Mechanistic And Kinetic Study Of RAFT Miniemulsion Polymerization

Colloidal instability, polymerization mechanism, and kinetic of RAFT (Reversible addition-fragmentation chain transfer) styrene miniemulsion polymerization were systematically investigated.Inspired from the theoretical simulations about the superswelling of particles by Luo et al., the influence of the interfacial tension, costablizer concentration, chain transfer constant of RAFT agent and its concentration, and surfactant levels on the colloidal stability during the polymerization was investigated. The experimental data showed that lowering interfacial tension, increasing co-stabilizer levels, using RAFT agent with lower chain transfer constant, lowering the concentration of RAFT agent, and increasing the surfactant levels all help to achieve the colloidal stability during polymerization, leading to a smooth polymerization course, low molecular weight distribution, and narrow particle size distribution. Based on the experimental observations about molecular weight and it distribution, particle size and its distribution, the physical model in the RAFT miniemulsion was revealed. In the very beginning of RAFT miniemulsion polymerization (<5% conversion), a small fraction of nucleated mini-droplets (particles) will absorb a huge amount of monomer from those non-nucleated droplets, leading to a large increase in size, i.e. superswelling. Meanwhile, a large number of droplets shrink and immerge into the large particles or droplets, which results in the low nucleation efficiency of RAFT miniemuslion polymerization. During this swelling process, the miniemulsion would become unstable once the swelling degree is too high. Two kinds of particles: polymer particles and oligomer particles would come into being in the case of lower swelling degree. Two kinds of particles have different polymerization kinetics and RAFT concentrations, which broadens the molecular weight distribution together with the particle size distribution.Both the number of particles (N_p) and average number of radical ((?)) per particle were formulated. The relationship between N_P and SDS (Sodium DodecylSulfate) concentration is found to be N_P (?) [SDS]^0.426, the power of which is decreasedfrom the unit of the regular miniemulsion polymerization due to the superswelling. The kinetic model of RAFT miniemulsion polymerization was derived based on a proper modification on Smith-Ewart equation. In a simplified zero-one case, the average number of propagating radicals per particle can be described byn|-_RAFT^-1=N|-_blank^-1 +2K[RAFT]₀ , where K is the RAFT equilibrium coefficient. Itwas found that the rate retardation is an intrinsic kinetic property of RAFT (mini)emulsion polymerization. The miniemulsion polymerization of styrene was also carried out with styrene oligomers of 1-phenylethyl phenyl-dithioacetate (PS-PEPDTA) and 2-cyranoprop-2-yl dithiobenzoate (PS-CPDB) as the RAFT agents.The experimental n|- data were well described by the theory. The K values were estimated to be 314 Lmol^-1s^-1 for PS-CPDB and 22 Lmol^-1s^-1 for PS-PEPDTA. Thefragmentation rate coefficients appeared to be in the order of magnitude of 10⁴-10⁵s^-1Batch bulk, batch miniemulsion, and semi-continuous miniemulsion polymerization mediated by RAFT were compared in terms of the synthesis of high molecular weight polymer. It turned out that batch RAFT polymerization either in bulk or in miniemulsion is difficult to synthesize polystyrene of high molecular weight with low PDI. We surprisingly found that the PDI increases significantly with increase of the targeted molecular weight. Instead, it was demonstrated that a two-step semi-batch RAFT miniemulsion polymerization is effective to synthesize higher molecular weight polymer with relative narrow molecular weight distribution.