AbstractN‐Methacryloyl‐N‐methylcrotonamide (MMCA) was prepared by the reaction of methacryloyl chloride withN‐methylcrotonamide. WhileN,N‐disubstituted methacrylamide and crotonamide, which can be considered as structual moieties of MMCA, do not homopolymerize, nevertheless MMCA yielded a homopolymer consisting of 5‐membered cyclic repeating units. MMCA was found to be more reactive than N‐propyldimethacrylamide in copolymerization with styrene and methyl methacrylate. Since the polymer radical adds to the methacrylamide moiety, the steric hindrance of a β‐methyl group in the crotonamide moiety must be less significant than that of the α‐methyl group of the methacrylamide moiety in the dimethacrylamide. The 5‐membered ring formation during the polymerization of MMCA was facilitated by the favorable conformation of the three carbon atoms concerned in the ring closure to achieve coplanarity, which is required for the fast 5‐membered ring closure. The coplanarity of the three carbon atoms and reluctance to homopolymerize of the reacting double bonds seem to be the minimum requirements for forming of a polymer consisting of solely 5‐member