ELSEVIER

Polymer Degradation and Stability 60 (I 998) 2 15-2 16

Q 1998 Published by Elsevier Science Limited. All rights reserved

Printed in Northern Ireland PII: s0141-3910(97)00080-3 0141-3910/98/S19.00

Comment: Oxygen charge-transfer complexes as peroxidation

initiators in polymers

Gerald Scott Aston University, Birmingham B4 7ET, UK

(Received 23 November 1996)

A number of papers published recently in this journal have invoked oxygen charge-transfer com- plexes (CTCs) to explain intiation phenomena in polymers. i4 This is of course a valid assumption if experimental evidence is available to show that oxygen CTCs lead to free-radical formation in the presence of UV light. To the best of my knowledge, this has not so far been unequivocally demon- strated, but in the meantime by dint of repetition, the hypothesis has tended to assume a self-perpe- tuating validity, and CTCs have recently been invoked to explain phenomena that can be equally well explained by classical peroxidation chemis- try.The theory of charge-transfer initiation was widely discussed prior to 1970 but was discarded as an explanation of the initiation process occurring in polyolefins in the mid- 1970s because of a lack of experimental evidence and in favour of hydroper- oxides,5p8 which are always present in small amounts in commercial polymers either from the manufacturing procedure or more likely from the processing operation.9,10

The postulated CTC initiation reactions for polyethylene as I understand them are:3

followed by:

H202 k 2HO. 2 - %H2- 2 - CH - +2H20. (2)

215

Reactions l(a) and l(b) are unlikely on energetic grounds. Even ‘02 is not known to undergo this reaction. Verification of these steps is therefore required. This would involve exposing rigorously purified polyolefins without hydroperoxides or other adventitious sensitisers (e.g. carbonyl com- pounds, catalyst residues, ozone, singlet oxygen, etc.) to oxygen and UV light and demonstrating the formation of free radicals by a sensitive tech- nique such as ESR. This would almost certainly require the presence of a spin-trap (e.g. phenyl-tert- butylnitrone, PBN) and it would be necessary to confirm that the spin-trap itself did not give radi- cals. (2-methyl-2-nitrosopentane, MNP does give alkyl radicals under these conditions.iO)

Reactions 1 and 2 have recently been invoked4 to explain the difference between molar oxygen absorption and the lower molar formation of mac- romolecular oxidation products in HALS-stabi- lised relative to unstabilised polyolefins. It is not clear why the postulated oxygen CTC initiation mechanism should be more important in the stabi- lised than in the unstabilised system. It appears to be invoked simply to explain the loss of oxygen from the system as water. However, in the same paper, the authors also show an increased forma- tion of CO and CO2 from HALS-stabilised poly- olefins than from unstabilised polyolefins. Both increased oxygen absorption and increased elim- ination of volatile oxidation products are entirely consistent with the presently understood mechan- ism of HALS action, which involves the aminoxyl- catalysed formation of ketones9 followed by the

216 G. Aston

PCOOO..PCOOOH PCOOH +___-______----------___-----___----___;

IX

I 4 I I I

ON<

-CH,CHCH,- >NO.+ -CH>COCH,- - 1

(PON<) y

-CH2;HCH2-((P.)

hf;;r;;sh ;::Tes 1

-CHzC0.+CH2- - POOH I I I

Scheme 1. Catalytic mechanism of piperidinoxyl radicals ( > NC )

L,t L, 1 \

! i \ i \ \ i \ , 1 \ \ ! \ \ \ , \ I

/ \)2 o ,,<j

// \ \ -LH, + co.co2 \ ,

-CH,C, ,PCd,O., ;

\ \ 02/PH 00. I I \ I , \ I I \ I \ \ I

I

PH I , , ', P.+POOH /' \ 0 I' \ /

"lLl, 1

f'.+-CH,C 'OOH

(PCOOOH)

0,IPH

1 021PH

P.+ POOH.etc

P. t POOH. CIC

well-established Norrish elimination of low-mole- cular-weight oxygen-containing fragments (e.g. CO, COZ, aldehydes and ketones). Peracyloxyl radicals (PCOOO.) and peracids (PCOOOH) are also produced as by-products in the same process to continue the catalytic chain reaction. Several moles of oxygen may thus absorbed in each cata- lytic cycle, but only a fraction of this remains in the polymer (Scheme 1). The results described by Gijsman and Dozeman provide valuable con- firmation of the catalytic piperidinoxyl photo- stabilisation mechanism and are potentially capable of measuring the stoichiometric inhibition coefficients (f) of aminoxyls.

REFERENCES

1. 2.

3.

4.

5.

6. I. 8.

9.

10.

Gugumus, F., Polym. Degrad. Stab., 1991, 34, 205. Gijsman, P., Hennekens, J. and Tummers, D., Polym. Degrad. Stab., 1993, 39, 225. Gijsman, P., Hennekens, J. and Jannssen, K., Polvm. Degrad. Stab., 1994, 46, 63. Gijsman, P. and Dozeman, A., Polym. Degrad. Stab., 1996, 53, 45. Carlsson. D. J., Garton, A. and Wiles, D. M., Macro- molecules, 1976, 9, 695. Scott, G., Am. Chem. Sot. Symp. Ser., 1976, 25, 340. Chakraborty, K. B. and Scott, G., Polymer, 1977, 18, 98. Carlsson, D. J., Garton, A. and Wiles, D. M., Develop- ments in Polymer Stabilisation--1, ed. G. Scott, Applied Science Publication, Chapter 7, 1979. Scott, G., in Atmospheric Oxidation and Antioxidants, Vol. 2, ed. G. Scott. Elsevier, Amsterdam, Chapter 8. 1993. Scott, G., Polym. Degrad. Stab., 1995, 48, 315.