Ultraviolet Light Induced Reactions in Polymers

tion of mixtures of isobutyl vinyl ether and acrylonitrile (7)5 presumably as a .... polymerization of butadiene, methyl methacrylate and acrylonitril...
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1 Participation of Excited Species in Propagation Step in Photopolymerization NORMAN

G. GAYLORD

Gaylord Research Institute, Inc., New Providence, N.J. 07974

The use of ultraviolet light in polymerization processes takes advantage of the fact that exposure of organic molecules to irradiation results in the generation of reactive species, e.g. excited species, at ambient or lower temperatures. The role of UV light has generally been considered to be restricted to the initiation of polymerization. However, propagation may also involve, or even be limited to, species generated by photo­ -excitation. UV Light in Polymerization Initiation When a polymerizable monomer is exposed to UV light in the presence of a photosensitizer, polymerization may "be initiated by radicals generated by dissociation of the excited sensitizer, e.g. benzoin methyl ether. Alternatively, the excited photosen­ sitizer may transfer its excitation energy to the monomer which, in turn, undergoes excitation and radical initiated polymeriza­ tion. [p-p]* --> P·

P-P

P-M.

M-M.

P-P + M* --> M.

[P-P]*

(1) (2)

The initiation of polymerization in the presence of benzo­ phenone requires the presence of a hydrogen donor such as a sol­ vent or polymer. The excited sensitizer abstracts a hydrogen from the donor resulting in the formation of a radical which initiates monomer polymerization. Ρ

*

Ρ

S-M.

PH + S.

(3)

Polymerization of monomers occurs in the absence of a photosensitizer when the functional groups in the monomer undergo 1

2

UV LIGHT INDUCED REACTIONS IN POLYMERS

excitation upon exposure to l i g h t of the appropriate wavelength, i.e. energy l e v e l . Although the result of photoactivation of a monomer is gen­ e r a l l y the generation of a r a d i c a l species, ionic polymerization has been reported where a c a t i o n i c a l l y polymerizable monomer, i.e. isobutylene, underwent photoionization in the vapor state and the ionized fragments were separated from t h e i r electrons by an electric field (1). Irradiation of a donor monomer-electron acceptor charge transfer complex, e.g. N-vinylcarbazole-sodium chloroaurate (2), -nitrobenzene (2) or -p-chloroanil (3) and β-propiolactone-uranyl nitrate (4), results i n the initiation of cationic polymerization. Irradiation of an acceptor monomer-electron donor charge transfer complex i n i t i a t e s anionic polymerization i n the case of nitroethylene-tetrahydrofuran (5) and r a d i c a l polymerization i n the case of methyl methacrylate-triphenylphosphine (6). Free r a d i c a l copolymerization is i n i t i a t e d upon UV i r r a d i a ­ t i o n of mixtures of isobutyl vinyl ether and a c r y l o n i t r i l e (7) presumably as a result of photoexcitation of the comonomer charge transfer complexes. The excited complexes dissociate into ionradicals which initiate r a d i c a l propagated copolymerization. 5

JVE-^AN]—[VETTAN]

VEÎ

+

aAN

+

bVE

*VET

+

TAN

00

(5)

* VE- (AN -co-VE^)

Mixtures of styrene and a c r y l o n i t r i l e also y i e l d free r a d i ­ c a l copolymers under UV i r r a d i a t i o n (Table I) (8). Table I. Photoinitiated Copolymerization of Styrene and Acrylonitrile Light Dark 2537 A. 3500 A. 3500 A. Dark

Time, hr. 2k.O 0.5 0.5 6.0 18.0 1.0 2k.O

Conversion,$ 0.0 0.5 0.9 12.2 31.6 1.8 2.3

S/AN mole r a t i o Found Theory

58/U2

59Al

S/AN mole r a t i o = 1; 30°C Although i n i t i a t i o n by monomer ion-radicals may also be operative i n t h i s case, alternatively, homopolymerization of excited comon­ omer complexes may occur to a limited extent, due to the low con­ centration of such complexes, followed by ion pair coupling or dissociation to i n i t i a t e r a d i c a l copolymerization.

