Ultraviolet Light Induced Reactions in Polymers

2 Benzoin Ether Photoinitiated Polymerization of Acrylates (1)

Downloaded by UNIV OF ARIZONA on March 10, 2017 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch002

S. PETER PAPPAS, ASHOK K. CHATTOPADHYAY (2), and L E L A N D H . CARLBLOM Department of Polymers and Coatings, North Dakota State University, Fargo, N. D. 58102

Our interest in the photochemistry of benzoin ethers was prompted by two major reasons: (1) a discrepancy existed between recent mechanistic studies on the photochemistry of benzoin ethers and earlier reports on the benzoin ether photo initiated polymerization of reactive monomers, and (2) the extensive, commercial utilization of benzoin ethers as photo initiators in uv curable coatings and printing inks warranted further investigation of this discrepancy. The photochemistry of benzoin ethers (α-alkoxy-α-phenyl -acetophenones) has been examined in considerable detail, recently, by quenching (3, 4), sensitization (4), CIDNP (5), and radical scavenging (6) studies. These investigations indicate that benzoin ethers undergo a facile, photocleavage (Norrish type I) to yield benzoyl and benzyl ether radicals, as shown in eqn 1. This α-scission is not retarded by conventional triplet

+

(1)

quenchers which led to the suggestion (3) that reaction occurs via the excited singlet state. However, the sensitization (4) studies are best interpreted in terms of triplet reactivity. The (3,4) constitutes an facility of α-cleavage (k 10 sec ) important reason for the effectiveness of benzoin ethers as photoinitiators for uv curing in air (7) since scission is not quenched by oxygen or reactive monomers, such as styrene (8). Unfortunately, the facility of cleavage does not prevent air -inhibition of uv curing by reaction of the initiator or growing polymers radicals with oxygen (7). Based on the photochemistry of eqn 1, one would reasonably predict that, in the presence of reactive monomer, a conven-10

-1

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

2.

PAPPAS E T A L .

13

Benzoin Ether Photoinitiated Polymerization

t i o n a l r a d i c a l c h a i n p o l y m e r i z a t i o n w o u l d o c c u r , i n i t i a t e d by the b e n z o y l and b e n z y l e t h e r r a d i c a l s , as r e c e n t l y p r o p o s e d (9). O n e w o u l d f u r t h e r p r e d i c t that a m a x i m u m of two i n i t i a t o r f r a g m e n t s w o u l d be i n c o r p o r a t e d p e r p o l y m e r m o l e c u l e d e p e n d i n g u p o n the r e l a t i v e extent of t e r m i n a t i o n by d i s p r o p o r t i o n a t i o n a n d c o u p l i n g . H o w e v e r , these p r e d i c t i o n s a r e not i n a c c o r d w i t h e a r l i e r s t u d i e s . U t i l i z i n g C - l a b e l l e d b e n z o i n m e t h y l e t h e r as photo i n i t i a t o r i n the p o l y m e r i z a t i o n of m e t h y l m e t h a c r y l a t e ( M M A ) , M o c h e l and c o w o r k e r s r e p o r t e d that 12-14 r a d i o a c t i v e f r a g m e n t s o r m o l e c u l e s of i n i t i a t o r w e r e i n c o r p o r a t e d p e r p o l y m e r (10). N o r a d i o a c t i v i t y w a s i n c o r p o r a t e d when the p o l y m e r i z a t i o n w a s i n i t i a t e d t h e r m a l l y w i t h (X , ( X ' - a z o b i s i s o b u t y r o n i t r i l e ( A I B N ) i n the p r e s e n c e of r a d i o l a b e l e d b e n z o i n m e t h y l e t h e r ( B E ^ . F u r t h e r m o r e , b a s e d on q u a n t u m y i e l d data f o r b e n z o i n , it w a s e s t i m a t e d that l e s s than 2 quanta a r e u t i l i z e d / p o l y m e r m o l e c u l e f o r m e d . S u b s e q u e n t l y , these r e s u l t s t o g e t h e r w i t h u n p u b l i s h e d d a t a w e r e c i t e d as e v i d e n c e f o r c o p o l y m e r i z a t i o n of p h o t o e x c i t e d b e n z o i n e t h e r s and b e n z o i n w i t h r e a c t i v e m o n o m e r s , s u c h as M M A and s t y r e n e (11). This pro p o s a l i s c o n s i s t e n t w i t h the f i n d i n g that no r a d i o a c t i v i t y was i n c o r p o r a t e d w h e n the p o l y m e r i z a t i o n w a s i n i t i a t e d t h e r m a l l y w i t h A I B N i n the p r e s e n c e of B E (10). H o w e v e r , the i n c o r p o r a t i o n of 12-14 p h o t o e x c i t e d b e n z o i n e t h e r m o l e c u l e s by the a b s o r p t i o n of l e s s than 2 quanta r e q u i r e s that about 7 e x c i t e d m o l e c u l e s be p r o d u c e d / q u a n t u m a b s o r b e d . T h i s w o u l d constitute a c h a i n p r o c e s s f o r l i g h t a b s o r p t i o n f o r w h i c h we a r e u n a w a r e of any precedent. We f e l t that the u n i q u e n e s s of these r e s u l t s w a r ranted a reinvestigation.


