The free-radical polymerization of hydrophobic monomers in emulsions is an industrially and scientifically useful means of producing polymers. Resulting products from traditional emulsion polymerizations typically have quite wide distributions of molecular weights and even relatively simple architectures such as A-B blocks are impossible to synthesize. Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization techniques allow unprecedented control over the molecular architecture of polymers made by free-radical polymerization. RAFT/emulsion polymerizations have considerable technical potential, for example to "tailor-make" material properties or to eliminate added surfactant from surface-coating formulations. However, considerable difficulties have been experienced in using RAFT in emulsion polymerization systems.

The successful use of the living radical polymerization technique RAFT is first described for the seeded emulsion polymerization of styrene using the benzyl-stabilized RAFT agent 2-phenylprop-2-yl phenyldithioacetate (PPPDTA). RAFT-mediated polymerization is seen to give both control over the molecular weight and a narrow polydispersity product. The presence of RAFT agent in the monomer droplets at the commencement of polymerization is postulated to be the cause of previous RAFT/emulsion attempts being unsuccessful. The use of gamma-initiation of RAFT/emulsion systems is also described; the relaxation behavior on removal from the radiation source gives information about radical loss processes. A reduction in the rate of polymerization and long inhibition periods are observed that are dependent on the concentration of RAFT agent in both chemically- and gamma-initiated systems. The characteristic times for gamma-relaxations are also seen to be much shorter in the presence of RAFT agents.

Chain-length dependent termination is shown to play an important role in RAFT-mediated emulsion polymerization, with the RAFT agent changing the length of the propagating radical as a function of conversion. At low conversion, the termination rate coefficients are higher than in the absence of RAFT and zero-one kinetics is applicable to the system; at high conversion, termination is slower and pseudo-bulk kinetics are more appropriate. The observed increase in the number of radicals per particle as polymerization progresses is consistent with the influence of chain-length dependent termination, as is the observed increase in the timescale for relaxation with the increasing length of the dormant chains.

A method is described by which a suitable average rate coefficient for termination may be selected for the Smith-Ewart population balance equations. In some situations, it is possible to easily calculate the Smith-Ewart parameter for termination from the chain-length distribution of radicals analytically, while various numerical techniques (including integration and Monte Carlo simulation) may be used more generally. RAFT/emulsion systems are shown to have greatly reduced compartmentalization compared to their non-RAFT analogues. The RAFT-induced exit of radicals was estimated to lead to a ~400-fold increase in the rate coefficient for radical exit from the particles, which is consistent with the rapid relaxations observed in gamma-relaxation experiments.

The inhibition period of RAFT/emulsion systems is shown to be adequately modeled by zero-one kinetics, once the RAFT-induced exit of radicals, the exit of the re-initiating group from the particle, and the specificity of the re-initiating group to the initial RAFT agent are included.

With the models developed here for RAFT/emulsion systems, strategies for improving the performance of reactions are developed, including the use of lower-activity RAFT agents to improve the compartmentalization of the system. The use of oligomeric adducts to the initial RAFT agent are shown to improve the rate of polymerization by reducing the termination rate coefficients in the system.

Generated automatically with Publist v. 1.3b

Last edited: Friday September 10, 2010

Valid XHTML 1.1 Valid CSS 2