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Photo-stabilising action of metal chelates in polypropylene—Part III: Thermal antioxidant action and its relationship to photo-stabilisation
Polymer Degradation and Stability 5 (1983) 55 63
Photo-stabilising Action of Metal Chelates in Polypropylenc Part III: Thermal Antioxidant Action and its Relationship to Photo-stabilisation Norman S. Allen,* Alfonso Chirinos-Padron* & John H. Appleyardt * Department of Chemistry. t Department of Physics, Mathematics and Computing. John Dalton Faculty of Technology, Manchester Polytechnic, Chester Street, Manchester M1 5GD, Great Britain (Received: 22 April, 1982)
A BS TRA C T The thermal antioxidative behaviour of three metal chelates in polypropylene film has been examined using hydropero xide analysis and infra-red spectroscopy. All three chelates inhibit both hydroperoxide and carbonyl group formation during oxidation. Their thermal antio xidant efficiencies follow closely their photo-stabilising effects, suggesting that similar mechanisms of protection are involved. The rates of photo-decomposition of the metal chelates measured by second order derivative absorption spectroscopy were found to be independent of initial hydroperoxide concentration, indicating that the)' are ineffective macroalkoxy and hydroxy radical traps. The results suggest that metal chelates compete effectively with oxygen for macroalkyl radicals thermally and photochemically.
INTRODUCTION In Parts I 1 and II 2 we showed that metal chelates owe their photostabilising effect in polypropylene to their ability to scavenge macroalkyl radicals or to give products which are themselves effective stabilisers. N o evidence for excited state quenching could be found using aromatic carbonyl compounds as sensitisers. 1 55 Polymer Degradation and Stability 0141-3910/83/0005-0055/$03.00 © Applied Science Publishers Ltd, England, 1983. Printed in Great Britain
56
Norman S. Allen, Alfonso Chirinos-Padron, John H. Appleyard
In this work we have also found the metal chelates to be effective inhibitors of hydroperoxide formation during processing and this will also have an important effect on light stability. In view of this we have examined here the thermal antioxidant properties of metal chelates in polypropylene during oven ageing at 140 °C by monitoring both carbonyl and hydroperoxide formation. The effect of oven ageing on light stability has also been examined. The results confirm that the photo-protective behaviour of the metal chelates follows closely their thermal antioxidant action. EXPERI M ENTA L Materials
Polypropylene powder containing no commercial additives was supplied by ICI (Petrochemical and Plastics Division) Ltd. The stabilisers Irganox 1425 and Irgastab 2002 (calcium and nickel derivatives of bis(ethyl-3,5-ditert-butyl-4-hydroxybenzyl phosphonate) and Cyasorb UV 1084 (nickel(II)2,2'-thiobis(4-tertoctylphenylato)n-butylamine) were supplied by Ciba-Geigy (UK) Ltd, Manchester, and the American Cyanamid Company, Rotterdam, The Netherlands, respectively. The additives (0.1% w/w) were solvent blended into the polymer powder using dichloromethane and, after drying, the polymer was pressed into film 200-300 pm thick at 200°C. Irradiation
Films of polymer were exposed in a Microscal apparatus (Microscal Ltd, London) utilising a 500W high pressure Hg/W lamp (RH--ambient; temperature--50 °C; 2's > 300 nm). Rates of photo-oxidation were monitored by the well established carbonyl index method using a Pye-Unicam Model 3-300 infra-red spectrometer.3'4 Thermal oxidation
Polymer films were thermally oxidised in an oven at 140 °C and samples were removed after 1, 3 and 6 h for hydroperoxide analysis. The thermal stability of the films was also monitored using carbonyl index.
57
Photostabilising action of metal chelates--Ill
UV visible absorption spectra Second order derivative absorption spectra of the additives were measured using a Perkin-Elmer Model 554 absorption spectrometer. Hydroperoxide analysis Hydroperoxide concentration in the films was determined by an iodometric procedure, outlined in Part I, using cumene hydroperoxide as a standard (see also Carlsson and WilesS).
