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 polymeri-zation of 2-oxazoline(OXZ) were investigated. Equimolar reactions of OXZ with poly-merization 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 BFaOEt2 in acetonitrile at 40°C, the BFa-OXZ complex was isolated in a quantitative yield. On the other hand, in the reaction of OXZ with CHal at 80°C, no oxazolinium ion was detected, but N-(p-iodo-ethyl)-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 CHal. Thus, the propagating species of the OXZ polymerization by CHal 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. KEY WORDS 2-Oxazoline / Cationic Polymerization / Poly(N-for-mylethyleneimine) / Propagating Species / Oxazolinium Ion / N-(p-iodoethyl)-N-methylformamide / Menschutkin Reaction / Cationic polymerization of 2-substituted-2-oxazolines has been widely studied, 1-4 and the following schemes involving the oxazolinium ion has been assumed without direct experimental support. On the other hand, the polymerization of un-substituted 2-oxazoline(OXZ) was not reported before our recent paper. 5 We first succeded in the cationic polymerization of OXZ by the isolation of the pure monomer and the use of +