Polyolefins photolysis during

Additives which act as light filters, light absorbers and quenchers of excited states are called photo-stabilizers. Efficient stabilization against the light requires the elimination of free radicals appearing during polyolefin photolysis. 

The first degradable carbon-chain polymer was synthesised by Brubaker of the Dupont Company as early as 1950. This was a copolymer of ethylene and carbon monoxide (E-CO) which has since been extensively studied by photochemists, notably by J. E. Guillet and his co-workers at Toronto University. It was seen in Chapter 3 (Scheme 3.6) that macro-molecular ketones are formed by peroxidation of polyolefins and by subsequent photolysis they play an important role in the reduction of molecular weight of polyethylene during environmental exposure. 

In spite of the numerous studies reported on photooxidation of polyolefins, the detailed mechanism of the complete process remains unresolved. The relative contribution by species involved in photoinitiation, the origins of the oxidative scission reaction, and the role played by morphology in the case of photoreactions in solid state are not completely understood. Primary initiator species in polyethylenes [123] and polypropylenes [124] are believed to be mainly ketones and hydroperoxides. During early oxidation hydroperoxides are the dominant initiator, particularly in polypropylene, and can be photolyzed by wavelengths in solar radiation [125]. Macro-oxy radicals from photolysis of polyethylene hydroperoxides undergo rapid conversion to nonradical oxy products as evidenced by ESR studies [126]. Some of the products formed are ketones susceptible to Norrish I and II reactions leading to chain scission [127,128]. Norrish II reactions predominate under ambient conditions [129]. Concurrent with chain scission, crosslinking, for instance via alkoxy macroradical combination [126], can take place with consequent gel formation [130,131]. 

The macroradical formed to reaction (16) interacts with oxygen, leading to a normal chain reaction and the formation of hydroperoxides. Accordii to the data of [85], the quantum yield of carbonyl groups in the photolysis of polyethylene is no greater than 0.1. This means that the kinetic chains in photooxidation are not very long. In view of this, termination of the chains by means of antioxidants is hindered in practice, since in the case of short chains and a high rate of initiation, effective inhibition is impossible. The formation of unsaturated compoimds during photolysis [reactions (15) and (16)] facilitates further oxidation of the polyolefin. 

It is well known that olefins may form complexes with some metals [80, 81]. Mercuric acetate can form complexes with polyolefins selectively, being removed by acid treatment [82]. Such complexes have been used in the separation of unsaturated fatty acids from mixtures [83]. Subbarao [84] has protected olefins aginst reduction using silver complexes, and Cope used reversible platinum complex formation [85, 86] for the resolution of medium ring trans olefins. Landesburg [87] used the iron tricarbonyl complex of the alcohol (13) to protect it during oxidation to the ketone (14) which was regenerated from the complex by photolysis [88a]. 

Mechanisms involved in the photolysis of thermally oxidized polyolefins during processing... 

Hydroperoxides represent, as it is generally accepted, ones of the most reactive species in the polymer photodegradation [41]. They can be induced during the processing of polymer, as well as during the subsequent exposure of polymers to light and heat in the presence of air. The present transitional metals accelerate the decomposition by radicalic process of the accumulation of hydroperoxides. In the case of polyolefin, the hydroperoxides photolysis leads to the formation of some carbonyl compounds and/or alcoxy radicals. 

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