Photodegradation and Photostabilization of Coatings - ACS Publications

4 Preparation of P o l y m e r i c U l t r a v i o l e t Stabilizers

Downloaded by UNIV OF CINCINNATI on February 18, 2015 | http://pubs.acs.org Publication Date: April 8, 1981 | doi: 10.1021/bk-1981-0151.ch004

DAVID A. TIRRELL Department of Chemistry, Carnegie-Mellon University, Pittsburgh, PA 15213

A consideration of conventional ultraviolet stabilizers used to prevent or retard the photooxidation of organic polymers, shows that overall effectiveness is often severely limited by poor long-term performance. This is not, in general, a result of photodecomposition of the stabilizer, but rather of physical loss of the stabilizer from the matrix polymer through diffusion, through extraction or through phase separation processes. The solubility of the stabilizer in the matrix is often less than the minimum effective concentration, leading to stabilizer migration, and exposure to solvents and/or high temperatures in processing or in use can accelerate stabilizer loss. These problems are particularly acute in the stabilization of thin films and coatings. It is primarily the prospect of reduced mobility and volatility, with the expected improvement in long-term perfor mance, which motivates the preparation of polymeric ultraviolet stabilizers. Work in this area was reviewed thoroughly by Bailey and Vogl in 1976 (1), and more recently by the author (2). The present paper describes recent synthetic work involving four classes of effective ultraviolet stabilizers: salicylate esters (I), 2-hydroxybenzophenones (II), a-cyano-3-phenylcinnamates (III) and hydroxyphenylbenzotriazoles (IV). In each HO II

I

IV 0097-6156/81/0151 -0043$05.00/0 © 1981 American Chemical Society In Photodegradation and Photostabilization of Coatings; Pappas, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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case, the approach has i n v o l v e d the p r e p a r a t i o n o f a modified s t a b i l i z e r which i s i n f a c t a s u b s t i t u t e d styrene, and two c l a s s i c a l styrene syntheses have been employed (Scheme I ) . I n Scheme l a , the aromatic r i n g o f the s t a b i l i z e r i s a c e t y l a t e d HOCHCH

CH=CH

3

r ^ - O l

1) CH C0C1 Q

2) NaBH,


4

>

jell

2

3

)

m

S

0

/

^

la BrCHCH_

CH=CH

ft

lb Scheme I i n a F r i e d e l - C r a f t s r e a c t i o n , and r e d u c t i o n to the b e n z y l i c a l c o h o l followed by dehydration produces the polymerizable s t a b i l i z e r d e r i v a t i v e . T h i s route i s p a r t i c u l a r l y u s e f u l i n the synthesis o f r e l a t i v e l y v o l a t i l e compounds (e.g. the s a l i c y l a t e s ) s i n c e the dehydration i s extremely r a p i d and the product can be d i s t i l l e d immediately from the h o t r e a c t i o n f l a s k . The sequence i n Scheme l b has been more g e n e r a l l y u s e f u l . The a c c e s s i b i l i t y o f e t h y l - s u b s t i t u t e d s t a b i l i z e r precursors suggests b e n z y l i c bromination w i t h N-bromosuccinimide followed by base-catalyzed dehydrobromination. T e r t i a r y amines were found to be p a r t i c u l a r l y e f f e c t i v e i n the syntheses described i n t h i s paper. R a d i c a l polymerizations o f v i n y l - s u b s t i t u t e d u l t r a v i o l e t s t a b i l i z e r s were accomplished with a z o b i s i s o b u t y r o n i t r i l e (AIBN) as i n i t i a t o r , w i t h c a r e f u l e x c l u s i o n o f oxygen. Copolymerization was a l s o r e a d i l y achieved. The f o l l o w i n g s e c t i o n s d e s c r i b e i n d e t a i l the p r e p a r a t i o n o f polymeric u l t r a v i o l e t s t a b i l i z e r s from s a l i c y l a t e e s t e r s , 2-hydroxybenzophenones, a-cyano-3-phenylcinnamates and hydroxyphenylbenzotriazoles. S a l i c y l a t e E s t e r s . The methyl e s t e r s o f three isomeric v i n y l s a l i c y l i c acids (the 3-, 4- and 5 - v i n y l compounds) have been prepared, u s i n g the s y n t h e t i c routes o u t l i n e d above. Methyl 5 - v i n y l s a l i c y l a t e was prepared as shown i n Scheme I I (3). F r i e d e l - C r a f t s a c e t y l a t i o n o f methyl s a l i c y l a t e gave the 5 - a c e t y l d e r i v a t i v e i n 80% y i e l d . Blocking o f the phenol followed by NaBH^ r e d u c t i o n then provided methyl 5-(1-hydroxyethy1)acetyls a l i c y l a t e i n an o v e r a l l y i e l d of 70%. The hydroxyethyl compound was dehydrated over KHSO^ at 225°C/0.2 mm, and the