RESULTS AND DISCUSSION
Thermal oxidation One of the most significant features of the results is shown in Table 1. Here it is seen that, over a heating period of 6 h, all three chelates inhibit the formation of hydroperoxide groups, Cyasorb UV 1084 being the most effective, followed by Irgastab 2002 and then Irganox 1425. The high efficiency of the former could well be associated with the presence of the sulphur atom. It is interesting to note that even after 6 hours' heating both the nickel chelates held the hydroperoxide level to below that of the unheated control. Thus, inhibition of hydroperoxide formation is due either to the ability of the metal chelates to destroy them directly at 140 °C
TABLE 1 Effectof Thermal Oxidation at 140 °C on Hydroperoxide Concentration in Polypropylene Films Containing 0.1 ~o w/w Metal Chelates Additit~e Oh*
None Cyasorb UV 1084 Irganox 1425 Irgastab 2002 * Heating time.
75 12 26 38
POOH (~g/g) lh 3h
119 15 26 26
5 160 24 50 34
6h
13 950 24 357 53
58
Norman S. Allen, Alfonso Chirinos-Padron, John H. Appleyard
TABLE 2 Relative Initial Concentration of Metal Chelates in Polypropylene Films During Thermal Oxidation at 140°C Additive
Heating time (h) 1 3
0
Cyasorb UV 1084 Irganox 1425 Irgastab 2002
12.2 12.7 12.0
6.1 17.2 14.5
2.7 15.5 14.3
6
2.9 15.8 9.6
or to their ability to compete with oxygen for macroalkyl radicals. F r o m previous work 2 the latter would appear more likely. Relative initial concentrations of the chelates shown in Table 2 were variable with heating time. Cyasorb UV 1084 showed a gradual decrease while the other two showed an increase. These values are reliable since second order derivative spectroscopy was used to eliminate an interference from product absorption. The decrease observed for UV 1084 is evidently due to some thermal reaction whereas the increase observed for the other two could well be an aggregation effect. Low volatility
120 --
r
T
%_,o.o
I
r
1
r
t
/
x80 121 Z
~
6.0
g m U
1 2.0
o.(
_
J ~0
1 20
1 30 HEATING
1 40 TIME
L so
so
,n hours
Fig. 1. Ratesof thermal oxidation of polypropylenefilms(300 #m thick) containing x, no additive and 0.1 ~ w/w of O, Irganox 1425; II, Cyasorb UV 1084; A, Irgastab 2002, during heating in air at 140°C.
Photostabilising action of metal chelates--Ill
59
appears to be unlikely at the temperature used here and this is reflected by the carbonyl index data shown in Fig. 1. Carbonyl index data in Fig. 1 show that all three chelates are effective inhibitors of carbonyl formation. This is expected in view of the fact that the chelates inhibit hydroperoxide formation. The efficiencies of the chelates follow the order Irgastab 2002 > Cyasorb UV 1084 > Irganox 1425, the first of these being an extremely good antioxidant alone in the polymer. The effect of the nickel derivative compared with that of the calcium compound is interesting and could well be associated with the inactivity of the latter since, after 6 hours' heating there is no decrease in concentration (Table 2). Photo-oxidation
The photo-stabilities of unheated and heated films are shown in Figs 2 to 4 for Cyasorb UV 1084, Irganox 1425 and Irgastab 2002, respectively. Embrittlement times are shown in Table 3; these correspond to 0.06 carbonyl units. Taking the chelates individually, it is seen that Cyasorb UV 1084 shows quite a different pattern in behaviour compared with the other two. After 1 hour's heating at 140°C it shows a marked improvement in light stability followed by a decrease after further heating. Clearly, this chelate is destroyed thermally to give products which are more effective light stabilisers. This was the conclusion of our earlier work 1,2 on photo-stabilisation where we found that this particular chelate decomposed well before the onset of embrittlement. The data in Fig. 2 demonstrate this effect for Cyasorb UV 1084 for both unheated and heated films. TABLE 3 Effect of Thermal Oxidation at 140°C on Subsequent Photo-stability of Polypropylene Films Containing 0.1 °/o w/w Metal Chelates
Additire
None Cyasorb UV 1084 lrganox 1425 Irgastab 2002 * Heating time.
UV emhrittlement times Oh* lh 3h 6h 135 580 270 780
75 870 260 690
550 250 730
470 235 530
60
Norman S. Allen, Alfonso Chirinos-Padron, John H. Appleyard
I0( N ~ 8"C x ~ 6"0
¢ ~m4-O
JJ / /
2"£
:,- 16.0 -
z NI2,0~
E
~8.o~_,,=, ,
4.G i
0 IRRADIATION
TIME
in
hours
Fig. 2. Photo-oxidation in Microscal unit of polypropylene films (300prn thick) containing +, no additives unheated; x, no additives after 1 hour's heating at 140°C; 0.1 ~ w/w ofCyasorb UV 1084 after A, 0 hours'; ©, 1 hours'; O, 3 hours'; [], 6 hours' heating at 140°C. Lower graph shows the corresponding photo-decomposition of Cyasorb UV 1084 in the same prior unheated and heated films.
