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Abstract

Detailed studies have been made both of the photo-oxidation of polymer model compounds (ietralin and 2,6,10,14 tetramethylpenta-decane (TMPD)), in the near ultra-violet region of the spectrum, and of the inhibition of these reactions by a nickel chelate, bis-stilbenedithialato nickel (NiSDT).

In the Introduction, theories of the autoxidation of organic compounds are reviewed, with special reference to photo-oxidation, Aspects of photo-initiation, including theories of singlet oxygen activity are also covered. Mechanisms of the inhibition of autoxidation are also described and theories of the action of radical-trapping agents, hydroperoxide-decomposing agents, multifunctional antioxidants, screening agents and photo-stabilisers are outlined. Particular attention is paid to previous work involving NiSDT, the photo-stabiliser investigated in the present study.

In the Experimental Section, a description is given of the apparatus and procedures used to follow the photo-oxidation of the polymer model compounds, as well as of the methods of producing monochromatic light. Accounts are also included of the preparation of the reactants and reagents used in the photo-oxidation reactions and of the methods used for actinometry and product analysis.

The Results and Discussion Section describes the experimental measurements made on tetralin and TMPD using both polychromatic and monochromatic light. Degassed tetralin was also found to undergo a photochemical reaction under nitrogen. With regard to the photooxidation of pure tetralin, it is concluded that the triplet excited state of this compound may contribute to initiation by promoting the homolytic decomposition of the hydroperoxide.

Studies of the inhibition by NASDT of the photo-oxidation of both polymer model compounds were also carried out. Both NiSDT and its p-substituted derivatives were found to undergo a photochemical reaction in various solvents, even in the absence of oxygen. The post—induction period steady rate of oxygen uptake in the inhibited photo-oxidation of both tetralin and THPD was analysed, end the formation of a reaction product which is a catalyst for hydroperoxide has been proposed.

Studies of the rate of loss of NiSDf during the induction period - showed that the inhibitor was being lost in a non-radicel trapping process. Possible mechanisms are discussed, but a definite conclusion was not reached. It is suggested that the chelate could generate alkyl radicals by either a thermal or photo-chemical reaction of an intermediate complex with alkylperoxy radicals.

It was concluded that more data is required, both concerning the mechanism of inhibition of thermal autoxidation by NiSDT and about the mechanism and products of the photo-chemical loss of the chelate in order to clarify the reactions of NiSDI during photooxidation reactions. Possible mechanisms for the radical trapping and hydroperoxide decomposing reactions of NiSDT are discussed.

It was suggested that NiSDT may act as an effective 'photo-stabiliser in polymer systems if the independent photochemical “loss of the chelate did not occur in the solid matrix.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QD Chemistry
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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