Polymer Stabilization and Degradation - American Chemical Society

15

Downloaded by UNIV OF NEW SOUTH WALES on August 26, 2015 | http://pubs.acs.org Publication Date: June 14, 1985 | doi: 10.1021/bk-1985-0280.ch015

Polymerizable, Polymeric, and Polymer-Bound (Ultraviolet) Stabilizers 1

O T T O V O G L , A N N CHRISTINE A L B E R T S S O N , and ZVONIMIR J A N O V I C

2

Polytechnic Institute of New York, Brooklyn, NY 11201

A number of polymerizable ultraviolet stabilizers have been synthesized and their homo- and copolymerization studied. The initial work consisted of the synthesis of vinyl derivatives of methyl salicylate (three isomers), 2,4-dihydroxybenzophenone, and ethyl α-cyano-β-phenylcinnamate. More recently, vinyl derivatives (two isomers) and one isopropenyl derivative of 2(2-hydroxyphenyl)2H-benzotriazole have been prepared. A number of benzotriazoles with more than one benzotriazole ring in the molecule, or compounds with both benzo(or aceto)phenone and 2(2-hydroxyphenyl)2H-benzotriazole groups in one molecule, have also been synthesized. Acryloyl and methacryloyl derivatives of benzotriazolesubstituted polyphenols have been prepared and homo- and copolymerized. Other polymerizable ultraviolet stabilizers and antioxidants have also been synthesized and incorporated into polymeric structures.

Many polymers undergo thermal and thermal oxidative degradation during fabrication processing and require stabilization. Over longer periods of time and at ambient temperature, polymers also deteriorate in the solid state (aging, weathering) through autooxidation and photooxidation. Stabilization against these two types of deterioration is also necessary. In outdoor applications, where the materials NOTE: This paper was written in cooperation with Eberhard Borsig, Amitava Gupta, Shanjun Li, Zohar Nir, Witold Pradellok, Fu Xi, and Shohei Yoshida. Current address: Department of Polymer Technology, Royal Institute of Technology, Stockholm, Sweden. Current address: INA-Research Institute, Zagreb, Yugoslavia. 1

2

0097-6156/85/0280-0197$06.00/0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


198

POLYMER STABILIZATION AND DEGRADATION

are exposed to u l t r a v i o l e t solar r a d i a t i o n above 2 9 0 nm, the energy of t h i s r a d i a t i o n i s p r i m a r i l y absorbed by functional groups, p a r t i c u l a r l y carbonyl groups (impurities i n polymers:carbonyl groups are impurities i n p o l y o l e f i n s , but major components i n a c r y l i c s , for example); t h e i r excited states can lead to the i n i t i a t i o n of photochemical reactions and, u l t i m a t e l y , to polymer degradation. Fortuna t e l y , the quantum y i e l d s of many photochemical reactions are low, but over a period of years, degradation occurs. Some of t h i s degradation i s aided by moisture; the t o t a l exposure to the environment i s c a l l e d weathering. The deterioration of p l a s t i c materials: oxidat i o n , chain cleavage, c r o s s l i n k i n g , and the elimination of small molecules, leads to the loss of the desirable properties and to the f a i l u r e of the polymer. P l a s t i c s are commonly protected against such deterioration (l-k) by the addition of antioxidants against thermal oxidation, and of u l t r a v i o l e t s t a b i l i z e r s which can absorb the damaging r a d i a t i o n and re-emit i t i n a harmless form or function by trapping r a d i c a l s or decomposing hydroperoxides. The s t a b i l i z e r s must be effective over a long period of time. It i s important that the s t a b i l i z e r s do not v o l a t i l i z e , be leached out, or can otherwise be removed from the p l a s t i c m a t e r i a l . It i s also important that the s t a b i l i z e r be d i s t r i b u t e d i n the polymeric matrix where i t i s most needed, p a r t i c u l a r l y on the surface of the materials. These conditions require that the s t a b i l i z e r s be comp a t i b l e , p a r t i c u l a r l y within the amorphous f r a c t i o n of the polymer. In semicrystalline materials, the s t a b i l i z e r s are substantially excluded from the c r y s t a l l i n e part and the spherulites of such polymers. Low molecular weight molecules r e a d i l y diffuse through the amorphous portion of the polymer; even higher molecular weight materials often move through the amorphous matrix. In p o l y o l e f i n s , the s o l u b i l i t y of s t a b i l i z e r s i s r e l a t i v e l y low because most s t a b i l i z e r s , both antioxidants and u l t r a v i o l e t s t a b i l i z e r s , are aromatic compounds of r e l a t i v e l y high p o l a r i t y which are not very compatible with the p a r a f f i n l i k e hydrocarbon polymers. S i g n i f i c a n t amounts of s t a b i l i z e r s are often l o s t from polymeric materials because of v o l a t i l i z a t i o n during f a b r i c a t i o n or because of exudation, leaching, or solvent extraction during end use. This problem i s especially severe with semicrystalline polymers which have r e l a t i v e l y small amorphous fractions and with a r t i c l e s having a high surface-to-volume r a t i o , such as films or f i b e r s . M o b i l i t y and compatibility of chemical compounds i n a polymer matrix are functions of molecular weight, molecular s i z e , and r e l a t i v e p o l a r i t y of the s t a b i l i z e r s (antioxidants, u l t r a v i o l e t s t a b i l i z e r s ) . To increase compatibility and decrease the loss of s t a b i l i z e r s , high molecular weight products having several s t a b i l i z i n g groups i n the same molecule have been synthesized. Molecules with s t a b i l i z e r c h a r a c t e r i s t i c s have been used as capping agents for o l i gomers or low molecular weight polymers, or have been attached to compatibilizing groups such as l i n e a r a l k y l groups of 1 2 - 1 5 carbon atoms for s t a b i l i z e r s to be used i n p o l y o l e f i n s . A l l t h i s work has l e d t o s t a b i l i z e r s that are less v o l a t i l e , more compatible, and are consequently less r e a d i l y l o s t during f a b r i cation and exposure t o the environment. The myth that s t a b i l i z e r s must have r e l a t i v e l y high mobility to be effective has, however, p e r s i s t e d , and only recently have indications become compelling that

