Modeling the Influence of Diffusion-Controlled Reactions and Residual Termination and Deactivation on the Rate and Control of Bulk ATRP at High Conversions

Shiping Zhu,Ali Rabea

doi:10.3390/polym7050819

Abstract

In high-conversion atom transfer radical polymerization (ATRP), all the reactions, such as radical termination, radical deactivation, dormant chain activation, monomer propagation, etc. could become diffusion controlled sooner or later, depending on relative diffusivities of the involved reacting species. These diffusion-controlled reactions directly affect the rate of polymerization and the control of polymer molecular weight. A model is developed to investigate the influence of diffusion-controlled reactions on the high conversion ATRP kinetics. Model simulation reveals that diffusion-controlled termination slightly increases the rate, but it is the diffusion-controlled deactivation that causes auto-acceleration in the rate (“gel effect”) and loss of control. At high conversions, radical chains are “trapped” because of high molecular weight. However, radical centers can still migrate through (1) radical deactivation–activation cycles and (2) monomer propagation, which introduce “residual termination” reactions. It is found that the “residual termination” does not have much influence on the polymerization kinetics. The migration of radical centers through propagation can however facilitate catalytic deactivation of radicals, which improves the control of polymer molecular weight to some extent. Dormant chain activation and monomer propagation also become diffusion controlled and finally stop the polymerization when the system approaches its glass state.

Highlights

Controlled radical polymerization (CRP), such as stable free radical polymerization (SFRP) [1], atom transfer radical polymerization (ATRP)(or equivalently, controlled reversible-deactivation radical polymerization) [2,3], and reversible addition fragmentation chain transfer (RAFT) polymerization [4], has attracted great attention in synthesis of well-controlled functional polymers.Among different CRP methods, ATRP is of particular interest for its high potential of applications in various areas [5]
Control over chain microstructure such as molecular weight distribution (MWD) and polymerization system livingness depend on the polymerization recipes and conditions, which determine the rates of radical termination, radical deactivation, dormant chain activation, and monomer propagation
We develop a kinetic model that includes all the possible reactions at high conversions and examine their effects on the polymerization rate and the control of polymer molecular weight in a bulk ATRP using methyl methacrylate (MMA) as a model system

Summary

Introduction

Controlled radical polymerization (CRP) (or equivalently, reversible-deactivation radical polymerization, RDRP), such as stable free radical polymerization (SFRP) (or equivalently, aminoxyl-mediated radical polymerization, AMRP) [1], atom transfer radical polymerization (ATRP). In the conventional free radical polymerization, diffusion-controlled monomer propagation is responsible for the termination of polymerization at an incomplete conversion (termed “glass effect”) It is much more complicated in CRP processes. Schematic presentation of (a) termination by propagation, (b) deactivation by propagation, and (c) termination by activation/deactivation, which can possibly occur in ATRP at high conversions when small reactants experience diffusion limitations. We develop a kinetic model that includes all the possible reactions at high conversions and examine their effects on the polymerization rate and the control of polymer molecular weight in a bulk ATRP using methyl methacrylate (MMA) as a model system

Theoretical Development

Diffusion-Controlled Reactions

Residual Termination and Deactivation

Parameters and Initial Conditions

Results and Discussion

Conclusions