Degradation random chain scission

Unlike the poly(alkyl methacrylates) that degrade by random chain scission, PMMA undergoes degradation through unzipping when heated. 

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

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. 

It is well known (11) that for degradation by random chain scission, the following relationship is valid ... 

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]. 

For both polyethylene and its many copolymeric variants and polypropylene, the main thermal degradative routes follow initial random chain scission. These reactions are only slightly affected by the differences in the physical structure such as crystallinity, but are influenced by the presence of impurities. However, it is largely true that while these may influence the proces-sibility and long-term stability of respective polyolefins, they may have little or no effect on the flammability. 

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. 

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 ... 

The essentially linear Arrhenius plot for the temperatures 130 t 150 , and 1T0 C demonstrates that random chain scission for polyethylene in the melt does behave in an Arrhenius manner. While we are not able to determine if the degradation behaves Arrhenlusly in the solid state, we can conclude that there is a definite discontinuity in the linear behavior between the two states. This discontinuity precludes the use of high temperature melt experiments to determine rate constants at temperatures below 90°C. 

The following conclusions may now be drawn for random chain scission degradation of polyethylene with the obvious qualifications that only limited data are reported herein. 

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]. 

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