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Excited Species in Propagation Step

[S-*AN]

> (SÎTAN)

3

v (S-AN) S 7AN X

(6)

+

^y (S-AN) S-AN( S - A N ) s TAN

^ ^ ^ S ( S - A N ) S·

X

(7)

X

+

(8)

AN

> (S-AN) S-AN- ( S - C O - A ^ )

(S-AN) S-AN- + aS + bAN x

x

* (S-AN) S-(S

(S-AN) S- + aS + bAN

X

x

-co-AI^)

(9) (10)

Thus, i n these cases, UV r a d i a t i o n provides a method f o r t h e i n i t i a t i o n o f r a d i c a l or i o n i c polymerization. A f t e r the i n i t i a ­ t i o n s t e p , c h a i n propagation proceeds as though i n i t i a t i o n r e ­ s u l t e d from the use o f c o n v e n t i o n a l c a t a l y s t s f o r r a d i c a l and ionic polymerization. UV L i g h t i n P o l y m e r i z a t i o n

Propagation

P h o t o a c t i v a t e d Copolymerization. Although p o l y m e r i z a t i o n and c o p o l y m e r i z a t i o n g e n e r a l l y i n v o l v e t h e a d d i t i o n o f a monomer t o a r e a c t i v e c h a i n end, t h e ground s t a t e charge t r a n s f e r complex generated by t h e i n t e r a c t i o n o f an e l e c t r o n donor monomer and a s t r o n g e l e c t r o n acceptor monomer, a c t s as a s i n g l e u n i t and, upon e x c i t a t i o n o f t h e complex, b o t h monomers enter t h e c h a i n . D

+ A

i ^ A ]

|i>Î:Âj

~(DA) D TA

Jllïâ^

X

+

(11) -(DA) N

(12)

The N - v i n y l p y r r o l i d o n e - m a l e i c anhydride charge t r a n s f e r com­ p l e x undergoes homopolymerization upon p h o t o e x c i t a t i o n i n a i r t o y i e l d the a l t e r n a t i n g copolymer (9). Complexation o f a r e l a t i v e l y poor e l e c t r o n a c c e p t i n g monomer w i t h a Lewis a c i d o r organoaluminum compound converts the accept­ or monomer t o a stronger e l e c t r o n acceptor, thus promoting the f o r m a t i o n o f ground s t a t e comonomer complexes. The l a t t e r under­ go p h o t o e x c i t a t i o n and homopolymerization. A

+ MX

D

+ A...MX - — * |p-*A...Mx]

JDÎ7A...MXJ

ν A...MX

(13)

> - ( D A ) - + nMX n

|pÎTA...Mx]

(lh)

(15)

Complexation o f methyl methacrylate and a c r y l o n i t r i l e w i t h t r i e t h y l a l u m i n u m converts these poor e l e c t r o n a c c e p t i n g monomers i n t o stronger e l e c t r o n a c c e p t o r s . Ground s t a t e charge t r a n s f e r complexes a r e generated when styrene i s added t o these monomers i n the presence o f t r i e t h y l a l u m i n u m . P h o t o e x c i t a t i o n o f t h e

4

UV

L I G H T INDUCED REACTIONS IN

POLYMERS

complexes under UV i r r a d i a t i o n r e s u l t s i n the formation of exc i p l e x e s which homopolymerize t o y i e l d equimolar copolymers over a wide range of comonomer r a t i o s ( l O ) . Thus, UV l i g h t c o n t r o l s the propagation step by promoting the formation of polymerizable s p e c i e s , i . e . comonomer e x c i p l e x e s . As shown i n Table I I , the p h o t o a c t i v a t e d c o p o l y m e r i z a t i o n of styrene and methyl methacrylate i n the presence of A l E t ^ y i e l d s equimolar, a l t e r n a t i n g copolymers when the i n i t i a l monomer charge contains excess styrene. However, when the comonomers are pre­ sent i n equimolar r a t i o , the copolymer i s r i c h i n methyl meth­ acrylate. Table I I . P h o t o a c t i v a t e d P o l y m e r i z a t i o n of S - M M A . . . A l E t ^ Charge S/MMA/A1

mole r a t i o

a

Copolymer mole r a t i o Found Theory S/MMA

Conversion, °j 0

50/50/10

I.7U

37/63

50/50

70/30/5

2.79

51Λ9

60/UO

80/20A 85/15/3 90/10/2

2.83 2.95 3.19

51Λ9 51Λ9 50/50

70/30 75/25 8U/16

* 500W tungsten lamp; 5°C; Based on l / l S/MMA

[ÂlEt Ί = 10 mmoles; 2 hr L 41

Since MMA...AlEt^ i s a stronger e l e c t r o n acceptor than uncomplexed MMA, i t forms charge t r a n s f e r complexes w i t h the l a t t e r as w e l l as w i t h styrene. MMA S

+

MMA..

