Termination of Surface Radicals and Kinetic Modeling of ATRP Grafting from Flat Surfaces by Addition of Deactivator

Abstract

Surface-initiated controlled radical polymerization has been widely used for grafting polymers from various surfaces. However, how the surface-constrained radicals are terminated remains to be further elucidated. In this work, a simple kinetic model is developed for surface-initiated atom transfer radical polymerization (SI-ATRP) with addition of excess deactivator in solution. The model describes the development of polymer layer thickness, as well as the concentrations of radical, dormant and dead chains. A simple but accurate analytical solution is obtained for the polymer layer thickness as a function of time. The model accounts for the effects of equilibrium constant, activator/deactivator concentration ratio, monomer concentration, grafting density and rate constants of propagation and termination. The model is verified with the experimental data of 2-methacryloyloxythyl phosphorylcholine (MPC), methyl acrylate, acrylamide, and N-isopropylacrylamide under various conditions. In correlating thickness versus time experimental curves at different catalyst concentrations, it is clearly demonstrated that the termination of radicals on surface is facilitated by diffusion of catalyst species in solution. Although radical chains are immobilized, radical centers “migrate” and terminate through activation and deactivation reactions. The termination rate constant is therefore proportional to catalyst concentration. It is also found that the termination is influenced by chain conformation and the rate constant is grafting density dependent.