In Photodegradation and Photostabilization of Coatings; Pappas, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

TIRRELL

4.

Polymeric

UV

45

Stabilizers

HOgHCH

CH=CH

3

1)

2

CH C0C1^

4) KHS0

3

4


Scheme I I phenol freed by treatment w i t h sodium methoxide i n methanol. The o v e r a l l y i e l d o f methyl 5 - v i n y l s a l i c y l a t e was 35%. Methyl 5 - v i n y l s a l i c y l a t e was polymerized and copolymerized w i t h v i n y l monomers, u s i n g AIBN as r a d i c a l i n i t i a t o r , apparently without i n t e r f e r e n c e by the p h e n o l i c hydroxy1 groups. The p r e p a r a t i v e route to the 3- and 4 - v i n y l compounds i s i l l u s t r a t e d i n Scheme I I I , f o r the p r e p a r a t i o n o f methyl 3-vinylsalicylate (4). CH CH 2

'

CH CH

3

2

1)

C0 ,K C0 2

2

3

2) CH OH,H SO/ 3) A c 0 3

9

1

2

4

A/°

3

, NBS

AC

I" A C0 CH

5) Et N,CH CN ) H ONa ^ 3

X

2

, I

4

3

6

C

3

3

L "CO^

Scheme I I I 2-Ethylphenol was carbonated i n the p o s i t i o n ortho t o the hydroxyl group by treatment w i t h C O 2 under pressure, i n the presence o f anhydrous K 2 C O 3 at 175°C. The y i e l d o f 3 - e t h y l s a l i c y l i c a c i d was 72%. E s t e r i f i c a t i o n w i t h methanol, a c e t y l a t i o n w i t h a c e t i c anhydride, and b e n z y l i c bromination w i t h N-bromosuccinimide a f f o r d e d methyl 3-(1-bromoethy1)acetyls a l i c y l a t e i n an o v e r a l l y i e l d o f 42%. NBS was s u p e r i o r to B r i n C C I 4 i n the b e n z y l i c bromination; use o f the l a t t e r reagent produced a mixture o f the d e s i r e d product p l u s methyl 3-(l-bromoe t h y l ) - a - b r o m o a c e t y l s a l i c y l a t e . Dehydrobromination w i t h t r i e t h y l a m i n e i n a c e t o n i t r i l e , followed by treatment w i t h sodium methoxide i n methanol, gave methyl 3 - v i n y l s a l i c y l a t e i n 22% y i e l d o v e r a l l from 2-ethylphenol. T r i e t h y l a m i n e i n a c e t o n i t r i l e proved to be a convenient dehydrobrominating agent; the r e a c t i o n was complete a f t e r f i v e hours, and i s o l a t i o n o f the product was f a c i l i t a t e d by p r e c i p i t a t i o n o f the amine hydrobromide and by the v o l a t i l i t y of the s o l v e n t . The y i e l d o f the dehydrobromination was 83% u s i n g t h i s reagent, v s . 56% w i t h t r i - n - b u t y l a m i n e i n dime thylacetamide. A s i m i l a r r o u t e — c a r b o n a t i o n , bromination, dehydrobrominat i o n — p r o d u c e d methyl 4 - v i n y l s a l i c y l a t e i n 28% y i e l d from 3-ethylphenol ( 5 ) . Three a l t e r n a t i v e routes to 3 - v i n y l s a l i c y l i c a c i d d e r i v a t i v e s were a l s o i n v e s t i g a t e d b r i e f l y , without success: carbonation o f 2