.AlEt

+

MMA..

.AlEtg

MMA. -.MMA.. .AlEt^j +

3

stTMMA..

.AlEt^J

(16) (17)

The c o p o l y m e r i z a t i o n of the two complexes y i e l d s an MMA-rich co­ polymer. However, when the i n i t i a l monomer charge contains ex­ cess s t y r e n e , there i s l i t t l e or none of the complex from Eq. (l6) and the equimolar copolymer r e s u l t s from the homopolymeriza­ t i o n of the comonomer complex i n Eq. (17)· The c o p o l y m e r i z a t i o n of styrene and a c r y l o n i t r i l e i n the presence of A l E t ^ under UV i r r a d i a t i o n y i e l d s equimolar, a l t e r ­ n a t i n g copolymers when the i n i t i a l comonomer charge i s equimolar or contains excess a c r y l o n i t r i l e and products w i t h compositions intermediate between t h a t of the equimolar copolymer and t h a t of the r a d i c a l copolymer when the i n i t i a l charge i s r i c h i n styrene (Table I I I ) ( l O ) . The intermediate compositions may represent mixtures of equimolar and r a d i c a l copolymers, b l o c k copolymers generated as shown i n Eq. (6)-(l0) or random copolymers r e s u l t i n g from c o p o l y m e r i z a t i o n of complexes and monomers.

1.

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5

Excited Species in Propagation Step

Table I I I . P h o t o a c t i v a t e d P o l y m e r i z a t i o n o f S-AN...AlEt^ Copolymer S/AN mole r a t i o Found Theory

Charge S/AN/AI

Conversion,

mole r a t i o 30/70/6 50/50/6 70/30/6 80/20/6 90/10/6

8.81 5.67 7.12 lh.33 27.10

5k/U6 59Al 67/33 7U/26 8k/l6

51Λ9 51Λ9 6V36 63/37 72/28

t 3500 A. Hg lamp; 30°C; l A l E t J ] == 12 mmoles; 2 h r L 4! Based on l / l S/AN Ethylaluminum s e s q u i c h l o r i d e (EASC) and d i c h l o r i d e are even more e f f e c t i v e than A l E t i n promoting the formation o f styrenemethyl methacrylate and s t y r e n e - a c r y l o n i t r i l e charge t r a n s f e r complexes. The c o n c e n t r a t i o n o f ground s t a t e complexes i s s u f f i ­ c i e n t l y high so t h a t a u t o e x c i t a t i o n occurs, p o s s i b l y through c o l l i s i o n , and p o l y m e r i z a t i o n proceeds even i n the dark. However, upon exposure t o UV l i g h t , t h e c o n c e n t r a t i o n o f e x c i p l e x e s and the r a t e o f homopolymerization are g r e a t l y increased t o y i e l d a l t e r n a t i n g copolymers over a wide range o f comonomer compositions (Table IV) (10). a

Table IV. P h o t o a c t i v a t e d P o l y m e r i z a t i o n o f S-AN...EASC Copolymer S/AN mole r a t i o Found Theory

