Role of Chain Transfer Agents in Free Radical Polymerization Kinetics

This study deals with modeling the propagation and the chain transfer reactions in the free radical polymerization of ethylene, methyl methacrylate (MMA), and acrylamide (AM). The chain transfer agents modeled in the free radical polymerization of ethylene are the experimentally widely used species such as ethylene, methane, ethane, propane, trimethylamine, dimethylamine, chloroform, and carbon tetrachloride. The role of 4-X-thiophenols as chain transfer agents in the polymerization of MMA and AM has been investigated. Geometry optimizations have been carried out with the B3LYP/6-31+G(d) methodology. Reaction rate constants are calculated via the standard transition-state theory with the B3LYP/6-311+G(3df,2p)//B3LYP/6-31+G(d), MPWB1K/6-311+G(3df,2p)//B3LYP/6-31+G(d), and M05-2X/6-311+G(3df,2p)//B3LYP/6-31+G(d) methodologies, which reproduce qualitatively the experimental trends for the chain transfer rate constants. The usage of simple continuum models with the MPWB1K/6-311+G(3df,2p)//B3LYP/6-31+G(d) methodology for the solvation energies has slightly improved the accurate prediction of the chain transfer constants. Polar interactions highly influence the barrier heights for chain transfer reactions in the FRP of ethylene, MMA, and AM. Calculated chain transfer rate constants in the FRP of MMA and AM correlate quite well with the Hammett constants.