1 4

X

T h e C - l a b e l l e d b e n z o i n m e t h y l e t h e r ( Β Ε χ ) , u t i l i z e d by M o c h e l ' s g r o u p (10), w a s p r e p a r e d by m e t h y l a t i n g b e n z o i n w i t h C - m e t h y l i o d i d e . In o r d e r to o b t a i n a d d i t i o n a l i n f o r m a t i o n , we a l s o p r e p a r e d doubly and t r i p l y l a b e l l e d a n a l o g s , B E and B E , as o u t l i n e d on S c h e m e 1. 1 4

1 4

2

3

I r r a d i a t i o n s w e r e c o n d u c t e d at 366 n m ( C o r n i n g 7-83 f i l t e r c o m b i n a t i o n ) i n a m e r r y - g o - r o u n d a p p a r a t u s in w h i c h the s a m p l e s r o t a t e d about a s t a t i o n a r y 4 5 0 - W a t t H a n o v i a m e d i u m p r e s s u r e l a m p f o r constant l i g h t e x p o s u r e . T h e s a m p l e s c o n s i s t e d of d e g a s s e d 5 m l s o l u t i o n s of the photo i n i t i a t o r s i n neat M M A c o n t a i n e d in P y r e x t u b e s . T w o i n i t i a t o r c o n c e n t r a t i o n s w e r e u t i l i z e d : 1.05 χ 10" and 4 . 1 1 χ Ι Ο " Μ , w h i c h c o r r e s ponded to 16 and > 99% l i g h t a b s o r p t i o n , r e s p e c t i v e l y . S a m p l e s w e r e i r r a d i a t e d to about 7% m o n o m e r c o n v e r s i o n s w h i c h r e q u i r e d 15 m i n f o r the o p t i c a l l y d e n s e s o l u t i o n s and 30 m i n f o r the tubes w i t h low i n i t i a t o r c o n c e n t r a t i o n . The resulting poly3

2

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

14

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

SCHEME 1 Ο OH PhC-CHPh

14

+

2

CH I

Ο 0 CH PhC-CHPh 1 4

Ag 0

2>

3

3

B Eχ


Ph

1 4

Ο

NaCN

CHO

OH

14 M

Ph

BE

1 4

14 1

C—

CHPh

BE

2

3

m e r s w e r e p r e c i p i t a t e d i n m e t h a n o l , and the n u m b e r of m o n o m e r s r e a c t e d / q u a n t u m a b s o r b e d (0 ) was d e t e r m i n e d by s i m u l t a n e o u s i r r a d i a t i o n of a b e n z o p h e n o n e / b e n z h y d r o l a c t i n o m e t e r f o r w h i c h 0 w a s t a k e n as 0 . 8 5 (12). T h e i n t e n s i t y of l i g h t a b s o r b e d w a s a p p r o x i m a t e l y 4 χ 10 q u a n t a / m i n at h i g h initiator concentration. m

17

A f t e r d e t e r m i n a t i o n of m o n o m e r c o n v e r s i o n s , the p o l y m e r s w e r e d i s s o l v e d i n benzene and r e p r e c i p i t a t e d two a d d i t i o n a l t i m e s p r i o r to d e t e r m i n a t i o n of i n t r i n s i c v i s c o s i t i e s ( [ η ] ) and specific a c t i v i t i e s . Intrinsic v i s c o s i t i e s were obtained in b e n z e n e at 3 0 ° C w i t h a n o . 1 U b b e l o h d e v i s c o m e t e r (solvent flow t i m e s w e r e 6 9 . 1 - 0.1 s e c ) . N u m b e r average m o l e c u l a r w e i g h t s ( M ) w e r e c a l c u l a t e d f r o m the e x p r e s s i o n : [ η ] = 8 . 6 9 χ 10" M (13). S p e c i f i c a c t i v i t i e s w e r e d e t e r m i n e d by _ liquid scintillation counting. P o l y m e r activities (A), 0 and M v a l u e s a r e p r e s e n t e d in T a b l e I. n

