A 1H NMR Investigation of Reversible Addition-Fragmentation Chain Transfer Polymerization Kinetics and Mechanisms. Initialization with Different Initiating and Leaving Groups

In situ 1H NMR spectroscopy was used to directly investigate the processes that occur during the early stages (the first few monomer addn. steps) of azobis(isobutyronitrile)-initiated reversible addn.-fragmentation chain transfer (RAFT) polymns. of styrene in the presence of cumyl dithiobenzoate at 70 and 84 DegC.The change in concn. 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 equiv. 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 obsd. during the initialization period. The rate-detg. step for the initialization process was the addn. (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 addn. rate coeff. 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 av. termination rate for the cumyl radicals than for the cyanoisopropyl radicals. The probable (small) difference in intermediate radical concn. is considered to be a less significant contributor to the length of the perio