Charge S/AN/AI

Conveys

mole r a t i o 12.5/87.5/5 25/75/5 37.5/62.5/5 50/50/5 62.5/37.5/5 75/25/5

U8/52 50/50 52/U8 52/1(8 5lA9 55A5°

10.7 15 Λ 16.5 21.5 17.1 12.3

c

c

^5/55 50/50 5h/U6 59Λ1 62/38 69/31

10 mmoles; 1 h r Ambient l i g h t ; 30°C; JEASCJ Based on l / l S/AN A l t e r n a t i n g s t r u c t u r e confirmed by N M R The i n f l u e n c e o f UV l i g h t on the course o f c o p o l y m e r i z a t i o n , i . e . on copolymer composition, i s d r a m a t i c a l l y shown i n the t e r p o l y m e r i z a t i o n o f butadiene, methyl methacrylate and a c r y l o n i t r i l e i n the presence o f ethylaluminum d i c h l o r i d e . The r e a c t i o n a c t u ­ a l l y i n v o l v e s t h e c o p o l y m e r i z a t i o n o f two complexes, i . e . B D - M M A . . . .EtAlClp and BD-AN.. . E t A l C l . When the r e a c t i o n i s con­ ducted i n the dark w i t h an i n i t i a l B D / M M A / A N = 6θ/2θ/20 molar p

6

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

charge, the terpolymer composition v a r i e s w i t h temperature, rang­ i n g from a B D / M M A . ^ A N composition o f 50/U1/9 at 0°C t o a U7/17/36 composition a t 50 C. I n c o n t r a s t , when the r e a c t i o n i s conducted under U V l i g h t , the composition i s e s s e n t i a l l y independent o f temperature and remains a t about 50/33/17 over the 0-50 C range (Table V ) ( l l ) . A p p a r e n t l y thermal a c t i v i a t i o n y i e l d s d i f f e r i n g r e l a t i v e amounts o f the copolymerizable e x c i t e d complexes as the temperature i s v a r i e d w h i l e U V l i g h t e x c i t e s a l l o f the ground s t a t e complexes whose r e l a t i v e concentrations are independent o f temperature. The y i e l d o f terpolymer i s a t a maximum between 330 and 350 myUL, c o n s i s t e n t w i t h the U V s p e c t r a which i n d i c a t e the formation o f a charge t r a n s f e r complex w i t h a b s o r p t i o n i n t h i s r e g i o n (12). Table V . T e r p o l y m e r i z a t i o n o f BD-MMA.-AN i n Presence of E t A l C l B D / M M A / A N / A 1 = 60/20/20/2 mole r a t i o Dark Temp., Terpolymer,mole

°c

BD

0 19 30 1+0 50

50 52 56 1+7 1+7

MMA.

1+1 26 20 18 17

AW

9 23 21+ 35 36

ratio MMA/AN

h.55 1.13 Ο.83

0.51 0.1+7

UV Light Temp., Terpolymer,mole °C

BD

MMA.

2 21.5 30 1+0 50

50 50 1+9 50 52

31+ 33 33 33 32

AN

16 17 18 17 16

p

ratio MMA./AN

2.12 I.9I+

1.83 1.9l* 2.02

In the t e r p o l y m e r i z a t i o n o f s t y r e n e , methyl methacrylate and a c r y l o n i t r i l e ( S / M M A . / A N = 50/25/25 mole r a t i o ) i n the presence o f EASC, the terpolymer composition i s approximately 50/36/lU, i n ­ dependent o f the temperature w i t h i n the range o f 10-90 C, whether the r e a c t i o n i s conducted i n the dark o r under U V r a d i a t i o n (10). However, the t e r p o l y m e r i z a t i o n r a t e i s i n c r e a s e d 2-5 times under UV light. The r e a c t i o n o f a conjugated diene such as butadiene o r i s o prene and an e l e c t r o n acceptor monomer such as m a l e i c anhydride proceeds through a ground s t a t e complex which undergoes c y c l i z a t i o n t o y i e l d the D i e l s - A l d e r adduct. However, under U V l i g h t , i n a i r o r i n the presence o f s e n s i t i z e r s , the adduct i s accompa­ n i e d by the equimolar, a l t e r n a t i n g copolymer which r e s u l t s from e x c i t a t i o n o f the ground s t a t e complex, f o l l o w e d b y homopolymer­ i z a t i o n o f the e x c i t e d complex (13), as shown i n Eq. ( l 8 ) . When the e l e c t r o n acceptor monomer i s a r e l a t i v e l y weak acceptor, e.g. a c r y l o n i t r i l e , the r e a c t i o n w i t h a conjugated diene such as butadiene, t o y i e l d the D i e l s - A l d e r adduct, pro­ ceeds s l o w l y . However, i n the presence o f aluminum c h l o r i d e , t h e a c r y l o n i t r i l e i s converted t o a s t r o n g e r e l e c t r o n acceptor and the formation o f the ground s t a t e complex and the adduct therefrom proceeds more r a p i d l y . When the r e a c t i o n i s c a r r i e d out under U V i r r a d i a t i o n , the y i e l d o f adduct i s g r e a t l y reduced and the e q u i -

1.

