A <SUP>1</SUP>H NMR Investigation of Reversible Addition−Fragmentation Chain Transfer Polymerization Kinetics and Mechanisms. Initialization with Different Initiating and Leaving Groups

In situ ¹H nuclear magnetic resonance spectroscopy was used to directly investigate the processes that occur during the early stages (the first few monomer addition steps) of azobis(isobutyronitrile)-initiated reversible addition−fragmentation chain transfer (RAFT) polymerizations of styrene in the presence of cumyl dithiobenzoate at 70 and 84 °C. The change in concentration of important dithiobenzoate species and monomer as a function of time was investigated. The predominant type of growing chain under the reaction conditions carries a cumyl end group. The initialization period (the period during which the initial RAFT agent is consumed) in the presence of cumyl dithiobenzoate in homogeneous media was significantly longer than for equivalent reactions using cyanoisopropyl dithiobenzoate as RAFT agent, and the rate of monomer conversion was correspondingly slower. Very strong fragmentation selectivity of the formed intermediate radicals (to form the tertiary propagating radical) was observed during the initialization period. The rate-determining step for the initialization process was the addition (propagation) of the initiator-derived and cumyl radicals to styrene, to form the corresponding single-monomer adducts. The greater length of this period with respect to the same reaction using cyanoisopropyl dithiobenzoate as RAFT agent is suggested to be a result of slower propagation due to a smaller addition rate coefficient of the cumyl radical (which was found to be the dominant propagation process during initialization) to styrene, than for the cyanoisopropyl radical, and to a higher average termination rate for the cumyl radicals than for the cyanoisopropyl radicals. The probable (small) difference in intermediate radical concentration is considered to be a less significant contributor to the length of the period.