Chain-length dependence in living/controlled free-radical polymerizations: Physical manifestation and Monte Carlo simulation of reversible transfer agents

A Monte Carlo model is used to show that chain-length dependent termination plays an important role in free-radical polymerization systems containing reversible transfer agents (RTAs) such as RAFT agents and alkyl halides. As dormant chains are activated through the reversible transfer reaction, the chain-length of the active species changes. By changing the length of the propagating radical, the RTA changes the behavior of the entire system, through the chain-length dependent termination reaction. The amount of polymer that may be produced before two radicals terminate is studied as a function of system parameters such as the transfer constant of the RTA, the concentration of the RTA, and the length of the dormant chains by modeling RTA-mediated polymerization, with emphasis on emulsion polymerizations. High transfer constant RTAs and short dormant chains exhibit significantly shorter radical lifetimes and hence have a slower overall rate of polymerization (seen as a lower average number of radicals per particle) than systems without RTA. Conversely, long dormant chains may lead to an extension of radical lifetimes and an increase in the number of radicals per particle. These effects are due to the change in the chain length of the polymeric radical because of transfer to dormant species, such that significant amounts of short-short termination are seen at low conversions and long-long termination is required at high conversions. This work also suggests several experimental techniques, including the use of oligomeric adducts to the RTA, that may offer solutions to the known problems of RTA systems in bulk, solution, and particularly emulsion polymerization.