Irganox 1425 shows only a small decrease in light stabilising efficiency, even after heating for 6 h. The rate of decomposition of this chelate is also independent of prior thermal oxidation and, in fact, much of the additive remains even after embrittlement (Fig. 3). Irgastab 2002 shows a gradual decrease in light stabilising efficiency after heating for 6 h apart from the 3-h heated sample when there is a small upward trend. Again, the rate of photo-decomposition of this chelate is independent of prior heating (Fig. 4). From the above results a number of important deductions can be made regarding the photo-stabilising mechanisms of metal chelates. The first and most important observation is that the thermal oxidative stabiliser effect of the metal chelates follows the order Irgastab 2002 > Cyasorb UV
Photostabilising action of metal chelates--Ill
×
~
/7
/
°I
61
S.0
I
I
312.(
I
.
.
.
.
z 8.c
4.0
~o,
0
i
I00
i
200
i
300
IRRADIATION TIME in houPs
Fig. 3. Photo-oxidation in Microscal unit of polypropylene films (300pm thick) containing +, no additives unheated; ×, no additives after 1 hour's heating at 140°C; 0.1 ~o w/w Irganox 1425 after A, 0 hours'; (3, 1 hours'; O, 3 hours'; and [3, 6 hours' heating at 140 °C. Lower graph shows the corresponding photo-decomposition of Irganox 1425 in the same prior unheated and heated films.
1084 > Irganox 1425 which is the same order as their light stabilising effect. Thus, the photo-chemical behaviour of these chelates reflects closely their thermal antioxidant behaviour. This is particularly so for Cyasorb UV 1084 which appears to decompose thermally and photochemically to give a product or products which are more effective stabilisers. The second observation is that the photo-stabilising efficiencies of these metal chelates are independent of initial hydroperoxide concentration and this is also reflected in their rates of photodecomposition. Thus, in agreement with our earlier work, the metal chelates are ineffective in scavenging macroalkoxy and hydroxy radicals and appear only to be effective in scavenging macroalkyl radicals. Their
62
Norman S. Allen, Alfonso Chirinos-Padron, John H. Appleyard
~
4.() Z,0~
&
00L
A
~
I
I
a
A
_
t
I
I
s~0
16
I
zoo
I
4oo IRRADIATION
600 TIME in hours
Fig. 4. Photo-oxidation in Microscal unit of polypropylene films (300#m thick) containing +, no additives unheated; x, no additives after 1 hour's heating at 140 °C; 0.1 ~ow/w Irgastab 2002 after A, 0 hours'; O, 1 hour's; O, 3 hours'; I-q, 6 hours' heating at 140 °C. Lower graph shows the corresponding photo-decomposition of Irgastab 2002 in the same prior unheated and heated films.
efficiency as thermal antioxidants confirms this conclusion. In future papers we will be examining the r61e of nickel chelates as singlet oxygen scavengers since this is also believed to be an important mechanism, particularly for chelates containing sulphur donor atoms. 6'v
ACKNOWLEDGEMENTS The authors thank Dr T. J. Henman of ICI (Petrochemical and Plastics Division) Ltd, Great Britain, for samples and helpful discussions, and the Venezulean Council for Scientific and Technological Research (CONICIT), for a grant to one of them (A. Chirinos-Padron).
Photostabil&ing action of metal chelates lI1
63
REFERENCES 1. N. S. Allen, A. Chirinos-Padron and J. H. Appleyard, Polym. Deg. andStab., 4, 223 (1982). 2. N. S. Allen, A. Chirinos-Padron and J. H. Appleyard, Polym. Deg. andStab., this issue. 3. N. S. Allen (Ed). Developments in polymer photochemistry--2, Applied Science Publishers Ltd, London, 1981. 4. J. F. McKellar and N. S. Allen, Photochemistry of man-made polymers, Applied Science Publishers Ltd, London, 1979. 5. D. J. Carlsson and D. M. Wiles, Macromolecules, 2, 583 (1969). 6. J. P. Guillory and C. F. Cook, J. Am. Chem. Soc., 95, 4885 (1968). 7. B. Felder and R. Schumacher, Angew. Makromol. Chemie, 31, 35 (1973).
