Random chain scission, mechanism

Polymers with only one substituent on alternating carbon atoms degrade according to the depolymerization and the random chain scission mechanisms [7]. Thus, Fig. 9.1 shows that the maximum quantity of styrene evolved by pyrolysis of polystyrene is attained at 725°C, whereas PMMA with two substituents on alternating carbon atoms degrades much faster and the maximum quantity of monomer released by pyrolysis is reached at a much lower temperature, as shown in Fig. 9.2 [7]. 

Various studies have been made on the effects of radiation on lactide/glycolide polymers (24,38,58). Gilding and Reed (24) reported the effect of y rays on Dexon sutures. Those results confirmed that deterioration of the sutures occurs but that random chain scission is not the primary mechanism. Number average-molecular weight Mn showed a dramatic decrease at doses above 1.0 Mrad. Thus, unzipping of the polymer chain appeared to be the more dominant process, at least in the case of polyglycolide. 

The two polymer substrates investigated as part of the study of DBDPO mixtures were polypropylene (PP) and linear high density polyethylene (HDPE). while both PP and HDPE decompose by similar random chain scission, radical mechanisms, chain transfer occurs much more teadily during the pyrolysis of PP because of the presence of the tertiary hydrogens. In addition, only primary chain end radicals are formed when the HDPE chain cleaves homolytically. Therefore, a comparison of the PP/DBDPO and the HDPE/DBDPO mixtures volatile product distributions was undertaken. 

PEG can be severely degraded in air. Its melting point and heat of fusion are reduced by as much as 13 °C and 32 kJ kg"1, respectively [81]. The thermal degradation of PEG in air follows a random chain scission oxidation mechanism, and could be suppressed by addition of an antioxidant, 2, 2,-methylene-bis (4-methyl-6-tert-butylphenol) (MBMTBP), due to the reaction of MBMTBP with ROO radicals formed in the propagation step [79]. Low-molecular-weight esters including formic esters are produced as the main products of the thermal degradation of PEG (Scheme 3.17) [80]. 

In cases where no additional oxygen is present, polystyrene can undergo nearly pure thermal degradation. The two prevalent mechanisms are sequential elimination of monomer units, which is called unzipping or depolymerization. In this case, styrene monomer is formed. Random chain scission can also occur. It is sometimes combined with unzipping at the reactive broken chain ends. At temperatures approaching 300 °C, up to 40 % of a polystyrene molecule can be converted to styrene monomer. 

In the PS cracking process both mechanisms are possible and therefore 60 wt% of monomer can be recovered. Cracking of PP, PE and other polyolefins occurs by random-chain scission and therefore a broad hydrocarbon spectrum is produced. 

A review and some new results on the thermal degradation of poly-p-xylylene have been presented by Jellinek and Lipovac [303]. Little volatile material is formed but appreciable amounts of dimer, trimer, tetramer and pentamer were isolated. Typical vacuum volatilization curves are given in Fig. 73. It has been proposed that the mechanism consists of random chain scission at abnormal structures in the chain, followed by a depropagation reaction resulting in low molecular weight polymer but very little monomer. 

Thermal degradation of plastic and rubber wastes in inert atmospheres has been extensively studied in the past. It is widely accepted that it takes place through radical mechanisms, two main pathways having been proposed depolymerization by end-chain cracking and random chain scission. In the first case, high concentrations of the starting monomer are obtained, but this mechanism is predominant only in the thermal degradation of a few polymers, such as PS and... 

Hagnauer and LaLiberte (13) recently studied the degradation of poly[bis(ra-chlorophenoxy)phosphazene] via gel permeation chromatography and solution viscosity measurements on samples aged at 165°C. The degradation mechanism was postulated to be random chain scission at weak links in the polymer backbone. No evidence was obtained for a depolymerization-type mechanism. 

Poly[(trifluoroethoxy) (octafluoropentoxy) (X)phosphazene] (X — cross-link site) PFAP(II), undergoes a loss in molecular weight at elevated temperatures (135°-200°C) that is detrimental to mechanical properties. The mechanism of molecular weight loss was found to be random chain scission at weak links in the PFAP(II) chain. These weak links are postulated to be phosphazane moieties which would be attacked by water, acids, and POH to produce PN chain scission. 

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