Abstract

The first goal of this dissertation is to implement reversible addition-fragmentation chain transfer (RAFT) in microemulsion polymerizations to produce stable latex nanoparticles containing low polydispersity polymers of predetermined molecular weight. The compartmentalized nature of microemulsion polymerizations necessitates the investigation of the impact of several key parameters on the control of the polymerization, namely the ratio of the chain transfer agent concentration to the initial concentration of micelles in the microemulsion, and the water solubilities of the monomer and chain transfer agent. In addition, a simple kinetic model is developed to assist in identifying the source of rate retardation and investigating the impact of the key parameters.

RAFT microemulsion polymerizations of butyl acrylate (BA) with the chain transfer agent methyl-2-(O-ethylxanthyl)propionate (MOEP) served as the starting point for addressing the goals of this dissertation. These polymerizations produced stable poly(BA) particles less than 30 nm in diameter that contained polymers of the predicted molecular weight. However, as expected, the rate of BA polymerization decreased significantly as the ratio of MOEP/micelle increased. Concurrently, the location of the rate maximum shifted to earlier conversions. The combined decrease and shift of the rate maximum indicated that the incorporation of RAFT may induce nonlinear monomer partitioning between the micelles and polymer particles. Therefore, the monomer partitioning profile as a function of conversion was investigated by measuring the microstructural evolution of the micelles and polymer particles using small angle neutron scattering (SANS). Fitting of the SANS spectra for polymerizing BA microemulsions at MOEP/micelle ratios from 0.3 to 4.9 showed that the concentration of monomer at the locus of polymerization does in fact decrease linearly. Therefore, nonlinear monomer partitioning is not the source of the observed rate retardation or shift of the rate maximum.

The effects of the water solubilities of the monomer and the chain transfer agent on the control of the polymerization were examined by replacing BA with the low water solubility monomer 2-ethylhexyl acrylate (EHA) and by extending the ethyl group on MOEP to a dodecyl group (MODP) to decrease the solubility without altering the functionality. Decreasing the water solubility of the monomer was expected to increase the control of the polymerization because the low water solubility polymers are partitioned into surfactant stabilized particles at a low degree of polymerization. However, the opposite result was observed because coalescence of the BA particles increased the rate of transport of the chain transfer agent to the locus of polymerization. Decreasing the water solubility of the chain transfer agent was expected to decrease the polydispersity of the polymers by maintaining a more uniform distribution of the chain transfer agent per particle. In addition the average chain transfer agent per particle ratio decreases, so the rate retardation should be alleviated. In contrast to the expected results, decreasing the chain transfer agent solubility produced a bimodal distribution of polymers due to propagation by both controlled and uncontrolled polymerization. Simulations of the molecular weight distribution showed that an initial average chain transfer agent per micelle ratio greater than five is necessary to minimize the probability of a polymer propagating in the absence of the chain transfer agent.

The measured RAFT microemulsion polymerization kinetics, polymer molecular weights and polydispersities, and latex particle sizes allowed for the identification of the key mechanisms so that a simplified kinetic model could be developed to describe RAFT microemulsion polymerization. The model demonstrates the significance of the rate of fragmentation of the intermediate macroRAFT radical and the rate of diffusion of the chain transfer agent to the locus of polymerization. The model was fit to the rate of BA polymerization with MOEP and the intermediate macroRAFT radical lifetime was found to be approximately twice the characteristic time for propagation. Therefore, slow fragmentation of the macroRAFT radical is responsible for the observed rate retardation.

In conclusion, RAFT microemulsion polymerization is capable of producing stable latex nanoparticles containing low polydispersity polymers of predetermined molecular weight. The key parameters for achieving controlled polymerization are the chain transfer agent per micelle ratio, and the water solubilities of the monomer and the chain transfer agent. Rate retardation arises from slow fragmentation of the macroRAFT radical, which must be addressed through the rational design of the chain transfer agent.