PHOTODEGRADATION


46

AND

PHOTOSTABILIZATION

OF

COATINGS

2- hydroxyacetophenone, F r i e s rearrangement o f a c e t y l s a l i c y l i c a c i d , and f o r m y l a t i o n o f s a l i c y l i c a c i d . None gave the d e s i r e d 3- s u b s t i t u t e d s a l i c y l i c a c i d i n h i g h y i e l d . The p o l y m e r i z a t i o n o f v i n y l s a l i c y l i c a c i d d e r i v a t i v e s might be expected to be complicated by termination or t r a n s f e r r e a c t i o n s i n v o l v i n g the phenolic hydroxy1 group. Our observat i o n s concerning t h i s p o i n t seem to p a r a l l e l those of Kato (6), who s t u d i e d the r a d i c a l p o l y m e r i z a t i o n of hydroxystyrenes w i t h AIBN an i n i t i a t o r . The k i n e t i c s and mechanism of the polymerizations o f meta- and para-hydroxystyrenes were found to be those of t y p i c a l r a d i c a l p o l y m e r i z a t i o n , w h i l e i n t r a m o l e c u l a r d e a c t i v a t i o n o f the growing chain was p o s t u l a t e d to account f o r reduced molecular weight and anomalous k i n e t i c behavior i n the p o l y m e r i z a t i o n o f the ortho-hydroxy compound. In the v i n y l s a l i c y l i c a c i d d e r i v a t i v e s prepared i n t h i s work, the hydroxyl group i s i n v o l v e d i n strong hydrogen-bonding w i t h the c a r b o n y l of the neighboring e s t e r o r a c i d f u n c t i o n , and the e f f e c t o f t h i s i n t e r a c t i o n on the r e a c t i v i t y of the phenol i s not known w i t h c e r t a i n t y . However, i-f one compares the molecular weights of the polymers o f the methyl e s t e r s o f 3-, 4- and 5 - v i n y l s a l i c y l i c a c i d s , the suggestion i s again that o n l y the hydroxyl group ortho to the growing chain end c o n t r i b u t e s s i g n i f i c a n t l y to termination or t r a n s f e r (Table I ) . Table I Inherent V i s c o s i t i e s of Poly(Methyl V i n y l s a l i c y l a t e s ) Polymer Poly(methyl 5 - v i n y l s a l i c y l a t e ) Poly(methyl 4 - v i n y l s a l i c y l a t e ) Poly(methyl 3 - v i n y l s a l i c y l a t e )

^inh 2.46 2.61 0.16

d4/g d^/g d^/g

(DMSO) (DMSO) (Benzene)

The data i n Table I are not d i r e c t l y comparable, since the v i s c o s i t y of the 3-isomer was determined i n benzene while the others were measured i n DMSO. In a d d i t i o n , the f i r s t two polymers were prepared i n bulk p o l y m e r i z a t i o n s , while the p o l y m e r i z a t i o n o f methyl 3 - v i n y l s a l i c y l a t e was c a r r i e d out w i t h the monomer d i l u t e d 1:1 w i t h benzene. Thus no c e r t a i n c o n c l u s i o n can be drawn; the data are, however, an i n d i c a t i o n of p o s s i b l e d i f f i c u l t y i n r a d i c a l p o l y m e r i z a t i o n of s u b s t i t u t e d styrenes b e a r i n g a phenol ortho t o the v i n y l group. f

2,4-Dihydroxy-4 -Vinylbenzophenone. 2,4-Dihydroxy-4' vinylbenzophenone was prepared i n 30% y i e l d from 4-ethylbenzoic a c i d as shown i n Scheme IV (7). A c y l a t i o n of r e s o r c i n o l by 4-ethylbenzoic a c i d gave 2,4-dihydroxy-4 -ethylbenzophenone i n 72% y i e l d . Attempted bromination of t h i s compound w i t h NBS y i e l d e d not the b e n z y l i c bromide, but r a t h e r ring-brominated products. Since r i n g f


TIRRELL

4.

