Photostabilising effect of bis(hindered piperidine) compounds: Part I—In photo-oxidative degradation of cis-1,4-poly(butadiene)

Polymer Photochemistry 3 (1983) 47-64

Photostabilising Effect of Bis(Hindered Piperidine) Compounds: Part I In Photo-Oxidative Degradation of cis-l,4-Poly(Butadiene) Y u n g Y u a n Y a n g t , J u l i a L u c k i , J a n F. R a b e k a n d B e n g t R ~ n b y Department of Polymer Technology, The Royal Institute of Technology, Stockholm, Sweden (Received: 9 March, 1982)

ABSTRACT The photostabilizing effect of three commercially available bis(hindered piperidines ): Tinuvin 144 ( bis(1,2,2,6,6-pentamethyl-4piperidinyl)-2-n-butyl-2-( 3,5-di-tert-butyl-4-hydroxy-benzyl) malohate; Tinuvin 292 (bis(1,2,2,6,6-pentamethyl-4-piperidinyl) sebacate ) and Tinuvin 770 ( bis( 2,2,6,6-tetramethyl 1-4-piperidinyl)sebacare) on the free radical photo-oxidative degradation of cis-l,4polybutadiene has been investigated. Their stabilising effects include possible mechanisms such as formation of bis(hindered nitroxyl radicals), which then scavenge polymer alkyl radicals produced during photo-oxidation. Bis(hindered piperidines) may also decompose hydroperoxide groups. The most effective stabiliser of the three was Tinuvin 144 and the least effective was Tinuvin 770. INTRODUCTION In spite of e n o r m o u s research the photodegradation, photo-oxidation and photostabilisation of unsaturated elastomers still offers m a n y unsolved p r o b l e m s J -5 The low stability of polydienes and their susceptibility to oxidation and photo-oxidation, including singlet oxygen oxidation, necessitates stabilisation of the p o l y m e r for practically all technical applications. t Visiting scientist from The Institute of Photographic Chemistry, Academia Sinica, Peking, China 47

Polymer Photochemistry 0144-2880/83/0003-0047/$3.00 © Applied Science Publishers Ltd, England, 1983, Printed in Northern Ireland

48

Yung Yuan Yang, Julia Lucki, Jan F. Rabek, Bengt R~nby

Hindered piperidines were found to be effective stabilisers for the industrially important polyolefins.6-19 Several of these compounds are commercially produced as ultra-violet stabilisers for bulk polymers and fibres. Their stabilising effectiveness depends upon their ability to form nitroxyl radicals, which then scavenge polymer alkyl radicals produced during photo-oxidation. In polyolefins they also inhibit the photo-reactions of carbonyl groups and react with hydroperoxide groups. In this paper, we have investigated the mode of action for three commercially available hindered piperidines for protecting cis-l,4polybutadiene against photo-oxidative degradation. EXPERIMENTAL The following bis(hindered piperidines) were investigated for photostabilising properties: O O

H

H

Bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (Tinuvin 770)

O O II fl O--C--(CH2)8--C--O

Bis(l,2,2,6,6-pentamethyl-4-piperidinyl) sebacate (Tinuvin 292)

o

Bis(•,2,2•6,6-pentamethy•-4-piperidiny•)-2-n-buty•-2-(3•5-di-tert-buty•-4-hydr•xy-benzy•)malonate (Tinuvin 144)