Photo-stabilising Action of Metal Chelates in Polypropylenc Part III: Thermal Antioxidant Action and its Relationship to Photo-stabilisation Norman S. Allen,* Alfonso Chirinos-Padron* & John H. Appleyardt * Department of Chemistry. t Department of Physics, Mathematics and Computing. John Dalton Faculty of Technology, Manchester Polytechnic, Chester Street, Manchester M1 5GD, Great Britain (Received: 22 April, 1982)
A BS TRA C T The thermal antioxidative behaviour of three metal chelates in polypropylene film has been examined using hydropero xide analysis and infra-red spectroscopy. All three chelates inhibit both hydroperoxide and carbonyl group formation during oxidation. Their thermal antio xidant efficiencies follow closely their photo-stabilising effects, suggesting that similar mechanisms of protection are involved. The rates of photo-decomposition of the metal chelates measured by second order derivative absorption spectroscopy were found to be independent of initial hydroperoxide concentration, indicating that the)' are ineffective macroalkoxy and hydroxy radical traps. The results suggest that metal chelates compete effectively with oxygen for macroalkyl radicals thermally and photochemically.
INTRODUCTION In Parts I 1 and II 2 we showed that metal chelates owe their photostabilising effect in polypropylene to their ability to scavenge macroalkyl radicals or to give products which are themselves effective stabilisers. N o evidence for excited state quenching could be found using aromatic carbonyl compounds as sensitisers. 1 55 Polymer Degradation and Stability 0141-3910/83/0005-0055/$03.00 © Applied Science Publishers Ltd, England, 1983. Printed in Great Britain
56
Norman S. Allen, Alfonso Chirinos-Padron, John H. Appleyard
In this work we have also found the metal chelates to be effective inhibitors of hydroperoxide formation during processing and this will also have an important effect on light stability. In view of this we have examined here the thermal antioxidant properties of metal chelates in polypropylene during oven ageing at 140 °C by monitoring both carbonyl and hydroperoxide formation. The effect of oven ageing on light stability has also been examined. The results confirm that the photo-protective behaviour of the metal chelates follows closely their thermal antioxidant action. EXPERI M ENTA L Materials
Polypropylene powder containing no commercial additives was supplied by ICI (Petrochemical and Plastics Division) Ltd. The stabilisers Irganox 1425 and Irgastab 2002 (calcium and nickel derivatives of bis(ethyl-3,5-ditert-butyl-4-hydroxybenzyl phosphonate) and Cyasorb UV 1084 (nickel(II)2,2'-thiobis(4-tertoctylphenylato)n-butylamine) were supplied by Ciba-Geigy (UK) Ltd, Manchester, and the American Cyanamid Company, Rotterdam, The Netherlands, respectively. The additives (0.1% w/w) were solvent blended into the polymer powder using dichloromethane and, after drying, the polymer was pressed into film 200-300 pm thick at 200°C. Irradiation
Films of polymer were exposed in a Microscal apparatus (Microscal Ltd, London) utilising a 500W high pressure Hg/W lamp (RH--ambient; temperature--50 °C; 2's > 300 nm). Rates of photo-oxidation were monitored by the well established carbonyl index method using a Pye-Unicam Model 3-300 infra-red spectrometer.3'4 Thermal oxidation
Polymer films were thermally oxidised in an oven at 140 °C and samples were removed after 1, 3 and 6 h for hydroperoxide analysis. The thermal stability of the films was also monitored using carbonyl index.
57
Photostabilising action of metal chelates--Ill
UV visible absorption spectra Second order derivative absorption spectra of the additives were measured using a Perkin-Elmer Model 554 absorption spectrometer. Hydroperoxide analysis Hydroperoxide concentration in the films was determined by an iodometric procedure, outlined in Part I, using cumene hydroperoxide as a standard (see also Carlsson and WilesS).
RESULTS AND DISCUSSION
Thermal oxidation One of the most significant features of the results is shown in Table 1. Here it is seen that, over a heating period of 6 h, all three chelates inhibit the formation of hydroperoxide groups, Cyasorb UV 1084 being the most effective, followed by Irgastab 2002 and then Irganox 1425. The high efficiency of the former could well be associated with the presence of the sulphur atom. It is interesting to note that even after 6 hours' heating both the nickel chelates held the hydroperoxide level to below that of the unheated control. Thus, inhibition of hydroperoxide formation is due either to the ability of the metal chelates to destroy them directly at 140 °C
TABLE 1 Effectof Thermal Oxidation at 140 °C on Hydroperoxide Concentration in Polypropylene Films Containing 0.1 ~o w/w Metal Chelates Additit~e Oh*
None Cyasorb UV 1084 Irganox 1425 Irgastab 2002 * Heating time.