Polymeric

UV

Stabilizers

47

OAc

OH


Scheme IV bromination w i t h NBS had p r e v i o u s l y been observed f o r aromatic compounds w i t h s t r o n g l y e l e c t r o n - d o n a t i n g s u b s t i t u e n t s (8), the phenols were blocked by a c e t y l a t i o n , and the bromination repeated. A 75% y i e l d o f 2,4-diacetoxy-4 -(l-bromoethyl)benzophenone was obtained. Dehydrobromination with t r i - n - b u t y l a m i n e i n DMAc, followed by treatment w i t h NaHCO^ i n aqueous methanol produced the d e s i r e d 2,4-dihydroxy-4 -vinylbenzophenone. 2,4-Dihydroxy-4 -vinylbenzophenone was converted to a homopolymer o f inherent v i s c o s i t y 0.57 djfc/g by p o l y m e r i z a t i o n w i t h AIBN i n dimethylformamide. The UV spectrum o f the polymer showed the three a b s o r p t i o n maxima c h a r a c t e r i s t i c o f 2,4dihydroxybenzophenones (at 324, 292 and 248 nm), although the e x t i n c t i o n c o e f f i c i e n t was depressed i n comparison w i t h the 4 - e t h y l analogue. The behavior o f 2,4-dihydroxy-4'-vinylbenzophenone i n r a d i c a l copolymerization w i t h styrene was q u i t e unexpected. The presence o f two p h e n o l i c hydroxyls again suggests p o s s i b l e t r a n s f e r o r t e r m i n a t i o n i n r a d i c a l p o l y m e r i z a t i o n , so we examined c a r e f u l l y the e f f e c t s o f adding small amounts o f 2,4dihydroxy-4 -vinylbenzophenone o r 2,4-dihydroxy-4 -ethylbenzophenone t o a r a d i c a l p o l y m e r i z a t i o n o f styrene. The r e s u l t s were dependent on the i n i t i a t o r used: w i t h AIBN, the p h e n o l i c compounds d i d not change the polymer molecular weight at a l l , w i t h i n experimental e r r o r , w h i l e w i t h benzoyl peroxide, an i n c r e a s e i n molecular weight was observed. T h i s was a s c r i b e d to the d i f f e r i n g r e a c t i v i t i e s o f the i n i t i a t o r r a d i c a l s , and i t was suggested that o n l y the benzoyloxy r a d i c a l from benzoyl peroxide r e a c t s by a b s t r a c t i o n o f the p h e n o l i c hydrogen atom. The n e t r e s u l t i s a decrease i n i n i t i a t o r e f f i c i e n c y and an attendant i n c r e a s e i n polymer molecular weight. In fact, a l l o f our work on r a d i c a l p o l y m e r i z a t i o n o f phenol-containing v i n y l monomers suggests that i n h i b i t i o n and t r a n s f e r problems are at most minor, i f AIBN i s used as i n i t i a t o r and oxygen i s c a r e f u l l y excluded from the r e a c t i o n mixtures (£). f

f

1

1

f

1

American Chemical Society Library 1155 16til St. N. W. Washington, D. C. 20038 In Photodegradation and Photostabilization of Coatings; Pappas, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

48

PHOTODEGRADATION

A N D PHOTOSTABILIZATION O F COATINGS

E t h y l 4-Vinyl-g-Cyano-3-Phenylcinnamate. Ethyl 4-vinyl-acyano-3-phenylcinnamate has been prepared r e c e n t l y by Sumida, Yoshida and V o g l (10). The route i s shown i n Scheme V. CH CH 2