Photostabilising effect of bis (hindered piperidine ) compounds : I

49

All of these three bis(hindered piperidines) are commerically produced by the Ciba-Geigy Corporation and were supplied by this manufacturer. Commercial samples of c i s - l , 4 - p o l y ( b u t a d i e n e ) (PB) (Ameripol CB200 from Goodrich Chemical Co.) were purified by dissolving in benzene (analytically pure grade) and precipitating with spectral ethanol in a nitrogen atmosphere. The procedure was repeated. Free radical photo-oxidation by molecular oxygen was carried out using a high pressure Xenon lamp type XBO 1600 W OFR from Osram which produces light in the range 280-800 nm. The PB samples were exposed to the UV radiation as films (c. 50/xm on sodium chloride or quartz plates) or in c. 1 wt % solution in benzene (analytically pure grade). Ultra-violet absorption spectra of bis(hindered piperidines) were made in methanol (spectrally pure) in 1 cm quartz cell using PerkinElmer 575 U V - V I spectrometer. Infra-red spectra were obtained from PB films cast from 1 wt % solution on sodium chloride plate using a Perkin-Elmer computerised 580B spectrometer. ESR spectra were determined with a Bruker-420 ESR spectrometer using accessories for solid samples, low temperature UV irradiation and temperature-controlled measurements. Kinetic measurements of viscosity number were carried out in a specially constructed viscometer described earlier. 2° The viscometer was irradiated with light from the bottom. Solution viscosities were measured in a water thermostat at 3 2 + 0 . 5 ° C . The solutions for viscosity measurements were made by dissolving PB in benzene to a concentration of 0.167 wt%. The solutions were filtered in order to remove microgel. Determination of molecular weight distribution (MWD) curves for polymer samples was made by gel permeation chromatography (GPC) with tetrahydrofuran (THF) as solvent (spectrally pure) using a Waters Associate instrument Model ALC-201 after removing gel from solutions. The gel content of the PB samples was determined by extracting 0.5 g (W1) with benzene for 2 4 h in a Soxhlet apparatus equipped with coarse Alundum thimbles (RA98) which retain particles larger than 20 txm. The weight of the insoluble portion (W2) was measured after drying to constant weight. The gel content (in wt%) was calculated a s IO0(W2]WI). Oxygen uptake was measured by a specially constructed oxygen

50

Yung Yuan Yang, Julia Lucki, Jan F. Rabek, Bengt Rtmby

pressure monitor with the pressure transducer (Validyne type P10) mounted into the metallic cell with quartz window which was immersed in a thermostatted water bath at 3 0 + 0 . 0 1 °C. Polymer film samples 2 cm in diameter were first pre-evacuated at 10 -5 torr and the cell was filled with pure oxygen at 760 torr. The cell was irradiated by U V radiation from the Xenon lamp in the bath. Oxygen uptake was monitored with a CD12 transducer indicator and an XY recorder.

RESULTS A N D DISCUSSION The absorption spectra of the bis(hindered piperidines) used are shown in Fig. 1. Tinuvin 292 and 770 have very weak UV absorption which is practically zero at > 2 5 0 n m , whereas Tinuvin 144 has extended absorption up to 300 nm due to the presence of a phenyl group. These absorption spectra show that bis(hindered piperidines) cannot act as UV absorbers since they do not absorb light in the U V region used in these experiments (i.e. >280 nm). PB is a very sensitive polymer for free radical photo-oxidation and singlet oxygen oxidation. 3-5 This process can be easily observed during the photoirradiation of polymer solution in the presence of air and monitoring the kinetic changes in the polymer solution viscosities (Fig. 2). The 2.1-

A 1.8-

•

I

1.2

0.6

i

200

i

250

i

300

nrn

Fig. 1. Absorption spectra of bis(hindered piperidines) in methanol: ( ) Tinuvin 144 (5 × 10-5M); ( - - - - ) Tinuvin 292 (1× 10-3M) and - - ' - - ) Tinuvin 770 (1 x 10-3M).

Photostabilising effect of bis( hindered piperidine ) compounds: I

51

1.0-

I~.sp Sp .

0.6 ¸

0.2-

110

210

310

40I

510 t(min)

Fig. 2. Change in viscosity ratio for PB (0"167M) in benzene-methanol solution (9 : 1) during UV irradiation in the presence of air: (----(3--) pure PB, (---B~) with Tinuvin 144 (5-7 x 10-4M), (----~--) with Tinuvin 292 (5.7 × 10 4M) and ( ~ ) with Tinuvin 770 (5.7 x 10-4M). rapid photo-oxidative degradation process in solution can be strongly suppressed by the addition of bis(hindered piperidines) or a concentration of 5"72 × 10 -4 M (Fig. 2). The gel p e r m e a t i o n chromatograms (GPC (Fig. 3) show that addition of bis(hindered piperidines) efficiently retards the degradation process of PB during the UV irradiation of polymer in solution (Fig. 3).

Yung Yuan Yang, Julia Lucki, Jan F. Rabek, Bengt Rdnby

52

g

I

49

m

I

J

Q

O

I

I

f

45

I

I

I

I

I

40

I

|

i

I

J

I

I

I

I

35

~

30

I

I

I

I

I

25

Counts

Fig. 3. Gel p e r m e a t i o n chromatograms of PB: ( ) before U V irradiation and ( . . . . . ) after U V irradiation in b e n z e n e - m e t h a n o l solution (9 : 1) in the presence of air and with ( - - - ) Tinuvin 144 (5.7× 10-4M), ( - - . . . . ) Tinuvin 292 (5.7 x 10-4M) and ( - - ' - - ) Tinuvin 770 (5-7 x 10-%a).