GAYLORD

7

Excited Species in Propagation Step

molar, a l t e r n a t i n g copolymer becomes t h e predominant product (ik) (Table V l ) , i n accordance w i t h t h e r e a c t i o n sequence shown i n Eq.

(19). Table V I . P h o t o a c t i v a t e d Copolymerization o f Butadiene and A c r y l o n i t r i l e i n Presence o f A l C l ^ and E t A l C l 2

AlCl Dark Adduct

BDJVN

Copolymer ( B D - A N ) A

23.5 0

J

n

UV

12.8 lk7.3

EtAlClp Dark UV

20.0

18.3

15.8

126.8

B D / A N / A 1 = 266/266/2.66 mmoles; 20°C; kO min Y i e l d i n 10~3 mmoles/min

8

UV

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

As shown i n Table V I , when the r e a c t i o n between butadiene and a c r y l o n i t r i l e i s c a r r i e d out i n t h e presence o f E t A l C l p , a u t o e x c i t a t i o n occurs even i n t h e dark t o y i e l d b o t h adduct and equimolar, a l t e r n a t i n g copolymer. N e v e r t h e l e s s , UV l i g h t pro­ motes t h e f o r m a t i o n o f the copolymer. Thus, i n these r e a c t i o n s e x c i t a t i o n under UV l i g h t r e s u l t s i n a change i n t h e course o f the r e a c t i o n from adduct f o r m a t i o n t o c o p o l y m e r i z a t i o n . P h o t o a c t i v a t e d Homopolymerization. The p a r t i c i p a t i o n o f e x c i t e d monomer has been proposed i n t h e p o l y m e r i z a t i o n o f e t h y l ­ ene under UV ( l 5 , l 6 ) as w e l l as gamma i r r a d i a t i o n (17) i n t h e presence o f t r a c e amounts o f oxygen. I t has been suggested t h a t under i r r a d i a t i o n , ethylene undergoes e x c i t a t i o n per se o r as a r e s u l t o f t h e p e r t u r b i n g e f f e c t o f oxygen, t o generate t r i p l e t e x c i t e d e t h y l e n e . The l a t t e r r e a c t s w i t h ground s t a t e ethylene t o generate a d i r a d i c a l dimer (l6) o r an e x c i t e d dimer (17). Propagation i n v o l v e s a d d i t i o n o f the dimer t o t h e growing c h a i n end which i s presumed t o be a r a d i c a l . I n one p r o p o s a l (17)> a d d i t i o n o f t h e e x c i t e d dimer r e s u l t s i n t h e i n c o r p o r a t i o n o f one ethylene u n i t i n t o t h e c h a i n and t h e r e g e n e r a t i o n o f e x c i t e d ethylene monomer.

I f t h e e x c i t e d ethylene dimer e x i s t s as an i o n r a d i c a l p a i r , propagation may i n c o r p o r a t e b o t h monomeric u n i t s o f the dimer, analogous t o t h e behavior o f comonomer charge t r a n s f e r complexes such as butadiene-maleic anhydride, o r o n l y one u n i t , as noted i n the homopolymerization o f N - v i n y l c a r b a z o l e i n t h e presence o f e l e c t r o n a c c e p t i n g monomers such as a c r y l o n i t r i l e and maleic anhydride. Although l o n g considered i n c a p a b l e o f homopolymerization, m a l e i c anhydride i s r e a d i l y polymerized under UV i r r a d i a t i o n i n the presence o f a p h o t o s e n s i t i z e r (l8,19)> The p o l y m e r i z a t i o n presumably i n v o l v e s propagation o f e x c i t e d m a l e i c anhydride (20). I t i s noteworthy t h a t t h e s t r u c t u r e o f p o l y ( m a l e i c anhydride) contains fused cyclopentanone and s u c c i n i c anhydride u n i t s , d e r i v e d from two i n t e r a c t i n g m a l e i c anhydride u n i t s . This sug­ gests t h a t propagation i n v o l v e s e x c i t e d dimer r a t h e r than monomer.

