Beyond inhibition : a 1H NMR investigation of the early kinetics of RAFT-mediated polymerization with the same initiating and leaving groups

In situ 1H NMR (NMR) spectroscopy has been used to directly investigate the processes that occur during the early stages (typically the first few monomer addn. steps) of an AIBN-initiated reversible addn.-fragmentation chain transfer polymn. of styrene in the presence of the RAFT agent cyanoisopropyl dithiobenzoate at 70 and 84 DegC. The change in concn. of important dithiobenzoate species as a function of time has been investigated. It was found that the reaction was extremely selective during the period of consumption of the initial RAFT agent (defined as the initialization period), with almost no prodn. of RAFT-capped chains of d.p. greater than unity until all of the cyanoisopropyl dithiobenzoate was converted to its single monomer adduct. The rate-detg. step for this process was found to be the addn. (propagation) of the cyanoisopropyl radicals to styrene. During the period where the initial RAFT agent was consumed, fragmentation of formed intermediate radicals strongly favored the prodn. of the tertiary cyanoisopropyl radicals, which were the only significant propagating species during that period. This led to a greater rate of propagation during that period, since the propagation rate coeff. for the cyanoisopropyl radical is greater than that of polystyryl radicals. It was found that inhibition effects can occur in the presence of RAFT agents in homogeneous media when the kp for initiator fragments is smaller than for long chain radicals, which is a result of this aspect of the RAFT mechanism.