The amounts of gel (wt%) formed during U V irradiation of solid PB films containing bis(hindered piperidines) are also much lower than in the case of pure U V irradiated polymer (Fig, 4). All of these results show that the most effective stabiliser is the Tinuvin 144 and the least effective is Tinuvin 770. All three however show a very pronounced influence on the photodegradation and photocrosslinking process which in this case occurs via a free radical mechanism. The kinetics of oxygen uptake of the PB films doped with different bis(hindered piperidines) (Fig. 5) show that all of these compounds have approximately the same effect on the rate of decrease of oxygen consumption (Fig. 5). Addition of bis(hindered piperidines) mark~liy decreases the rate of photo-oxidation processes of PB which was observed by monitoring the formation of hydroperoxy (OOH) groups (Fig. 6), carbonyl groups (Fig. 7) and the relative absorption intensity at 240 nm (Fig. 8).

Photostabilising effect of bis (hindered piperidine) compounds : I

53

100 -Gel%

J 80-

60

40

30

0

60

90

1~0 t(min)

Fig. 4. Change of gel content (%) formed during UV irradiation of solid PB films: ( ~ - - ) pure PB, and with: (--i---) Tinuvin 144, (---[3--) Tinuvin 292 and (----0--) Tinuvin 770. (Molar ratio of PB :Tinuvin = 100:0.34.)

The stabilising mechanism of the bis(hindered piperidines) is complicated. This type of stabiliser is considered to have a multifunctional role in m a n y polymers. It has b e e n postulated that the stabilising effect of bis(hindered piperidines) depends upon their ability to form stable m o n o and/or bis(hindered nitroxyl radicals) (Stage I) which then m a y scavenge polymer alkyl radicals (P') produced during photo-oxidative degradation of PB (Stage II): Stage I

+R" and HO" (R~--w-H, CH3)

+ Radical

R

0

(1)

54

Yung Yuan Yang, Julia Lucki, Jan F. Rabek, Bengt Rgmby Stage H

(2) 0

0 I P

Some authors suggest that the photostabilising effect of bis(hindered piperidines) increases with their oxidation to bis(hindered nitroxyl radicals) (Stage I). 21"22These free stable radicals easily scavenge alkyl and polymeric alkyl radicals but they are not reactive with alkyl peroxy and polymer peroxy radicals. 13' 15. 18, 23-27 Each hindered nitroxyl radical may scavenge several alkyl or polymer alkyl radicals, because it is regenerated by peroxy radicals according to the Stage III reaction. 24,28 Stage III

+ R O O P (or POOP)

(3)

0 0 I P The presence of these three stages can be evaluated from measuring E S R signals obtained from the formation (Stage I), reaction (steady state condition) (Stage II) and regeneration (steady state condition) (Stage III) of bis(hindered piperidine radicals) (Figs 9 and 10). The structures of m o n o and bis(hindered piperidinoxy radicals) have been publilshed in detail in several other p a p e r s 6-1°'13A5"17"29 and will not be discussed here. The E S R spectra of bis(hindered nitroxyl radicals) formed in a polymer matrix show the following changes: the spectrum becomes a superposition of signals of different linewidth, due to the presence of both radicals in a molecularly different state and radicals of restricted mobility. The intensity of the E S R signal from nitroxyl radicals depends on the nature of the N-substituent (-H or -CH3) of the piperidine ring (Figs 9 and 10). The concentrations of free radicals formed after 2 h irradiation of bis(hindered piperidines) (steady state condition for Reactions 1 and 2) are the following: Tinuvin 144---1.2 × 10 5M. Tinuvin 2 9 2 - - 1 . 9 × 10-SM and Tinuvin

Photostabilising effect of bis( hindered piperidine ) compounds:

I

55

6oI 0J

m t~

xl t~

I-a.