X = H or COOH

(23)

1.

GAYLORD

Excited Species in Propagation Step

9

C h e m i c a l l y Induced Photopolymerization i n the Absence o f L i g h t The i n t e r a c t i o n o f UV l i g h t with organic molecules i s not the o n l y method o f g e n e r a t i n g e l e c t r o n i c a l l y e x c i t e d s p e c i e s . The same e x c i t e d s p e c i e s t h a t a r e formed under i r r a d i a t i o n a r e produced by c e r t a i n chemical r e a c t i o n s i n the absence o f l i g h t . Such c h e m i c a l l y generated e x c i t e d s p e c i e s undergo t h e same chem­ i c a l r e a c t i o n s as t h e e x c i t e d s p e c i e s formed by t h e a b s o r p t i o n o f l i g h t , i n c l u d i n g t h e t r a n s f e r o f e x c i t a t i o n energy (21). The a l t e r e d WSt s p e c t r a c h a r a c t e r i s t i c o f c h e m i c a l l y induced dynamic p o l a r i z a t i o n (CIDNP) (22) are observed d u r i n g the photol y t i c decomposition o f v a r i o u s peroxides i n the presence o f p h o t o s e n s i t i z e r s a t ambient temperatures, as w e l l as the thermal decomposition o f these peroxides i n the absence o f l i g h t a t temp­ eratures where they have a short h a l f - l i f e . The r e a c t i o n s which are r e s p o n s i b l e f o r CLDNP are apparently s i m i l a r i n both cases. The p r e c u r s o r s o f such r e a c t i o n s are photοlytically and chemi­ c a l l y generated s p e c i e s , r e s p e c t i v e l y , capable o f t r a n s f e r r i n g energy t o s u i t a b l e a c c e p t o r s . The e x c i t a t i o n o f comonomer donor-acceptor complexes occurs i n the presence o f p e r o x i d e s , under c o n d i t i o n s where the l a t t e r are undergoing r a p i d decomposition. The peroxide-induced e x c i t e d complexes then undergo homopolymerization t o equimolar, a l t e r n a t ­ ing copolymers. Thus, styrene-methyl methacrylate, s t y r e n e a c r y l o n i t r i l e , a - o l e f i n - a c r y l o n i t r i l e and b u t a d i e n e - a c r y l o n i t r i l e y i e l d equimolar copolymers i n t h e presence o f A1C1- and/or organoaluminum h a l i d e s under these c o n d i t i o n s . E x c i t e d complexes are a l s o the polymerizable s p e c i e s i n the peroxide-induced copolymer­ i z a t i o n o f conjugated dienes and maleic anhydride as w e l l as i n the formation o f copolymers d u r i n g the r e t r o g r a d e rearrangement of eyelopentadiene-maleic anhydride D i e l s - A l d e r adducts. Ethylene and maleic anhydride undergo peroxide-induced homo­ p o l y m e r i z a t i o n under s i m i l a r c o n d i t i o n s , i . e . i n the presence o f peroxides undergoing r a p i d decomposition, presumably through the propagation o f e x c i t e d monomers or dimers. Summary I r r a d i a t i o n o f a s u i t a b l e monomer under UV l i g h t i n t h e presence o f a p h o t o s e n s i t i z e r or complexing agent r e s u l t s i n the i n i t i a t i o n o f c o n v e n t i o n a l r a d i c a l or i o n i c p o l y m e r i z a t i o n , wherein t h e monomer adds t o the propagating c h a i n end. I r r a d i a ­ t i o n o f ground s t a t e comonomer charge t r a n s f e r complexes r e s u l t s i n e x c i t a t i o n f o l l o w e d by homopolymerization, wherein the comono­ mer e x c i p l e x adds t o the propagating c h a i n end t o y i e l d equimolar, a l t e r n a t i n g copolymers, e.g. S-MMA. and S-AN i n t h e presence o f RJU. or RA1X. The t e r p o l y m e r i z a t i o n o f BD-MMA.-AN i n the presence or RAlClg y i e l d s terpolymers whose composition v a r i e s w i t h temp­ e r a t u r e i n the dark and i s independent o f temperature under UV light. The ground s t a t e comonomer complexes from BD-maleic

