The reaction of 3-methyl-2-methylene-1,3-oxazolidine (1a) and phenylisothiocyanate (PhNCS) gives 3-methyl-2-(phenylthiocarbamoyl)methylene-1,3-oxazolidine (3) whereas that of 2-isopropylidene-3-methyl-1,3-oxazolidine (1b) and PhNCS gives 1:1 alternating copolymers. It is assumed that the reaction of 1b and PhNCS forms a zwitterionic intermediate (2b), followed by the successive combination of 2b to give 1:1 alternating copolymers 4 and/or 5. Consequently, it was demonstrated that the copolymerization of 1b and PhNCS proceeds via a zwitterionic mechanism with complete ring-opening to afford the 1:1 alternating copolymer 5.