breaking carbon-carbon bond [ J_7 ] .

    In view of the continuum of 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 of compounds, the photochemistry of BDMB was i n v e s t i g a t e d .

    CIDNP Spectrum of BDMB. The CIDNP technique can provide much u s e f u l information on processes i n which r a d i c a l species are formed and has been used to determine the species r e s p o n s i b l e f o r the i n i t i a t o r ac-t i v i t y of compounds such as I and II [2] [3] [Ji_8] [ J _ 9 ]. The NMR spec-trum taken during p h o t o l y s i s of BDMB was obtained under s i m i l a r con-d i t i o n s as employed f o r the study of I and II [ ^ 0 ] [ 21_ ]. The enhanced s i n g l e t absorption at 9 , 6 3 ppm (Figure 4 ) i s assigned to 4-morpho-linobenzaldehyde I I I which must r e s u l t from an i n i t i a l oi -cleavage of BDMB to engender a benzoyl and aminoalkyl r a d i c a l p a i r (Figure 5 ) . The aldehyde p o l a r i z a t i o n , according to Kaptein's r u l e s [ 2 ! 2], and the ESR parameters f o r these two r a d i c a l s are i n f u l l agreement with a t r i p l e t s t a t e precursor. By comparing the spectra obtained i n d i f f e -r e n t solvents, i t was determined that the r a d i c a l p a i r i s formed es-s e n t i a l l y v i a an unimolecular process, thus r u l i n g out the p h o t o l y t i c decomposition of BDMB v i a intermolecular e l e c t r o n t r a n s f e r or other bimolecular r e a c t i o n s . The two quartets at 4 . 4 6 and 4 . 6 2 ppm which e x h i b i t enhanced absorptions are assigned to the o l e f i n i c protons of IVa and b. The s i n g l e t s a t 5 . 2 8 ppm (emission) and 5 . 3 3 ppm (enhanced absorption) are a t t r i b u t e d to the o l e f i n i c protons i n Va and b. The pattern of these s i g n a l s i s due to the combination of cage r e a c t i o n s l e a d i n g to products e x h i b i t i n g absorption p o l a r i z a t i o n and escape r e -actions f u r n i s h i n g the same products but e x h i b i t i n g emission po-l a r i z a t i o n . The spectrum i n deuterated cyclohexane (Figure 4 ) and the CIDNP experiments i n other solvents lead to the conclusion that the formation of Va and b occurs p r e f e r e n t i a l l y v i a escape r e a c t i o n s whereas f o r the p o l a r i z a t i o n s of IVa and b the c o n t r i b u t i o n of the 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 .

    Preparative Photochemistry. I r r a d i a t i o n of BDMB on a preparative scale [23^] leads to a product mixture that would be expected based upon the r e s u l t s of the CIDNP experiments (Figure 6 ) . In benzene, the main photoproducts are 4-morpholinobenzaldehyde I I I ( 2 1 %) and 1-phenyl-butan - 2-one VII ( 3 2 % ) . The l a t t e r compound i s be l i e v e d to a r i s e from h y d r o l y s i s of the i n i t i a l l y formed enamines during work-up. On the basis of our experiments, however, other pathways f o r i t s formation cannot be completely r u l e d out. The deaminated d e r i v a t i v e VIII was al s o i s o l a t e d i n 11 % y i e l d i n d i c a t i n g t h a t competing reac-t i o n s - i . e . N o r r i s h type II or d i r e c t -cleavage - a l s o take place. S u r p r i s i n g l y N,N-dimethyl -4-morpholinobenzamid VI was i s o l a t e d i n 1 9 %. The mechanism of the formation of VI w i l l be the subject of 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 of BDMB i n isopropanol a f f o r d s the same products 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 .

    Dodecanthiol IX has been o f t e n used as a scavenger f o r non-cage benzoyl r a d i c a l s [ 2 ] [ 2 5 J . I r r a d i a t i o n of BDMB i n benzene with a

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  • 8. DESOBRY ET AL. Novel Photoinitiator for Modem Technology 97

    4.46/4.62 ppm.q

    V C H ,

    BDMB

    CH, hv

    CH,

    CH, CH,

    CH, CH.

    ^ ^ 9 IVa, b Q ^ j H y K - H *

    III / 9.63 ppm.8

    5.32 ppm.8 ^0*** 5.28 ppm.8 I

    CH.CH,

    V

    Figure 5. Decomposition products of BDMB detected by CIDNP.

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  • 98 RADIATION CURING OF POLYMERIC MATERIALS

    large excess of IX afforded I I I i n 88 % y i e l d (Figure 7) as well as products r e s u l t i n g from the r e a c t i o n of the benzoyl r a d i c a l with 1-dodecanthiyl r a d i c a l s (X, 6 % ) . The trapping product of the -aminoalkyl r a d i c a l was a l s o obtained (XI, 50 %).

    The decomposition of various p h o t o i n i t i a t o r s i n t o carbon cente-red 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-tetramethylpiperidinoxyl) as a trapping agent. A d d i t i o n of a t h r e e f o l d excess of TMPO to a s o l u t i o n of BDMB r e s u l t s i n the f o r -mation of XII i n high y i e l d (91 %) (Figure 8). VII could a l s o be i s o -l a t e d . This product may a r i s e from an i n s t a b l e primary a d d i t i o n pro-duct (XIII) of TMPO and the o -amino-alky1 r a d i c a l which i s hydroly-sed during work-up. These r e s u l t s a l s o confirm t h a t the dominant pathway of decomposition i s t h a t of o -cleavage.

    Trapping Reactions with 2 - t - B u t y l a c r y l i c A c i d Methylester XIV. This trapping r e a c t i o n , which mimics the i n i t i a t i o n step of the polymeri-s a t i o n process, has been used to obtain information on the r e a c t i v i t y of the primary 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 of BDMB i n the presence of a t h r e e f o l d excess of XIV a f f o r d s the ben-z o y l d e r i v a t i v e XV i n 87 % y i e l d (Figure 9). Again VII was a l s o i s o -l a t e d (64 % ) , whereas no st a b l e a d d i t i o n product of cC -aminoalkyl r a -d i c a l could be i d e n t i f i e d . This r e s u l t suggests that the benzoyl r a -d i c a l i s mainly r e s p o n s i b l e f o r the polymerisation of v i n y l i c mono-mers and i s i n agreement with previous studies on b e n z i l k e t a l s and benzoin ethers [27]. But, as o -aminoalkyl 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 polymerization [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 devoted to the e l u c i d a t i...

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