10

UV LIGHT INDUCED REACTIONS IN

POLYMERS

anhydride and BD-AN-A1C1.- and - E t A l C l ^ undergo c y c l i z a t i o n t o y i e l d adducts i n the dark and e x c i t a t i o n followed by homopolymer­ i z a t i o n under UV l i g h t . E x c i t e d ethylene monomer or dimer par­ t i c i p a t e s i n the propagation step i n the p o l y m e r i z a t i o n o f e t h y l ­ ene under UV l i g h t i n the presence o f oxygen. Homopolymerization of maleic anhydride under UV l i g h t i n the presence o f a photosen­ s i t i z e r proceeds through propagation o f the e x c i t e d monomer or dimer. Literature Cited 1. Sparapany, J. J . , J. Amer. Chem. Soc. (1966) 8 8 , 1357. 2 . Tazuke, S., A s a i , M., Ikeda, S., Okamura, S., J . Polym. S c i . , Part Β (1967) 5 , 4 5 3 . 3 . Natsuume, T., Akana, Y., Tanabe, K., Fujimatsu, Μ., Shimizu, M., Shirota, Y., Hirata, H., Kusabayashi, S., Mikawa, Η., Chem. Commun. (1969) 189. 4. Sakamoto, M., Hayashi, Κ., Okamura, S., J . Polym. S c i . , Part Β (1965) 3 , 2 0 5 . 5 . I r i e , M., Tomimoto, S., Hayashi, K., J . Polym. S c i . , Part Β (1972) 1 0 , 699, 6 . Mao, T. J . , Eldred, R. J . , J . Polym. S c i . , Part A-1 (1967) 5 , 1741. 7. Tazuke, S., Okamura, S., J . Polym. S c i . , Part A-1 (1969) 7, 715. 8 . Gaylord, N. G., D i x i t , S. S., J . Polym. S c i . , Part Β (1971) 9, 823. 9 . Tamura, Η., Tanaka, Μ., Murata, Ν., B u l l . Chem. Soc. Japan (1969) 42, 3042. 10. Gaylord, N. G., D i x i t , S. S., M a i t i , S., Patnaik, Β. K., J . Macromol. S c i . (Chem.) (1972) A6, 1495. 11. Furukawa, J . , Kobayashi, E., Iseda, Y., A r a i , Y., J . Polym. S c i . , Part Β (1971) 9, 179. 1 2 . Furukawa, J . , Kobayashi, Ε., A r a i , Y., J . Polym. S c i . , Part Β (1971) 9 , 8 0 5 . 1 3 . Gaylord, N. G., M a i t i , S., D i x i t , S. S., J . Macromol. S c i . (Chem.) (1972) A 6 , 1521. 14. Furukawa, J . , Kobayashi, Ε., Haga, Κ., Iseda, Y., Polym. J . (1971) 2 , 4 7 5 . 15. Machi, S., Hagiwara, M., Kagiya, T., J . Polym. S c i . , Part Β (1966) 4, 1019. 16. Hagiwara, Μ., Okamoto, Η., Kagiya, T., Kagiya, T., J . Polym. S c i . , Part A-1 (1970) 8 , 3295. 17. Hagiwara, Μ., Okamoto, H., Kagiya, T., J . Polym. S c i . , Part A-1 (1970) 8 , 3303. 1 8 . Bryce-Smith, D., G i l b e r t , Α., Vickery, B., Chem. Ind. (Lon­ don) (1962 ) 2 0 6 0 . 1 9 . Lang, J . L., Pavelich, W. Α., Clarey, H. D., J . Polym. S c i . , Part A (1963) 1, 1123.

1. 20.

GAYLORD

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11

Gaylord, N. G., M a i t i , S., J . Polym. S c i . , Polym. L e t t . Ed. (1973) 11, 2 5 3 . 2 1 . White, Ε. H., Miano, J . D., Watkins, C. J . , Breaux, E. J . , Angew. Chem. Internat. Ed. (1974) 1 3 , 229. 2 2 . Lepley, A. R., Closs, G. L., eds., "Chemically Induced Mag­ n e t i c P o l a r i z a t i o n " , Wiley, New York, 1973.