New Paradigms in Free-Radical Polymerization Kinetics

Abstract

Free-radical polymerization (FRP) is inherently complex in that it has a chain-reaction mechanism. Matters are complicated further by various reactions being chain-length dependent in rate, meaning that kinetic schemes should distinguish each individual length of radical. What all this means is that microscopically accurate descriptions of FRP kinetics can involve thousands of species balance equations. Since modern computing power enables solutions to such large sets of equations to be obtained, it is argued that this approach should be employed in attempting to reach an understanding of FRP kinetics. Three examples of doing so are presented: (1) How chain-length-dependent termination explains that the rate law for FRP is actually non-classical; (2) How a ‘composite’ termination model enables reconciliation of long-chain termination data with what is known about termination of small-molecule radicals; and (3) How chain-length-dependent propagation (CLDP) explains unusual rate data from FRP carried out in the presence of high amounts of chain-transfer agent. It is also explained how these paradigm shifts have led to new and unexpected insights into FRP kinetics. Especially noteworthy are that the nature of cross-termination often has no effect on the rate law and that CLDP narrows the molecular weight distribution.