Carbocationic Polymerization in the Presence of Sterically Hindered Bases. II. Polymerization of a-Methylstyrene

The effect of 2,6-di-tert-butylpyridine (DtBP) on a-methyl-styrene (αMeSt) polymerization induced by the “H20”/BC13 initiating system has been studied. Yields, molecular weights, (MW's), and molecular weight distributions (MWD's) obtained in the presence of [DtBP] >1.0 × 10−3 M are 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 × 10−4 M 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 × 104 and Mw /Mn = 3.8, whereas in the presence of [DtBP] = 1.0 ×l(10−4, M M = 4.5 × 10−4 and Mw/Mn = 1.87. According to Mayo plots, a small but nonnegligible extent of chain transfer to monomer persists even in the presence of DtBP, e.g., ktr,M/kp = 6.4 × 10−4 and 2.8 × 10−4 at -30 and -50°C, respectively. This, by DtBP unscavengable chain transfer to monomer, is attributed to a concerted chain transfer process. A mechanism is proposed which explains the experimental facts.