Z II1 O lx O

0

I

0

110

I

20

I

30

410

I 50

I

60 t ( m i n )

Fig. 5. Kinetics of oxygen uptake of the solid PB films: ( - - ~ - - ) pure PB sample, and with: ( 4 - - ) Tinuvin 144, (--C]--) Tinuvin 292 and (---0--) Tinuvin 770 during UV irradiation in the presence of air. (Molar ratio of PB:Tinuvin= 100 : 0"34.) 7 7 0 - - 2 . 6 x 10-3M. A f t e r k e e p i n g t h e U V i r r a d i a t e d s a m p l e s in t h e d a r k t w o d a y s t h e c o n c e n t r a t i o n s of f r e e r a d i c a l s i n c r e a s e a n d a r e as follows: T i n u v i n 1 4 4 - - 3 - 6 x 10-sM, T i n u v i n 2 9 2 - - 7 . 8 x 10-sM a n d T i n u v i n 7 7 0 - - 7 . 4 x 10-3M. T h i s i n c r e a s e in t h e E S R signal f r o m t h e

56

Yung Yuan Yang, Julia Lucki, Jan F. Rabek, Bengt R~nby 0.8 IOOH

0.6

0.4-

G.2

0

. . . . 0

I 10

. . . .

I 20

. . . .

I 30

t(hr)

Fig. 6. Kinetics of the formation of hydroperoxy/hydroxy groups on solid PB films during UV irradiation in the presence of air: (---O--) pure PB, and with: ( ~ ) Tinuvin 144, (---~--) Tinuvin 292 and ( ~ ) Tinuvin 770. (Molar ratio of PB: Tinuvin = 100 : 0.34.) samples stored in the dark occurs from the regeneration process of free radicals (Reaction 3). These reactions also show that the stabilisation efficiency of bis(hindered piperidines) does not d e p e n d only on the concentration of bis(hindered nitroxyl radicals) formed. In the case of Tinuvin 770 the a m o u n t of free radicals p r o d u c e d was highest whereas this stabiliser had the smallest effect on the stabilisation of PB. The decrease in the rates of degradation and/or cross-linking of PB b y adding bis(hindered piperidines) can be explained by their reaction with p o l y m e r alkyl radicals f o r m e d during U V irradiation of p o l y m e r in solution or in the solid state (Figs 2-4). In the presence of air several p o l y m e r alkyl radicals react very fast with the oxygen present and form p o l y m e r peroxy radicals (POO'): P ' + Oz

> POO"

(4)

This process must c o m p e t e with reaction 2. These p o l y m e r peroxy radicals abstract a further hydrogen from

Photostabilising effect of bis(hindered piperidine) compounds:

57

I

T; 0

se - ~

,o

e~

°~ ._~

r~

0

¢¢) (.q

~x t..

I

'

0

I

~

'

I

~

I

I

,~

'

8~

I

e~

0

0 0 t~

°.

Yung Yuan Yang, Julia Lucki, Jan F. Rabek, Bengt R&nby

58

1.2 4 t

~A0

1.20-

1.12-

1.08-

1.00 0

I 10

210

3[0

4b

510

t(min)

Fig. 8. Change of the relative absorption intensity of the peak at 240 nm in solid PB films formed during UV irradiation in the presence of air: (--O--) pure PB sample, and with (---l---) Tinuvin 144, (--(]--) Tinuvin 292 and (---0--) Tinuvin 770. (Molar ratio of PB :Tinuvin = 100:0.34.)

the same or from the neighbouring macromolecule and form a h y d r o p e r o x i d e group ( - O O H ) POO'+ PH

~ P O O H + P"

(5)

The kinetics of formation of h y d r o p e r o x y groups in PB can be easily m o n i t o r e d by I R spectroscopy by measuring the f o r m a t i o n of an absorption p e a k at 3450 cm-l.3°Addition of bis(hindered piperidines) decreases r e m a r k a b l y the f o r m a t i o n of the h y d r o p e r o x y groups in the irradiated PB samples (Fig. 6). This process cannot be simply exp-

Photostabilising effect of bis( hindered piperidine ) compounds: I

59

3.50 -

E 3.00stage

o

I

stage

II

stage

Ill

~/~

/

c

g ¢ 2.50 uJ o

= 2.00

0

•

0

irradiation

3 0I

6 l0

~1 . --i -

1 2I 0

1 ~0

dark

I.... 2'~0 rain

I

~-~

1I

~ days

Fig. 9. Kinetics of the formation of ESR signals from bis(hindered nitroxyl radicals) during UV irradiation in benzene-methanol solution (9 : 1) of different bis(hindered piperidines) at the concentration of 1-66x 10-2M: (---O---) Tinuvin 144, (---O--) Tinuvin 292 and ( ~ - - ) Tinuvin 770.

