Chain scission purely random

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. 

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. 

Random scission is also typical of the degradation of many pol5miers by reaction of chain bonds with other reagents. Hydrolytic and other chemical reactions and biological attack frequently show purely random scission, provided that the accessibility of the chain to the degrading agent is not limited by, eg, crystallinity or water permeability. 

Recently, Sivalingham et al. [43] suggested that PCL underwent both random chain scission and specific chain end scission (elimination of monomer from the hydroxyl end of the polymer) simultaneously (a parallel mechanism) on non-isothermal heating and degraded by pure imzipping of the monomer from the lydroxyl end of the polymer chain on isothermal heating. 

The number of chain scissions is only about one or less at temperatures at/or below the ceiling temperature of 300°C. At temperatures higher than the ceiling temperature a significant number of chain scissions were obtained. Polymethylene shows a maximum of the volatilization rate at 25% conversion (pure random scission) (Fig. 18), while for branched PE no maximum in the rate of conversion is evidenced. There is therefore a good agreement between theory and experimental results. 

In the case shown in Figure 3.9, and were set as zero to represent pure random scission. The ratio kjk, reflecting the relative rate of autocatalytic hydrolysis to non-catalytic hydrolysis, was varied from 0 to inflnity to cover a wide range of different polymers. It can be observed from Figure 3.9 that in all the cases the accumulation of short chains is insignificant until the molecnlar weight reaches a very small value. This means any measurable mass loss cannot be expected before the device breaks apart. 

Figure 10.5 Ester bonds number of oligomers as a function of chain scission number for pure random scissions.

The theoretical treatments of controlled degradation are based on its radical character [189, 191, 192, 202] and chemically (peroxide) initiated degradation, or pure chain random scission, where Saito s integral is valid [202a]. 

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