FRP mechanisms and kinetics

Free-radical polymerization, like coordination polymerization discussed in Chapter 2, involves the sequential addition of vinyl monomer(s) to an active center. For FRP the active centers are free radicals. The increase in chain length is very rapid an individual chain is initiated, grows to high MW and is terminated in a few seconds or less. After termination, the high-MW polymer chain does not react further (barring side reactions such as chain transfer to polymer or terminal/internal double bond polymerization) and is considered dead . Dead chains have a residence time of minutes or hours in the reactor, such that the final polymer product is an intimate mixture of chains formed under time and/or spatially varying conditions. 

Polymerization rate and average polymer chain length are controlled by the relative rates of initiation, propagation, termination and chain transfer events in the system. Starting from these basic mechanisms, this section derives simple kinetic expressions for a single monomer system. Complicating (but industrially important) secondary reactions are then discussed, followed by extension of the kinetics to multi-monomer systems. Dispersed throughout are values for important FRP rate coefficients, and descriptions of how they are experimentally obtained. More extensive overviews of FRP kinetics and mechanisms are presented in references 1-3. 

Many commercial polymers, including polystyrene, PMMA and LDPE, are synthesized via homogeneous FRP of a single monomer. Homopolymer properties are controlled by average chain length and chain-length distribution as well as, in some cases, structural characteristics such as branching level. 

The free radicals that initiate polymerization are usually generated by thermal or photochemical homolytic cleavage of covalent bonds. (A redox (reduction oxidation) process 

Initiator decomposition Chain initiation Chain propagation Chain termination By combination By disproportionation Chain transfer To monomer 

---