lained by a decrease in the number of polymer alkyl radicals which can be scavenged by the bis(hindered nitroxyl radicals) (Reaction 2) but also by another mechanism described earlier. 8-1°'18"31"32 In this mechanism bis(hindered piperidines) may decompose hydroperoxide groups"

+POOH H

+PO" + H 2 0 H

HOOP Complex

(6)

60

"flung Yuan Yang, Julia Lucki, Jan F. Rabek, Bengt R&nby

A 2.50" E o c m c

._~ n-

2.00stage

I

stage

II

stage

III

/ r/

J J

Ul

Y

1.50 v O

1.00"

I 0

"-~1~"

i rrad ia t ion

~-

' 30

I 60

'

I 120

'

I 180

de rk

'

I I---I 240min

I 1

~-~

I 2 days

Fig. 10. Kinetics of the formation of ESR signals from bis(hindered nitroxyl radicals) during U V irradiation of PB in b e n z e n e - m e t h a n o l solution (9 : 1) (0.167M)

with different bis(hindered piperidines) at the concentration of 1-66 × 10-2M: (---0---) Tinuvin 144, (---Q--) Tinuvin 292 and (--O--) Tinuvin 770.

a n d finally f o r m b i s ( h i n d e r e d mechanism:

~

n i t r o x y l radicals) a c c o r d i n g t o t h e

+ PH/O2

+e"

*

(7)

OOH

(8) OOH

O

Photostabilising effect of bis (hindered piperidine) compounds: I

61

An antagonistic effect of antioxidants added to polypropylene stabilised by hindered piperidines has been observed. 832"18'32-34 A direct hydroperoxide decomposing effect of the hindered piperidines was not observed. 35 It has been shown previously that amines may form hydrogen-bonded associates with hydroperoxides of the type: 36 ROOH

R

R

Similar associates were found between bis(hindered piperidines) and tert-butylhydroperoxide.30 ESR spectroscopy studies show that hindered piperidines react stoichiometrically with the hydroperoxide groups generated by thermal oxidation of polymer to give stable nitroxyl radicals, l° The bis(hindered nitroxyl radicals) formed in reactions 2,3 and 8 may also form a hydrogen-bonded complex with hydroperoxides in solution. 15,19,30,37,38

~+PH+POOH O

'

i

~

(9)

O" HOOP

(PH)

Complex formation was also suggested between nitroxyl radicals and alkylperoxy radicals. 39 The possible formation of complexes NH: HOO- (Reaction 6) and NO': HOO- (Reaction 9) appears to be highly desirable to enhance the effectiveness of bis(hindered piperidines) as photostabilisers.4° The bis(hindered nitroxyl radicals) may also abstract a hydrogen from a polymer molecule and form a hydroxylamine type compound and polymer alkyl radical according to the following reaction:

O

OH

62

Yung Yuan Yang, Julia Lucki, Jan F. Rabek, Bengt R~nby

A study of the kinetics of the formation of carbonyl groups in photo-irradiated PB in the presence of bis(hindered piperidines) by I R spectroscopy by monitoring formation of the p e a k at 1720 cm -I (Fig. 7) shows that these c o m p o u n d s have a m a r k e d effect on this process. Carbonyl groups in photo-irradiated PB are f o r m e d by the beta-scission process, described earlier in some detail. 3 Because this process occurs via a free radical mechanism it is evident that the addition of bis(hindered piperidines) must influence it. Carbonyl groups are easily excited by U V light and m a y transfer their excitation energy to molecular oxygen to form singlet oxygen and/or to h y d r o p e r o x y groups to d e c o m p o s e them. It has been found that hindered piperidines inhibit the photosensitising effect of benz o p h e n o n e 1° and a n t h r a q u i n o n e 41 on p h o t o d e g r a d a t i o n of polymers probably by deactiviation of the excited c h r o m o p h o r e groups such as excited carbonyl and aromatics. 42-44 O n the other hand it has b e e n reported that phosphorescence from the excited carbonyl groups is not affected by the added hindered piperidines. 28 It was also reported that nitroxyl radicals are capable of quenching excited aliphatic ketones 45 and have a m a r k e d controlling influence on the product mixture resulting from the photolysis of ketones in the presence of oxygen. 35 H i n d e r e d piperidines m a y also prevent the isomerisation of a, /3-unsaturated ketones (e.g. f o r m e d during photo-oxidation of polypropylene), 6"25 and thus prevent their decomposition by light. M o n o ( h i n d e r e d piperidines) are less efficient as light stabilisers than the commercially available bis(hindered piperidines) such as Tinuvin 144, 292 and 7 7 0 . 45,46 It was previously r e p o r t e d that tetrasubstituted hindered amines are usually better U V stabilisers than tri-substituted ones. 9 A m o n g bicyclo tri-substituted amines the U V stabilisation activity appears to d e p e n d on the degree of hindrance a r o u n d the nitrogen atom and the way in which the two rings are fused together. In our research we have found that the most effective stabilisers for preventing photo-oxidative degradation of PB are Tinuvin 144 and Tinuvin 292 which have a methyl group as Nsubstituent on the piperidine ring in comparison with Tinuvin 770 which has h y d r o g e n as N-substituent. T h e Tinuvin 144 has an additional effect on stabilisation due to the presence in the structure of a hindered phenol unit, which acts as antioxidant. It has m u c h better photostabilising activity on PB than either Tinuvin 292 or 770.

