Mechanisms of Thermal Degradation Chain Growth Polymers

1 Mechanisms of Thermal Degradation Chain Growth Polymers 

During initiation, degradation begins at sites on the polymer that are susceptible to nucleophilic attack. These reactive sites are typically associated with nearby electronegative groups that enhance the reactivity of carbon atoms, as shown in Fig. 9.1. In general, we represent the initiation step as shown in Fig. 9.2. 

Initiation creates free radicals that can react with additional loci on the polymer chains in the propagation step, resulting in the transfer of the radicals from one chain to another. Since there are often many different types of reactive loci on a polymer chain, we typically have a variety of mechanistic steps to consider when describing propagation. Some of the propagation 

Another reaction mechanism that occurs in some chain-growth polymers is solvolysis. In this type of reaction, a species reacts with a C-Xbond, where X represents a halogen, and breaks it. Specifically, this becomes important vhen describing the degradation of polyvinyl chloride. Acidic species act to remove the chlorine atom, forming hydrochloric acid. 

Not only do we need to know ho w susceptible a polymer is to degradation, we also need to understand how quickly the polymer will succumb to its effects. From a processing standpoint, we need to see how long the polymer can be held at elevated temperatures before excessive degradation occurs. From an end-use perspective, we need to predict the lifetime of the final product as a function of the environment it will encounter. For these reasons degradation kinetics are extensively studied. 

---