5

0

n

,

7

6

m

n

I n i t i a t o r a c t i v i t i e s (a) t o g e t h e r w i t h a v e r a g e v a l u e s f o r p o l y m e r a c t i v i t i e s (A) a r e p r e s e n t e d i n T a b l e II. In m a r k e d c o n t r a s t to the e a r l i e r r e p o r t s (10, 11), it i s seen that p o l y m e r a c t i v i t i e s a r e l e s s than that of the c o r r e s p o n d i n g i n i t i a t o r i n e a c h c a s e . U n f o r t u n a t e l y , we c a n o f f e r no r a t i o n a l e f o r t h i s d i s c r e p a n c y . H o w e v e r , the p r e s e n t data a r e r e a d i l y i n t e r p r e t e d in t e r m s of b e n z o i n e t h e r p h o t o c l e a v a g e into b e n z o y l and b e n z y l e t h e r r a d i c a l s (eqn 1), w h i c h initiate M M A p o l y m e r i z a t i o n . T h e i n c o r p o r a t i o n data m a y be a n a l y z e d by eqns 2 - 4 , w h e r e i n a i , a and a r e f e r to the s p e c i f i c a c t i v i t i e s of i n i t i a t o r s B E i , B E and B E , r e s p e c t i v e l y , A A and A r e p r e s e n t the c o r r e s p o n d i n g p o l y m e r a c t i v i t i e s , w h i l e B a n d Ε a r e the n u m b e r 2

2

3

3

it

2

3


2.

PAPPAS E T AL.

T A B L E I.

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 of M M A

BE ( Μ χ 10 )

M χ ΙΟ"

3

B E j (1 .05)



BE

2

(1 .05)

BE

3

(1 .05)

B E χ (41.1)

BE

2

(41.1)

BE

3

(41.1)

a

A

n

Cm 1020 1100 a 953 1030 1000 1000 1070 1070 248 307 310 258 271 280 341 240 235 338 258 272 262 332 318

8.20 7,86 8,54 7.97 7.86 8.54 8.08 8.31 8.96 6.32 4.97 5.17 5.89 5.07 5.07 6.21 5.62 5.48 5.69 5.69 5.60 5.17 4.97 5.17

4

dpm/mol χ 1.73 1.71 1.63 3.10 3.18 3.39 4.82 4.86 5.00 2.59 2.46 2.52 2.87 2.53 2.47 2.94 4.62 4.17 3.58 6.78 6.49 6.30 7.00 7.15

T h e weight of the p r e c i p i t a t e d p o l y m e r w a s i n a d v e r t e n t l y not

r e c o r d e d i n this e x p e r i m e n t .


15

16

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

T A B L E II.

Specific Activities A

BE ( Μ χ 10 )

a

BEj (1.05) (41.1)

3.13

BE (1.05) (41.1)

6.24

BE (1.05) (41.1)

9.31

d p m / m o l χ 10

3

1.69 2.62


2

3.22 4.12

3

4.89 6.74

of b e n z o y l and b e n z y l e t h e r r a d i c a l s , r e s p e c t i v e l y , i n c o r p o r ated/polymer. In t h i s c o n t e x t , E _ m a y be d e r i v e d i n d e p e n d e n t l y Ai A A

3

2

=

=

£a

Ia

2

= 2

a E

(2)

x

( Ε + B)

( E + B)

(3)

+ (a - a ) Ε 3

(4)

2

f r o m eqn 2 and s i m u l t a n e o u s s o l u t i o n o f eqns 3 and 4 . T h e r e s u l t i n g Ε and Β v a l u e s at e a c h i n i t i a t o r c o n c e n t r a t i o n a r e p r o v i d e d i n T a b l e III. A v e r a g e v a l u e s f o r 0 , M and 0p, the c o r r e s p o n d i n g n u m b e r of p o l y m e r m o l e c u l e s p r o d u c e d / q u a n t u m a b s o r b e d , a r e a l s o p r e s e n t e d i n T a b l e III. T h e s e v a l u e s w e r e found to be independent of r a d i o l a b e l w i t h i n e x p e r i m e n t a l e r r o r , e s t i m a t e d a s ί 1 0 % . H o w e v e r , a s s h o w n , they a r e h i g h l y dependent o n i n i t i a t o r c o n c e n t r a t i o n . m

T A B L E III.

1.05 41.1

S u m m a r y of Data f o r M M A

0

[BE] Μ χ 10

n

3

1030 280

0

Ε

Β

8.25

1.26

0.54

0.49

5.49

0.52

0.85

0.48

n χ 10" M

4

T h e t o t a l i n c o r p o r a t i o n at low i n i t i a t o r c o n c e n t r a t i o n of a p p r o x i m a t e l y one i n i t i a t o r f r a g m e n t / p o l y m e r i s i n r e a s o n a b l e a g r e e m e n t w i t h v a l u e s o b t a i n e d o n the p o l y m e r i z a t i o n of M M A


2.

PAPPAS E T A L .


