Synthetic polymers are usually the mixture of molecules of different molecular weights. Since molecular weight and its distribution affect the fundamental properties of polymer materials, one of the most important subjects in synthetic chemistry is to establish facile and versatile procedures for the synthesis of polymers with regulated molecular weight, in view of the great demand for the design of polymer materials of elaborate functions. Up to now, however, the successful examples are limited. This paper describes our recent findings that some metalloporphyrins of aluminum are excellent catalyst of much wider applicability than the reported examples for the synthesis of polymers with regulated molecular weight. The polymer of epoxide or β-lactone, the copolymer of phthalic anhydride-epoxide or CO2-epoxide and their block copolymers are synthesized in the form of regulated molecular weight. The polymer formation takes place on the faces of metalloporphyrin.