Radical degradative fragmentation

McNeill and Rincon studied thermal degradation of PC by means of TGA, TVA, DSC, FT-IR, mass spectrometry (MS) and gas chromatogra-phy-MS (GC-MS) method (see Fig. 2.5). PC is stable up to 300°C. Above that temperature, small quantities of phenol and p-cresol were detected, at 375-400°C CO2 appeared and then at T > 455°C CO and CH4 were formed the peak on the DTG curve was at F = 462°C. At 500°C the main products are q chc dimer and bisphenol A, with small quantities of CO2, p-cresol, p-ethyl phenol, phenol, p-vinyl phenol, p-isopropyl phenol, CO and CH4. In the absence of air and moisture, degradation of PC proceeds by hemolytic decomposition of the polymer chain, radical reactions, fragmentations and molecular rearrangements. 

This section will describe recent ex situ (in the laboratory) and in situ (in an FC) ESR experiments ° fragmentation of model compounds (MCs) for the fluorinated membranes chemical reactions and crossover processes in an FC inserted in the ESR spectrometer membrane stabilization by Ce(III) and a competitive kinetics (CK) approach that allows ranking of membrane stability to attack by oxygen radicals. Degradation of polyaromatic FC membranes studied by direct ESR will also be included. 

The bond p- to the double bond of the unsaturated disproportionation product 2 is also weaker than other backbone bonds.10 30,32 31 However, it is now believed that the instability of unsaturated linkages is due to a radical-induced decomposition mechanism (Scheme 8.7).30 This mechanism for initiating degradation is analogous to the addition-fragmentation chain transfer observed in polymerizations carried out in the presence of 2 at lower temperatures (see 6.2.3.4, 7.6.5 and 9.5.2). 

The ionization and excitation may lead to chemical bond cleavage and production of highly reactive species, free radicals, ions and molecular fragments, which subsequently interact with each other and at last stable degradation products are created. This complex sequence of processes can deliberately be divided into two basic phases, the initial physical phase, in which the ion energy is dissipated to electrons and atoms, and the chemical one comprising interaction of the reaetive species and production of the final stable products. 

PBS (Figure 30) is an alternating copolymer of sulfur dioxide and 1-butene. It undergoes efficient main chain scission upon exposure to electron beam radiation to produce, as major scission products, sulfur dioxide and the olefin monomer. Exposure results first in scission of the main chain carbon-sulfur bond, followed by depolymerization of the radical (and cationic) fragments to an extent that is temperature dependent and results in evolution of the volatile monomers species. The mechanism of the radiochemical degradation of polyolefin sulfones has been the subject of detailed studies by O Donnell et. al. (.41). 

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