CH CH 2

CH=CH„

3

3

i) soci

2

2) B e n z e n e , A I C I 3 3) N C - C H - C 0 E t


2

2

Scheme V 4-Ethylbenzoic a c i d was converted to the a c i d c h l o r i d e , which was t r e a t e d w i t h A I C I 3 and benzene t o give 4-ethylbenzophenone i n 90% y i e l d o v e r a l l . Condensation w i t h e t h y l cyanoacetate a f f o r d e d e t h y l 4-ethyl-a-cyano-3-phenylcinnamate as an e s s e n t i a l l y 50/50 mixture o f the Z- and E-isomers. The y i e l d of the condensation was h i g h l y s e n s i t i v e t o r e a c t i o n c o n d i t i o n s , and was optimized at 75% w i t h portionwise a d d i t i o n o f the ammonium acetate c a t a l y s t . Bromination and dehydrobromination as described e a r l i e r then completed the p r e p a r a t i o n . The o v e r a l l y i e l d o f e t h y l 4-vinyl-a-cyano-3-pbenylcinnamate was 20%. Homopolymerization o f e t h y l 4-vinyl-a-cyano-3-phenylcinnamate w i t h AIBN i n benzene gave a s o l u b l e polymer o f inherent v i s c o s i t y 0.2 d#/g. There was no evidence f o r involvement o f the t e t r a s u b s t i t u t e d double bond i n the p o l y m e r i z a t i o n . Copolymerizations w i t h styrene and methyl methacrylate were a l s o s u c c e s s f u l . 2 -(5-Vinyl-2-hydroxyphenyl)benzotriazole. Yoshida and Vogl have r e c e n t l y prepared 2-(5-vinyl-2-hydroxyphenyl)benzotriazole by the route shown i n Scheme VI (11). t\ w *rn / u n CflLCH. 4) A c 0 CH=CH NO D NaN0 /HCl^ V 2 3 | j 2 2

2

a

2

2) 4 - e t h y l phenol 2 3) Zn/NaOH

5

>

)

'

N

x

B

"

" *

Scheme VI The hydroxyphenylbenzotriazole s t r u c t u r e was c o n s t r u c t e d by a c o u p l i n g o f the diazonium s a l t o f o - n i t r o a n i l i n e w i t h 4 - e t h y l phenol, followed by r e d u c t i o n o f the nitro-azobenzene to the b e n z o t r i a z o l e w i t h z i n c powder and NaOH. A f t e r b l o c k i n g o f the phenol by a c e t y l a t i o n , bromination and dehydrobromination were performed as d e s c r i b e d e a r l i e r , and treatment w i t h aqueous NaOH


4.

TIRRELL

Polymeric UV Stabilizers

49

provided 2-(5-vinyl-2-hydroxyphenyl)-benzotriazole in an overall yield of 20%. Homopolymerization and copolymerizations with styrene and methyl methacrylate have been accomplished.


Conclusions The incorporation of four different classes of important ultraviolet stabilizers into high polymer chains has been accomplished by synthesis of polymerizable, vinyl-substituted stabilizer derivatives followed by radical polymerization. Each of the derivatives may be regarded as a substituted styrene, and classical styrene syntheses have been employed. Radical polymerization of the phenolic monomers (salicylate esters, 2-hydroxybenzophenones and hydroxyphenylbenzotriazoles) proceeds normally with AIBN as initiator, at least when oxygen is carefully excluded. It is expected that polymeric ultraviolet stabilizers, perhaps in combination with conventional stabilizers^ will make an important contribution to photostabilization technology. Acknowledgement The author would like to thank Professor Otto Vogl for permission to read and use several manuscripts prior to publication. Literature Cited 1. Bailey, D. and Vogl, O. J. Macromol. Sci., Rev. Macromol. Chem., 1976, 14(2), 267. 2. Tirrell, D. Polymer News, in press. 3. Bailey, D.; Tirrell, D.; and Vogl, O. J. Polym. Sci., Polym. Chem. Ed., 1976, 14, 2725. 4. Iwasaki, M.; Tirrell, D.; and Vogl, O. J. Polym. Sci., Polym. Chem. Ed., in press. 5. Tirrell, D. and Vogl, O. Makromol. Chem., in press. 6. Kato, M. J. Polym. Sci., A-1, 1969, 7, 2175. 7. Bailey, D.; Tirrell, D.; Pinazzi, C.; and Vogl, O. Macromolecules, 1978, 11(2), 312. 8. Horner, L. and Winkelman, E. H. in W. Foerst, Ed. "Newer Methods of Preparative Organic Chemistry," Vol. 3, Academic Press, 1964, p. 176. 9. Tirrell, D.; Ph.D. Thesis, University of Massachusetts, 1978. 10. Sumida, Y.; Yoshida, S.; and Vogl, O. Polymer Preprints, 1980, 21(1), 201. 11. Yoshida, S. and Vogl, O. Polymer Preprints, 1980, 21(1), 203. RECEIVED

September 16, 1980.


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