Degradation, polymer random chain scission

Unlike the polyalkyl acrylates, which are thermally degraded by random chain scission, polyalkyl methacrylates unzip when heated, and excellent yields of the monomers are produced when the polymers of the lower homologues are heated. When higher homologues are heated, there is also some thermal degradation of the alkyl substituents. 

Thermal degradation occurs when a polymer is exposed to an elevated temperature in an inert atmosphere under exclusion of other compounds. The resistance against such degradation depends on the nature and the inherent thermal stability of the polymer backbone. There are three main types of thermal degradation depolymerization, random chain scission, and unzipping of substituent groups. 

Polymer degradation typically occurs via random chain scission, depolymerization, or both, resulting in a loss of chain length and properties associated with polymer length. 

Polypropylene (PP). Pyrolysis of PP is favored by the branched structure of the polymer the thermal degradation also proceeds in this case via a random-chain scission, but the influence of the temperature on the product spectrum is more pronounced than in the case of PE [43, 31], At temperatures as low as 515°C, Predel and Kaminsky [26] found that PP pyrolysis leads to the production of 6.8% of gases, 36.7% of oils, 21.6% of hght waxes and 34.6% of heavy waxes. At these low temperatures the main compounds in the gas fraction are propene and butenes (about 51 and 17% in [26]), but at higher temperatures these products are converted into others [43]. Ponte et al. [31] found a remarkable... 

The model chosen to describe the degradation of polyethylene was random chain scission. Lenz (3,) in his section on degradation reactions of polymers cites work which supports the contention that polyethylene does thermally degrade in a random chain scission manner as opposed to depolymerization. For this model a statistical treatment has been developed by Montroll and Simha (). The extent of reaction may be related to the number average molecular weight by ... 

Weight loss by the thermal degradation or thermooxidative degradation of a polymer itself (as opposed to the volatilization of small molecules which might have been trapped in the polymeric structure) invariably requires the breakage of chemical bonds. Once chemical bonds start to break, reactive chain ends and other free radicals are created, and degradation can proceed either by depolymerization or by random chain scission [11,12]. 

Poly-a-methylstyrene and polymethylmethacrylate degradations show strong similarities. Thus, both polymers depolymerize at relatively low temperatures. Almost pure monomer is obtained as volatile product Depolymerization of polymethylmethacrylate, however, is initiated at unsaturated chain ends below 250°C, whereas poly-a-methylstyrene undergoes mainly random-chain scission. 

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. 

There are basically three types of thermal degradation reactions for vinyl polymers [36,37] (1) nonchain scission (2) random chain scission and (2) depropagation. In practice, mechanisms 2 and 3 blend into one another, with many polymers showing evidence of both processes. 

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