Quantitative Analysis of the Effect of Azo Initiators on the Structure of α-Polymer Chain Ends in Degenerative Chain-Transfer-Mediated Living Radical Polymerization Reactions

The effects of azo initiators on the structures of the α-polymer chain ends in organotellurium-, organostibine-, organobismuthine-, and organoiodine-mediated living radical polymerizations (TERP, SBRP, BIRP, and IRP, respectively) and reversible addition–fragmentation chain transfer radical polymerization (RAFT) were quantitatively analyzed for the first time. These polymerization methods predominantly, or exclusively, follow the degenerative chain transfer (DT) mechanism for the activation and deactivation of dormant and radical species. An external radical species is required to initiate the polymerization under DT conditions, and the influx of radical species decreases the α-end-group fidelity. The effect was examined for polymerizations of styrene (St), n -butyl acrylate (BA), N -vinylpyrrolidone (NVP), methyl methacrylate (MMA), and isoprene (Ip) in the presence of different chain transfer agents (CTAs) and azo initiators, such as 2,2′-azobis(isobutyronitrile) (AIBN), 1,1′-azobis(cyclohexane-1-carbonitrile) (ACHN), and 1-[(1-cyano-1-methylethyl)azo]formamide (V-30), and the formed polymers 2 and 3, which possess α-end structures derived from the CTA and azo initiator, respectively, were analyzed by using matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS). The amount of 3 was negligible when the rate of propagation ( k p ) was sufficiently high, such as in the polymerizations of BA, MMA, and NVP. The formation of 3 was considerable when k p was low, such as in the polymerizations of St and Ip, but was considerably reduced by carrying out the polymerization at high temperatures using an azo initiator decomposed at high temperatures. Furthermore, the formation of 3 was completely disappeared when the initiating radical was supplied by the reversible termination (RT) mechanism in TERP. The results indicate that α-polymer chain end structure is better controlled when both RT and DT are involved. The effect of azo initiator on the synthesis of a block copolymer was also examined, and pure diblock copolymer PMMA- b -PSt was obtained when both RT and DT were involved.