The catalytic effect of carbon on free radical polymerization, mediated by benzoyl peroxide initiation, has been previously reported by our group [1,2]. These studies have concluded that the carbon nanoparticles are involved in a mechanism that catalyzes the dissociation of the initiator on their surface. Consequently, the polymerization proceeds at temperature much below those normally used for the monomers considered. For methyl methacrylate, near complete conversion is achieved in less than three hours at 50 °C, and polymerization can be even carried out at refrigerator temperatures overnight. The current study aims at a systematic quantification of the accelerated kinetics, with the broader objective of elucidating the catalytic reaction mechanism. Kinetic data from free radical polymerization of different vinyl monomer such as glycidyl methacrylate (GMA), methyl methacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA) at different reaction parameters are compared and discussed. The effect of carbon content is studied and elucidated, as an initial increase of catalytic activity is observed at low carbon percentages followed by a decrease after a maximum is reached; this results from the competition between acceleration of dissociation and inhibition of polymerization, which becomes appreciable at higher levels. In all these systems, the formation of nanocomposites with superior dispersion of the carbon nanoparticles in the polymer matrix is observed; results on the characterization of these composites will be presented elsewhere.

It was interesting to note that the catalytic activity of the carbon particles is retained when these are present in a rubber formulation. Pulverized recycled rubber tire microparticles were tested and shown to catalyze the polymerization. As a matter of fact, the initial discovery of this catalytic effect occurred during the preparation of composites based on the pulverized rubber particles. Value-added materials can be obtained by synthesizing interpenetrating polymer networks (IPNs) based on pulverized rubber particles. Screening of the various additives in the rubber formulation for their effect on polymerization clearly showed that only carbon black is responsible for the catalysis. It was, however, interesting to note that the magnitude of the catalytic effect (normalized to the proper carbon content) is higher in the presence of rubber microparticles than when carbon nanoparticles are used. After excluding any possibility of another chemical catalytic activity effect, it was concluded that the near-perfect dispersion of the carbon black nanoparticles in the rubber formulation matrix creates a compartmentalization effect that interferes with the termination of radicals attached to one particle with those on other particles, thus severely impacting the rate of termination and consequently accelerating the net rate of polymerization. This of course is similar to the Tromsdorff effect observed in many radical polymerization systems as the viscosity of the medium increases with conversion. In experiments aimed at confirming this hypothesis, we conducted the polymerization in the presence of the carbon nanoparticles and varying amounts of poly(methyl methacrylate) added to the formulation. Results confirm the synergetic coupling between the reactive catalytic effect and the diffusional limitation effect. Quantification of the kinetics is reported for the proposed polymerization systems.

References:

1. Fouad Teymour, Sung Hun Kim, Amit Sachdeva, CARP: Carbon Accelerated Radical Polymerization—Free Radical Polymerization On Carbon Nanoparticles. Polymer Reaction Engineering VI, Engineering Conferences International, May 21-26, 2006, Halifax, Nova Scotia, Canada.

2. Ruohua Xiong, Fouad Teymour, and Hamid Arastoopour, Carbon Black Acts as Catalyst in Making Amphiphilic IPNs Based on Pulverized Rubber Particles, AIChE 2006 Annual Meeting, San Francisco, CA.