Nonchain scission

Nonchain-scission reactions resulting, for example, from the appHcation of heat, involve elimination of a small molecule — usually a pendant group — leaving the backbone essentially unchanged (Equation 9.4). 

Vinyl polymers are particularly susceptible to thermal degradation. A typical example is rigid PVC, which is impossible to process under commercially acceptable conditions without the use of thermal stabilizers. Unstabilized PVC imdeigoes dehydrochlorination near the melt processing temperature. This involves liberation of hydrochloric acid and the formation of conjugated double bonds (polyene formation). The intense coloration of the degradation products is due to polyene formation. A second example of a polymer that undergoes nonchain-scission reaction is poly(viriyl acetate) or PVAc. When heated at elevated temperatures, PVAc can liberate acetic acid, which is followed by polyene formation. 

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. 

Nonchain scission refers to reactions involving pendant groups that do not break the backbone. Typical of such reactions are dehydrochlorination of poly( vinyl chloride) (Equation 1.57), elimination of acid from poly(vinyl esters)—for example, poly(vinyl acetate) (Equation 1.58)—and elimination of alkene from poly(alkyl acrylate)s (Equation 1.59). 

The first two reactions lead to highly colored residues, indicating that the double bonds formed in the polymer backbone are primarily conjugated. Such elimination reactions are not satisfactory for synthesizing polyacetylene, however, since side reactions also occur. But nonchain scission has been... 

The depolymerization reaction cannot be stabilized practically. The same is true for random chain scission. However, the depolymerization can be slowed down by antioxidants. For this reason heat stabilizers have become very important in the plastics industry, mainly for halogen-containing polymers that decompose by nonchain scission. 

Chain Scission A degradation mechanism (300-500°C) involving rupturing of bonds in the backboned of the polymer chain, while nonchain scission is concerned with all other bond-breaking reactions caused by pyrolysis. 

The method only models those photon absorptions resulting in chain scissions. Other nonchain scission processes are ignored. This reduces the number of rate constants which would otherwise have to be estimated. 

---