Polyolefin oxidation mechanism

Degradation of polyolefins such as polyethylene, polypropylene, polybutylene, and polybutadiene promoted by metals and other oxidants occurs via an oxidation and a photo-oxidative mechanism, the two being difficult to separate in environmental degradation. The general mechanism common to all these reactions is that shown in equation 9. The reactant radical may be produced by any suitable mechanism from the interaction of air or oxygen with polyolefins (42) to form peroxides, which are subsequentiy decomposed by ultraviolet radiation. These reaction intermediates abstract more hydrogen atoms from the polymer backbone, which is ultimately converted into a polymer with ketone functionahties and degraded by the Norrish mechanisms (eq. 

Fundamental understanding of the chemistry of polyolefin oxidation and the mechanisms of their stabilisation has been pivotal in achieving the current body of knowledge and the development of the vast structural diversity of stabiliser molecules that are available today. This chapter aims to project, with mechanistic perspectives, a technological overview of recent progress made in the stabilisation of polyolefins that are suitable for human-contact applications. 

Scheme 1 Simplified auto-oxidation mechanism of polyolefins (PH is polymer substrate)...

Polyolefins.— The mechanism(s) of polyolefin photo-oxidation continue to be shrouded in complexities. Recently Geuskens has attempted to answer questions concerning the sensitized photolysis of hydroperoxides in these polymers in the presence of carbonyl groups. In contrast with the theory proposed above,it is suggested that a complex is formed and photolysed to produce a carboxylic acid and an ether directly (Scheme 19) rather than free hydroxyl radicals. 

Polyolefins.—Polyolefin oxidation continues to be a subject of considerable controversy. Wiles and co-workers have written a comprehensive review of polyolefin photo-oxidation mechanisms with particular emphasis on the role of... 

A. Carton, D.). Carlsson, D.M. Wiles, Photo-oxidation Mechanisms in Commercial Polyolefins, in N.S. AUen (ed.). Developments in Polymer Photochemistry - I, Applied Science, London (1980), Chapter 4, p. 93. 

Scoponi M, PradeUa F, Carassiti V. Photodegradable polyolefins. Photo-oxidation mechanisms of innovative polyolefin copolymers containing double bonds. Coord Chem Rev 1993 125 219-230. 

In isothermal determinations the shapes of CL intensity on time are individualize for different oxidation mechanisms involving specific free radicals. If polyolefins generate sigmoid form of CL curves [04G1], for polyamides [03C1] or polyurethanes [07J1] an early peak is recorded in CL intensity measurements followed by parabolic decrease. In any case, the CL intensity values on the first five minutes of measurements define the initial state of oxidation rather than the instant decomposition process. Either isothermal CL measurements or nonisothermal CL investigations are reliable and accurate tools for... 

A Garton. In NS Allen, ed. Photo-Oxidation Mechanisms of Commercial Polyolefins in Developments in Polymer Photochemistry, Vol. 1. London Applied Science, 1980, chap 4. 

Nocardia and P. aeruginosa were shown to break the cw-PI chain by an oxidative mechanism since aldehyde groups were found to accumulate during microbial degradation. This is always the first product formed during the abiotic peroxidation of cw-PI and the evidence suggests that the bacteria initiate a radical-chain peroxidation. This will be discussed further in the context of polyolefin biodegradation. 

Garton, A., Carlsson, D.J., and Wiles, D. M. (1980) Photo-oxidation mechanisms in commercial polyolefins, in Developments in Polymer Photochemistry, 1 (ed. N.S. Allen), Applied Science,... 

Norrish type II mechanism for photo-oxidation of polyolefins. 

A detailed study of the mechanism of the insertion reaction of monomer between the metal-carbon bond requires quantitative information on the kinetics of the process. For this information to be meaningful, studies should be carried out on a homogeneous system. Whereas olefins and compounds such as Zr(benzyl)4 and Cr(2-Me-allyl)3, etc. are very soluble in hydrocarbon solvents, the polymers formed are crystalline and therefore insoluble below the melting temperature of the polyolefine formed. It is therefore not possible to use olefins for kinetic studies. Two completely homogeneous systems have been identified that can be used to study the polymerization quantitatively. These are the polymerization of styrene by Zr(benzyl)4 in toluene (16, 25) and the polymerization of methyl methacrylate by Cr(allyl)3 and Cr(2-Me-allyl)3 (12)- The latter system is unusual since esters normally react with transition metal allyl compounds (10) but a-methyl esters such as methyl methacrylate do not (p. 270) and the only product of reaction is polymethylmethacrylate. Also it has been shown with both systems that polymerization occurs without a change in the oxidation state of the metal. 

Hitachi Cable Ltd. (35) has claimed that dehydrogenation catalysts, exemplified by chromium oxide—zinc oxide, iron oxide, zinc oxide, and aluminum oxide—manganese oxide inhibit drip and reduce flammability of a polyolefin mainly flame retarded with ATH or magnesium hydroxide. Proprietary grades of ATH and Mg(OH)2 are on the market which contain small amounts of other metal oxides to increase char, possibly by this mechanism. 

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