14

u t i l i z i n g C-labeled AIBN (14). The data are indicative of a conventional polymerization, initiated equally effectively by ben zoyl and benzyl ether radicals, and terminated predominately by disproportionation of the growing polymer ends. Since the maxi mum number of polymers/quantum absorbed i s 2 for termination by disproportionation, the value of 1.26 for 0p at low i n i t i a t o r concentration corresponds to a minimum quantum efficiency of 63%. This signifies that at least 63% of the absorbed light results i n polymer formation. At the high i n i t i a t o r concentration, i n i t i a l conversion of monomer to polymer increased by a factor of 1.6 and ^ decreased, as expected. However, as shown i n Table III, these changes were accompanied by a drop i n quantum efficiency to 25% (0 = 0.52), together with an increase i n incorporation of radioactivity (1.33 as compared to 1.03 i n i t i a t o r fragments/polymer). The reduced quantum efficiency may be attributed to self-reactions of the i n i t i a t o r radicals, as well as to chain termination by i n i t i a t o r radicals, i . e . , primary radical termination. The increase i n incorporation of radioactivity constitutes additional support for primary radical termination. The discrepancy i n the incorporation of benzoyl (B) and benzyl ether radicals (E) at high i n i t i a t o r concentration appears to be significant. Further studies on this interesting finding are i n progress. Similar studies are also being carried out with methyl acrylate (MA). In this case, the samples, which were irradiated i n t r i p l i c a t e , consisted of degassed 8 ml solutions of the photoi n i t i a t o r s and MA (25% by volume) i n benzene. Intrinsic v i s c o s i ties were obtained i n benzene at 35°C with a Cannon-Ubbelohde size 50 viscometer, and number average molecular weights were calcu lated from the expression: [η] = 1.28 χ ΙΟ" * M^- * (15). The pertinent results are summarized i n Table IV. As shown i n Table IV, the total incorporation values are greater than 2 at both i n i t i a t o r concentrations. Since MA termi nates predominately by combination (16), the presence of more than 2 i n i t i a t o r fragments/polymer i s suggestive of an additional mode of incorporation. A possible mechanism i s H-abstraction from polymer by i n i t i a t o r radicals which provides radical sites for chain branching. However, further discussion of these find ings must await confirmation of the molecular weight data by more direct means. p


17

1

711


18

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

a

TABLE IV. Summary of Data for MA [BE] (Μ χ 10 )

Κm

1.04

2560

3

41.0

Μ χ ΙΟ" η

5

"Ρ

Ε

Β

b

4.48

0.51

1.4

1.3

C

2.60

0.35

1.8

1.9

1040

estimated error ±10%.


15% monomer conversions. 10% monomer conversions.

ACKNOWLEDGMENT. We wish to express our appreciation to the Alcoa Foundation for financial assistance and to Dr. Zeno Wicks, J r . for helpful comments. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

16.

Taken, i n part, from the Ph.D. Thesis of A.K.C., North Dakota State University, 1974. Present address: DeSoto, Inc., 1700 So. Mt. Prospect Road, Des Plaines, I l l i n o i s 60018. Heine, H.-G., Tetrahedron Lett. (1972) 4755. Pappas, S. P. and Chattopadhyay, A. K., J . Amer. Chem. Soc. (1973) 95, 6484. Dominh, T., Ind. Chim. Beige. (1971) 36, 1080; Chem. Abstr. (1972) 76, 126080p. Ledwith, Α., Russell, P. J . and S u t c l i f f e , L. H., J . Chem. Soc., Perkin Trans. 2 (1972) 1925. Pappas, S. P., Progr. Org. Coatings (1974) 2 (4), 333. Heine, H.-G., Rosenkranz, H.-J., and Rudolph, H., Angew. Chem., Int. Ed. Engl. (1972) 11, 974. Hutchison, J . and Ledwith, Α., Polymer (1973) 14, 405. Mochel, W. E., Crandall, J . L. and Peterson, J . H., J . Amer. Chem. Soc. (1955) 77, 494. Bevington, J . C., "Radical Polymerization," Academic Press, New York, 1961, p. 77. Pappas, S. P., Alexander, J . E. and Zehr, R. D., Jr., J . Amer. Chem. Soc. (1970) 92, 6927. Fox, T. G., Kissinger, J . B., Mason, H. E. and Shuele, Ε. Μ., Polymer (1962) 3, 71. Bevington, J . C., Melville, H. W., and Taylor, R. P., J . Polym. Sci. (1954) 14, 463. Sen, J . N., Chatterjee, R. and P a l i t , S. R., J . S c i . Ind. Research (India) (1952) 11b, 90; Chem. Abstr. (1952) 46, 7847a. Bamford, C. H., Dyson, R. W. and Eastmond, G. C., Polymer (1969) 10, 885.


Ultraviolet Light Induced Reactions in Polymers

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