Isomerization Polymerization of 2-Oxazoline. II. Propagating Species and Mechanism of Unsubstituted 2-Oxazoline Polymerization

Abstract

The propagating species and the mechanism of the cationic polymerization of 2-oxazoline(OXZ) were investigated. Equimolar reactions of OXZ with polymerization catalysts were directly followed by NMR spectra. In the polymerization with methyl tosylate(MeOTs) catalyst, the propagating species was shown to be an oxazolinium ion. In the reaction of OXZ with BF3OEt2 in acetonitrile at 40°C, the BF3–OXZ complex was isolated in a quantitative yield. On the other hand, in the reaction of OXZ with CH3I at 80°C, no oxazolinium ion was detected, but N-(β-iodoethyl)-N-methylformamide 3 was isolated. 3 was taken to have been derived from the ring-opening reaction of unstable oxazolinium iodide which was transiently formed by the reaction of OXZ with CH3I. In addition, 3 itself caused the OXZ polymerization and gave a similar polymer yield to that of the polymerization by CH3I. Thus, the propagating species of the OXZ polymerization by CH3I was assumed to be N(CHO)CH2CH2N(CHO)CH2CH2I. The initiating behavior of alkyl halides and the effect of solvents upon the OXZ polymerization demonstrated a close-relationship between the OXZ polymerization by alkyl halide catalyst and the Menschutkin reaction.