75 12 26 38
POOH (~g/g) lh 3h
119 15 26 26
5 160 24 50 34
6h
13 950 24 357 53
58
Norman S. Allen, Alfonso Chirinos-Padron, John H. Appleyard
TABLE 2 Relative Initial Concentration of Metal Chelates in Polypropylene Films During Thermal Oxidation at 140°C Additive
Heating time (h) 1 3
0
Cyasorb UV 1084 Irganox 1425 Irgastab 2002
12.2 12.7 12.0
6.1 17.2 14.5
2.7 15.5 14.3
6
2.9 15.8 9.6
or to their ability to compete with oxygen for macroalkyl radicals. F r o m previous work 2 the latter would appear more likely. Relative initial concentrations of the chelates shown in Table 2 were variable with heating time. Cyasorb UV 1084 showed a gradual decrease while the other two showed an increase. These values are reliable since second order derivative spectroscopy was used to eliminate an interference from product absorption. The decrease observed for UV 1084 is evidently due to some thermal reaction whereas the increase observed for the other two could well be an aggregation effect. Low volatility
120 --
r
T
%_,o.o
I
r
1
r
t
/
x80 121 Z
~
6.0
g m U
1 2.0
o.(
_
J ~0
1 20
1 30 HEATING
1 40 TIME
L so
so
,n hours
Fig. 1. Ratesof thermal oxidation of polypropylenefilms(300 #m thick) containing x, no additive and 0.1 ~ w/w of O, Irganox 1425; II, Cyasorb UV 1084; A, Irgastab 2002, during heating in air at 140°C.
Photostabilising action of metal chelates--Ill
59
appears to be unlikely at the temperature used here and this is reflected by the carbonyl index data shown in Fig. 1. Carbonyl index data in Fig. 1 show that all three chelates are effective inhibitors of carbonyl formation. This is expected in view of the fact that the chelates inhibit hydroperoxide formation. The efficiencies of the chelates follow the order Irgastab 2002 > Cyasorb UV 1084 > Irganox 1425, the first of these being an extremely good antioxidant alone in the polymer. The effect of the nickel derivative compared with that of the calcium compound is interesting and could well be associated with the inactivity of the latter since, after 6 hours' heating there is no decrease in concentration (Table 2). Photo-oxidation
The photo-stabilities of unheated and heated films are shown in Figs 2 to 4 for Cyasorb UV 1084, Irganox 1425 and Irgastab 2002, respectively. Embrittlement times are shown in Table 3; these correspond to 0.06 carbonyl units. Taking the chelates individually, it is seen that Cyasorb UV 1084 shows quite a different pattern in behaviour compared with the other two. After 1 hour's heating at 140°C it shows a marked improvement in light stability followed by a decrease after further heating. Clearly, this chelate is destroyed thermally to give products which are more effective light stabilisers. This was the conclusion of our earlier work 1,2 on photo-stabilisation where we found that this particular chelate decomposed well before the onset of embrittlement. The data in Fig. 2 demonstrate this effect for Cyasorb UV 1084 for both unheated and heated films. TABLE 3 Effect of Thermal Oxidation at 140°C on Subsequent Photo-stability of Polypropylene Films Containing 0.1 °/o w/w Metal Chelates
Additire
None Cyasorb UV 1084 lrganox 1425 Irgastab 2002 * Heating time.
UV emhrittlement times Oh* lh 3h 6h 135 580 270 780
75 870 260 690
550 250 730
470 235 530
60
Norman S. Allen, Alfonso Chirinos-Padron, John H. Appleyard
I0( N ~ 8"C x ~ 6"0
¢ ~m4-O
JJ / /
2"£
:,- 16.0 -
z NI2,0~
E
~8.o~_,,=, ,
4.G i
0 IRRADIATION
TIME
in
hours
Fig. 2. Photo-oxidation in Microscal unit of polypropylene films (300prn thick) containing +, no additives unheated; x, no additives after 1 hour's heating at 140°C; 0.1 ~ w/w ofCyasorb UV 1084 after A, 0 hours'; ©, 1 hours'; O, 3 hours'; [], 6 hours' heating at 140°C. Lower graph shows the corresponding photo-decomposition of Cyasorb UV 1084 in the same prior unheated and heated films.