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


15.

VOGLETAL.

Polymerizable Ultraviolet Stabilizers

199

high mobility of low molecular weight compounds i s not e s s e n t i a l for the effectiveness of s t a b i l i z e r s i n polymeric materials. In f a c t , incorporation of polymerizable s t a b i l i z e r s into polymer chains has been found to make them e f f e c t i v e , e s p e c i a l l y i n long-term exposure. Sometimes a f l e x i b l e , rather short spacer group between the polymer backbone chain and the active s t a b i l i z e r moiety i s necessary; t h i s general approach has become the most up-to-date target of i n v e s t i g a tions. Recently, epoxy and o l e f i n monomers with spacer groups between the functional group and the polymerizable groups have been synthesized and t h e i r polymerization established; functional epoxides and olefins polymerize well when the functional ester group i s separated from the polymerizable group by a spacer group of at least three methylene groups ( 5 , 6 ) , onto which the polymer s t a b i l i z e r groups can now be attached. Polymerizable and Bound U l t r a v i o l e t S t a b i l i z e r s When u l t r a v i o l e t r a d i a t i o n i s absorbed by a macromolecule which i s excited to a higher energy state, the excited compound or the macromolecule usually returns to the electronic ground state because the energy i s dissipated by photophysical pathways by: (a) r a d i a t i o n less conversion to the ground state and release of v i b r a t i o n a l energy (heat), (b) emission of the energy of longer wavelengths (fluorescence or phosphorescence), (c) transfer of the energy to another molecule, as i n the case of r a d i c a l trappers or hydroperoxide decomposers. T y p i c a l u l t r a v i o l e t absorbers are s a l i c y l a t e esters, 4-aminobenzoate esters, o/-cyano-[3-phenylcinnamate esters, 2-hydroxybenzophenones, 2(2-hydroxyphenyl)2H-benzotriazoles (Figure l ) , and more recently diaminodiphenyloxamides. Some pigments and various types of carbon blacks are also known as effective u l t r a v i o l e t s t a b i l i z e r s . Another mechanism by which the s t a b i l i z a t i o n of the polymers can be accomplished i s by quenching the excited state of the molecules. Very few compounds are known where the energy of the excited state i s transferred to another molecule; n i c k e l chelates are the best-known examples. Review of Approaches. Over the l a s t two decades, observations have been made suggesting that polymers containing u l t r a v i o l e t s t a b i l i z i n g groups d i r e c t l y attached to the polymer chain may be very effective i n elastomeric or amorphous (glassy) polymers. Some experiments have shown that such s t a b i l i z e r s incorporated by grafting d i d not diminish the a c t i v i t y of the s t a b i l i z e r s . Most attempts i n the past to prepare higher molecular weight u l t r a v i o l e t s t a b i l i z e r s have used the reaction of a preformed polymer with an u l t r a v i o l e t - a b s o r b i n g small molecule, e . g . , of polymeric methacrylic acid salts with the 2-bromoethyl ether of the 4-hydroxyl group of 2,4-dihydroxybenzophenone (2). Probably the f i r s t polymerizable u l t r a v i o l e t absorbers for gene r a l use were a c r y l o y l or methacryloyl derivatives of the k-hydroxy group of 2,4-dihydroxybenzophenone (7)• A c r y l i c and methacrylic esters have also been prepared from 2(2-hydroxyphenyl)2H-benzotriazole s with a phenolic hydroxyl group i n the carbocyclic ring of a benzotriazole group (8).




