The kinetics of addition and fragmentation in reversible addition fragmentation chain transfer polymerization

Abstract

High-level ab initio calculations of the forward and reverse rate coefficients have been performed for a series of prototypical reversible addition fragmentation chain transfer (RAFT) reactions: R· + S=C(Z)SCH3 → R-SC·(Z)SCH3, for R = CH 3, with Z = CH3, Ph, and CH2Ph; and Z = CH 3, with R = (CH3), CH2COOCH3, CH2Ph, and C(CH3)2CN. The addition reactions are fast (ca. 106-108 L mol-1 s-1), typically around three orders of magnitude faster than addition to the C=C bonds of alkenes. The fragmentation rate coefficients are much more sensitive to the nature of the substituants and vary from 10-4 to 107 s-1. In both directions, the qualitative effects of substituents on the rate coefficients largely follow those on the equilibrium constants of the reactions, with fragmentation being favored by bulky and radical-stabilizing R-groups and addition being favored by bulky and radical-stabilizing Z-groups. However, there is evidence for additional polar and hydrogen-bonding interactions in the transition structures of some of the reactions. Ab initio calculations were performed at the G3(MP2)-RAD//B3-LYP/6-31G(d) level of theory, and rates were obtained via variational transition state theory in conjunction with a hindered-rotor treatment of the low-frequency torsional modes. Various simplifications to this methodology were investigated with a view to identifying reliable procedures for the study of larger polymer-related systems. It appears that reasonable results may be achievable using standard transition state theory, in conjunction with ab initio calculations at the RMP2/6-311+G(3df,2p) level, provided the results for delocalized systems are corrected to the G3(MP2)-RAD level using an ONIOM-based procedure. The harmonic oscillator (HO) model may be suitable for qualitative "order-of-magnitude" studies of the kinetics of the individual reactions, but the hindered-rotor (HR) model is advisable for quantitative studies. Show more