Photostabilising effect of bis(hindered piperidine) compounds : I

63

CONCLUSIONS Commercially available bis(hindered piperidines) (Tinuvin 144, 292 and 770) can be used as efficient stabilisers against photo-oxidative degradation and/or cross-linking of cis-l,4-polybutadiene in solution and also in the solid state, The mechanisms of stabilisation are the scavenging of p o l y m e r free radicals by bis(hindered nitroxyl radicals) and simultaneous decomposition of h y d r o p e r o x y groups formed.

REFERENCES 1. Rhnby, B. and Rabek, J. F., Photodegradation, photo-oxidation and photostabilisation o[ polymers, Wiley, London, 1975. 2. McKellar, J. F. and Allen, N. S., Photochemistry of man-made polymers, Applied Science Publishers, London, 1979. 3. Rabek, J. F., Lucki, J. and R~nby, B., Europ. Polym. J., 15 (1979) 1089. 4. Lucki, J., Rabek, J. F. and R~nby, B., Europ. Polym. J., 15 (1979) 1101. 5. Golub, M. A., NASA Technical Memorandum No. 78604, June 1979. 6. Allen, N. S., Polym. Photochem., 1 (1981) 243. 7. Bfilint, G., Rockebauer, A., Kelen, T., Tfidos, F. and Jokay, L., Polym. Photochem., 1 (1981) 139. 8. Allen, N. S., Gardette, J. L. and Lemaire, J., Polym. Photochem., 1 (1981) 111. 9. Son, P. W., Polym. Degrad. Stabil., 2 (1980) 295. 10. Allen, N. S., Polym. Degrad. Stabil., 2 (1980) 179. 11. Allen, N. S. and McKellar, J. F., Brit. Polym. J., 9 (1977) 302. 12. Allen, N. S., Polym. Degrad. Stabil., 2 (1980) 129. 13. Carlsson, D. J., Grattan, D. W. and Wiles, D. M., Polym. Degrad. Stabil., 1 (1979) 69. 14. Allen, N. S., Polym. Degrad. Stabil., 3 (1980) 73. 15. Hodgeman, D. K. C., J. Polym. Sci., 18 (1980) 533. 16. Balint, G., Kelen, T., Ttidos, F. and Reh:tk, A., Polym. Bull., 1 (1979) 647. 17. Allen, N. S. and McKellar, J. F., Europ. Polym. J., 16 (1980) 553. 18. Chakraborty, K. B. and Scott, G., Chem. Ind., (London), (1978) 237. 19. Carlsson, D. J., Gordon, A. and Wiles, D. M., Develop. Polym. Stabilis., 1 (1979) 219. 20. Rabek, J. F. and R~nby, B., J. Polym. Sci., A12 (1974) 273. 21. Carlsson, D. J., Grattan, D. W., Supranchuk, T. and Wiles, D. M., J. Appl. Polym. Sci., 22 (1978) 2217. 22. Ivanov, V. B., Shlapintokh, V. Y., Kvostach, V. Y., Shapiro, O. M. and Rozantsev, E. G., J. Photochem., 4 (1975) 313.

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Yung Yuan Yang, Julia Lucki, Jan F. Rabek, Bengt R&nby

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