- Chain-length dependence in living polymerizations: physical manifestation and Monte Carlo simulation of RAFT in emulsion polymerization. SW Prescott, MJ Ballard, E Rizzardo and RG Gilbert. 24th Australian Colloid and Surface Science Student Conference, Sunset Cove, SA, February 2004.
Recent progress has been made in the application of living polymerisation techniques (such as reversible addition-fragmentation chain transfer, RAFT) to emulsion polymerisation by developing a good mechanistic understanding of the systems. The free-radical polymerisation of hydrophobic monomers in emulsions is the method of choice for implementing most freeradical polymerisations on an industrial scale, particularly in producing surface coatings. Resulting products from classical emulsion polymerisations typically have quite wide distributions of molecular weights, and even relatively simple architectures such as A-B blocks are impossible to synthesise. RAFT polymerisation techniques have been the focus of a great deal of recent work as they allow unprecedented control over the molecular architecture for polymers made by free-radical polymerisation. RAFT/emulsion polymerisations have considerable technical potential: to “tailor-make” material properties, to eliminate added surfactant from surface coatings, and so on. However, considerable difficulties have been experienced in using RAFT in emulsion polymerisation systems.
A Monte Carlo model is used to show that chain-length dependent termination plays an important role in free radical polymerisation systems containing RAFT agents. 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 RAFT agent changes the behaviour 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 RAFT agent, the concentration of the RAFT agent and the length of the dormant chains by modelling RAFTmediated polymerisation. It is found that high transfer constant RAFT agents and short dormant chains exhibit significantly shorter radical lifetimes, and hence have a slower overall rate of polymerisation (seen as a lower average number of radicals per particle) than systems without RAFT agent. 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 due to 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 a number of experimental techniques, including the use of oligomeric adducts to the RAFT agent, that may offer solutions to the known problems of RAFT-mediated polymerisations in bulk, solution and particularly emulsion polymerisation.
Last edited: Friday September 10, 2010
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