(424c) Determination of the Dominant Nucleation Mechanism for Oil-Soluble Initiators in Miniemulsion Polymerization Using Kinetic Monte Carlo Simulations

The use of oil-soluble initiators in free-radical miniemulsion polymerization has increased due to their ability to generate radicals primarily within monomer droplets. Existing theories concerning the nucleation mechanism for oil-soluble initiators suggest that a single radical must be formed within a particle for propagation to occur, despite the fact that an oil-soluble initiator molecule decomposes to form two radicals. According to existing theories, the primary source of nucleating radicals may be either within the particle [1], or from the small amount of initiator in the aqueous phase [2].

At the nanometer size of miniemulsion particles, concentration variables do not adequately describe the reactions in a particle, and use of such variables may lead to the inaccurate description of mechanisms within a particle. Application of the kinetic Monte Carlo algorithm [3] allows for the simulation of individual reactive events, such as propagation or termination, while tracking macroscopic observables, such as conversion and molecular weight. Simulation results are qualitatively compared to the conversion rate and the peak of the molecular weight distribution for two sets of experimental data for styrene miniemulsion polymerization, one taken from Alduncin and Asua [4], and another collected by ourselves.

Comparison of the simulation results to Asua's experimental data shows that desorption is insignificant at his experimental conditions. Our experimental data were collected at a smaller particle size and a lower temperature, and comparison of the simulation results to our data indicates that desorption of radicals is significant at our experimental conditions. Desorption of radicals is shown to be more significant at lower temperatures, when the propagation rate is lower, and at smaller particle sizes, where a radical may reach the particle surface more quickly. Absorption of radicals is shown not to be necessary for qualitative agreement with either set of experimental data.

[1] Asua, J.M., Rodriguez, V.S., Sudol, E.D., El-Aasser, M. S. “The Free-Radical Distribution in Emulsion Polymerization Using Oil-Soluble Initiators.” Journal of Polymer Science Part A: Polymer Chemistry, 1989, 27, 3569 – 3587.

[2] Nomura, M. and Suzuki, K. “The Kinetic and Mechanistic Role of Oil-Soluble Iinitiators in Micro- and Macroemulsion Polymerizations.” Industrial and Engineering Chemistry Research, 2005, 44, 2561 – 2567.

[3] Fichthorn, K.A. and Weinberg, W.H. “Theoretical Foundations of Dynamic Monte-Carlo Simulations.” Journal of Chemical Physics, 1991, 95, 1090 – 1096.

[4] Alduncin, J.A. and Asua, J.M. “Molecular-Weight Distributions in the Miniemulsion Polymerization of Styrene Initiated by Oil-Soluble Initiators.” Polymer, 1994, 35, 3758 – 3765.