Irganox 1425 shows only a small decrease in light stabilising efficiency, even after heating for 6 h. The rate of decomposition of this chelate is also independent of prior thermal oxidation and, in fact, much of the additive remains even after embrittlement (Fig. 3). Irgastab 2002 shows a gradual decrease in light stabilising efficiency after heating for 6 h apart from the 3-h heated sample when there is a small upward trend. Again, the rate of photo-decomposition of this chelate is independent of prior heating (Fig. 4). From the above results a number of important deductions can be made regarding the photo-stabilising mechanisms of metal chelates. The first and most important observation is that the thermal oxidative stabiliser effect of the metal chelates follows the order Irgastab 2002 > Cyasorb UV
Photostabilising action of metal chelates--Ill
×
~
/7
/
°I
61
S.0
I
I
312.(
I
.
.
.
.
z 8.c
4.0
~o,
0
i
I00
i
200
i
300
IRRADIATION TIME in houPs
Fig. 3. Photo-oxidation in Microscal unit of polypropylene films (300pm thick) containing +, no additives unheated; ×, no additives after 1 hour's heating at 140°C; 0.1 ~o w/w Irganox 1425 after A, 0 hours'; (3, 1 hours'; O, 3 hours'; and [3, 6 hours' heating at 140 °C. Lower graph shows the corresponding photo-decomposition of Irganox 1425 in the same prior unheated and heated films.
1084 > Irganox 1425 which is the same order as their light stabilising effect. Thus, the photo-chemical behaviour of these chelates reflects closely their thermal antioxidant behaviour. This is particularly so for Cyasorb UV 1084 which appears to decompose thermally and photochemically to give a product or products which are more effective stabilisers. The second observation is that the photo-stabilising efficiencies of these metal chelates are independent of initial hydroperoxide concentration and this is also reflected in their rates of photodecomposition. Thus, in agreement with our earlier work, the metal chelates are ineffective in scavenging macroalkoxy and hydroxy radicals and appear only to be effective in scavenging macroalkyl radicals. Their
62
Norman S. Allen, Alfonso Chirinos-Padron, John H. Appleyard
~
4.() Z,0~
&
00L
A
~
I
I
a
A
_
t
I
I
s~0
16
I
zoo
I
4oo IRRADIATION
600 TIME in hours
Fig. 4. Photo-oxidation in Microscal unit of polypropylene films (300#m thick) containing +, no additives unheated; x, no additives after 1 hour's heating at 140 °C; 0.1 ~ow/w Irgastab 2002 after A, 0 hours'; O, 1 hour's; O, 3 hours'; I-q, 6 hours' heating at 140 °C. Lower graph shows the corresponding photo-decomposition of Irgastab 2002 in the same prior unheated and heated films.
efficiency as thermal antioxidants confirms this conclusion. In future papers we will be examining the r61e of nickel chelates as singlet oxygen scavengers since this is also believed to be an important mechanism, particularly for chelates containing sulphur donor atoms. 6'v
ACKNOWLEDGEMENTS The authors thank Dr T. J. Henman of ICI (Petrochemical and Plastics Division) Ltd, Great Britain, for samples and helpful discussions, and the Venezulean Council for Scientific and Technological Research (CONICIT), for a grant to one of them (A. Chirinos-Padron).
Photostabil&ing action of metal chelates lI1
63
REFERENCES 1. N. S. Allen, A. Chirinos-Padron and J. H. Appleyard, Polym. Deg. andStab., 4, 223 (1982). 2. N. S. Allen, A. Chirinos-Padron and J. H. Appleyard, Polym. Deg. andStab., this issue. 3. N. S. Allen (Ed). Developments in polymer photochemistry--2, Applied Science Publishers Ltd, London, 1981. 4. J. F. McKellar and N. S. Allen, Photochemistry of man-made polymers, Applied Science Publishers Ltd, London, 1979. 5. D. J. Carlsson and D. M. Wiles, Macromolecules, 2, 583 (1969). 6. J. P. Guillory and C. F. Cook, J. Am. Chem. Soc., 95, 4885 (1968). 7. B. Felder and R. Schumacher, Angew. Makromol. Chemie, 31, 35 (1973).