3-Ethyl-3-methacryloyloxymethyloxetane (EMO) was easily polymerized by dimethyl 2,2 ' -azobisisobutyrate (MAIB) as the radical initiator through the opening of the vinyl group. The initial polymerization rate (R-P) at 50 degreesC in benzene was given by R-P = k[MAIB](0.55) [EMO](1.2). The overall activation energy of the polymerization was estimated to be 87 kJ/mol. The number-average molecular weight (M-n) of the resulting poly(EMO)s was in the range of 1-3.3 x 10(5). The polymerization system was found to involve electron spin resonance (ESR) observable propagating poly(EMO) radicals under practical polymerization conditions. ESR-determined rate constants of propagation (k(P)) and termination (k(t)) at 60 degreesC are 120 and 2.41 x 10(5) L/mol s, respectively-much lower than those of the usual methacrylate esters such as methyl methacrylate and glycidyl methacrylate. The radical copolymerization of EMO (M-1) with styrene (M-2) at 60 degreesC gave the following copolymerization parameters: r(1) = 0.53, r(2) = 0.43, Q(1) = 0.87, and e(1) = +0.42. EMO was also observed to be polymerized by BF3OEt2 as the cationic initiator through the opening of the oxetane ring. The M-n of the resulting polymer was in the range of 650-3100. The cationic polymerization of radically formed poly(EMO) provided a crosslinked polymer showing distinguishably different thermal behaviors from those of the radical and cationic poly(EMO)s.