Polymerizations of methyl methacrylate by bis(pentamethylcyclopentadienyl)samarium hydride, [Sm(C5Me5)2H]2, in toluene were found to afford the polymers of M(a) > 100 X 10(3) with extremely low polydispersity, M(w)/M(n) = 1.02. Since the M(n) increases with an increase of the conversion while M(w)/M(n) remains intact, the present polymerization is estimated to proceed in a living fashion. The stoichiometric reaction of [SmH(C5Me5)2]2 with MMA in a 1:2 ratio gives rise to the formation of an eight-membered ring intermediate, in which a Sm-enolate bond exists at the one end and at the other end a Sm --- OC bond exists between the Sm and penultimate MMA unit as confirmed by X-ray work. Furthermore highly syndiotactic polymerization (95.3%) proceeded by lowering the reaction temperature to -95-degrees-C. Methylbis(pentamethylcyclopentadienyl)lanthanide complexes such as YbMe(CrMe5)2(THF) and LuMe(CrMe5)2(THF) in addition to trimethylaluminum complexes of organolanthanide(III) like Yb(C5Me5)2(mu-Me)2AlMe2, Y(C5Me5)2(mu-Me)2AlMe2, and Lu(Cr5Me5)2(mu-Me)2AlMe2 behaved in a fashion similar to that of [SmH(C5Me5)2]2. Organolanthanide(II) complexes such as Yb(C5Me5)2(THF)2, SM(C5Me5)2(THF)2, and Yb(indenyl)2(THF)2 also start the living polymerizations although their initiator efficiencies are much smaller (35%) than those obtained by organolanthanide(III) complexes (>90%).