Radical and anionic polymerizations of methyl and tert-butyl alpha-(allyloxymethyl)acrylates (AMA and ABA, respectively) were undertaken to see whether five-membered ring formation through head-to-head and tail-to-tail additions can occur irrespective of the polymerization mechanism. Both the monomers yielded highly cyclized polymers with a five-membered ring as a repeating cyclic unit in their radical polymerizations. Lithium compounds such as n-butyl- and tert-butyllithiums yielded highly isotactic uncyclized poly(AMA)s, while tert-butylmagnesium chloride afforded polymers with a considerably higher degree of cyclization (70%), the repeating cyclic units of which consist of a five-membered ring. Poly(ABA)s obtained through an anionic mechanism were found to have a five-membered ring as a repealing cyclic unit and degrees of cyclization higher than 80% irrespective of the initialer used. The structure of the anionically obtained cyclopolymers was fundamentally the same as those formed through a radical mechanism. These results along with those obtained from methyl and tert-butyl alpha-(propoxymethyl)acrylates. one of the monofunctional counterparts of AMA and ABA, respectively, led to the conclusion that anionic cyclopolymerizations proceed in the case where interaction of a countercation with the propagating chain end and the monomer is weak.