Kinetic modelling of the reversible addition–fragmentation chain transfer polymerisation of N-isopropylacrylamide

Abstract

Reversible addition–fragmentation chain transfer (RAFT) polymerisation is a complex process involving multiple fundamental reaction steps that makes estimation of rate coefficients and exact kinetic simulation problematical. In this paper, we show that the homopolymerisation of N-isopropylacrylamide (NIPAM) can be kinetically modelled using a much-simplified method involving only a few kinetic parameters. The experimental polymerisation of NIPAM was performed with 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid as the RAFT agent and 4,4′-azobis(4-cyanopentanoic acid) as the initiator in dioxane at three different temperatures (60, 70 and 80 °C) with the initial [RAFT]:[initiator] ratio kept at 5:1 and different initial [monomer]:[RAFT] ranging from 100:0.2 to 100:1. Our method for the kinetic simulation makes use of a pseudo-concentration of polymer, in which all chains are assumed to have the same molar mass equal to the average molar mass Mn. With these approximations, the apparent rate coefficients for propagation, termination and initiator decomposition (kp,app, kt,app and kd,app respectively) and three fitting parameters, αp (ratio of propagating radicals to macroRAFT agent), αkp (ratio krein,app/kp,app), and αkt (exponent describing the chain length dependence of kt) were optimised. Our modelling results adequately simulated the experimental polymerization kinetics. Values of kt,app, kd,app, αp, αkp, αkt appeared essentially independent of [monomer]:[RAFT] and the values kt,app, kd,app and αkt were consistent in magnitude with literature values of the corresponding kt, α and kdf. Retardation associated with the RAFT process is encompassed in the value of kp,app, which was strongly dependent on [monomer]:[RAFT], with higher kp,app and Rp being observed for higher [monomer]:[RAFT].