The effect of 2,6-di-tert-butylpyridine (DtBP) on a-methyl-styrene (αMeSt) polymerization induced by the “H20”/BC13initiating system has been studied. Yields, molecular weights, (MW's), and molecular weight distributions (MWD's) obtained in the presence of [DtBP] >1.0 × 10−3Mare invariant with DtBP concentration. Results are explained by the concept of terminative chain transfer, i.e., a process in which the proton which arises in the course of chain transfer to monomer is intercepted by DtBP so that the kinetic chain is terminated. Polymerization goes to completion in the presence of very small amounts of DtBP, i.e., 4.8 × 10−4M or less; however, MW's and MWD's of the poly(a-methylstyrene) (PaMeSt) formed are still profoundlyjiffected. Thus, in the absence of DtBP, Mn= 1.2 × 104and Mw/Mn= 3.8, whereas in the presence of [DtBP] = 1.0 ×l(10−4,MM = 4.5 × 10−4and Mw/Mn= 1.87.