280 300

350

Wovelength 6XI0T M 4

gure 1.

400

(nm)

In C H C I . 0 . l c m 2

2

Typical ultraviolet

path

absorbers.



15. VOGL ET AL.

Polymerizable Ultraviolet Stabilizers

201

A l l y l groups have also been introduced i n the 3 p o s i t i o n of 2 , 4 dihydroxybenzophenone. Nucleophilic displacement reaction of the chloride i n 4-chloromethylstyrene by phenolates of 2 , 4 - d i h y d r o x y benzophenone ( 9 ) or 2(2-hydroxy-5-methylphenyl)4'-hydroxybenzotriazole also provided a method of producing styrene-type polymerizable ultraviolet stabilizers ( 9 , 1 ° ) • Another polymerizable group was introduced by allowing the nonhindered phenolic hydroxyl group i n k p o s i t i o n of 2,4-dihydroxybenzophenone to react with polyepichlorohydrin. A l l y l o x y , vinyloxy, v i n y l s u l f o n y l , and acryloamido groups have also been u t i l i z e d i n making polymerizable l i g h t s t a b i l i z e r s ( 1 0 ) . When s t a b i l i z i n g groups, including u l t r a v i o l e t - a b s o r b i n g groups, are introduced into polymers by polymer reactions, they are accompanied by side reactions causing color formation. By using an azo i n i t i a t o r (which contains an u l t r a v i o l e t - a b s o r b i n g group, 2-hydroxybenzophenone) as polymerization i n i t i a t o r , i t was possible to i n t r o duce a s t a b i l i z e r at the end of the polymer chain ( l ) . Ultraviolet-absorbing groups were also modified i n such a way that they could be incorporated into condensation polymers, p a r t i c u l a r l y into polyesters, polyamides, and polyurethanes by using t r i and t e t r a - s u b s t i t u t e d 2-hydroxybenzophenones with polymerizable hydroxyl and carbonyl groups i n the k and k' p o s i t i o n s , leaving thus the hydrogen-bonding hydroxyl group i n the orthoposition to the c a r bonyl group. Very recently i t was found that substituted diaminophenyloxamides are effective u l t r a v i o l e t s t a b i l i z e r s . We had now prepared regular copolyoxamides with 4,4'-diaminodiphenyloxamide and 3 * 3 ' - d i aminodiphenyloxamide with various d i a c i d chlorides ( l l ) . Polymerizable U l t r a v i o l e t S t a b i l i z e r s - Miscellaneous Types. In our research on polymerizable u l t r a v i o l e t s t a b i l i z e r s , we have decided to prepare styrene-type monomers i n which the v i n y l (or isopropenyl) group i s d i r e c t l y attached to the phenyl group of the s t a b i l i z e r , which might be polymerized s i m i l a r l y to styrene. These monomers can indeed be polymerized and copolymerized successfully with styrene, a c r y l i c and methacrylic acid derivatives 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 the r a d i c a l i n i t i a t o r ( 1 2 - 1 4 ) . Syntheses of styrene derivatives are normally c a r r i e d out by two general methods ( 1 2 ) : (a) Dehydration of a 1-hydroxyethyl group attached to the benzene r i n g , which i n turn i s e a s i l y obtained by F r i e d e l - C r a f t s acylation followed by sodium borohydride reduction. The dehydration technique requires that the reaction be c a r r i e d out i n a good vacuum and the product formed be s u f f i c i e n t l y v o l a t i l e to be removed immediately from the r e a c t i o n , otherwise polymerization (or oligomerization) occurs which reduces the y i e l d of the monomer, (b) Bromination of the e t h y l group with N-bromosuccinimide followed by dehydrobromination of the 1-bromoethyl d e r i v a t i v e , conveniently c a r r i e d out with a l i p h a t i c t e r t i a r y amines i n aprotic solvents, such as a c e t o n i t r i l e or dimethylacetamide (Structure l ) . For the preparation of methyl v i n y l s a l i c y l a t e s ( 3 - , 5-isomers) both synthetic routes were used. Methyl 5 - v i n y l s a l i c y l a t e was synthesized i n about 5 0 $ o v e r a l l y i e l d from methylsalicylate by the f i r s t route (dehydration) ( 1 5 ) , the synthesis of methyl 3 - v i n y l s a l i cylate ( 1 6 ) and methyl 4 - v i n y l s a l i c y l a t e ( 1 7 ) s t a r t i n g from 2 - e t h y l -



202 CH,

CH,

CHt

UBS

B r

0

CCU

I

CH,

H

DM Ac

I

Polymer Stabilization and Degradation - American Chemical Society

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