PURPOSE: To readily obtain the subject polymer useful as a molding material by a single-stage reaction by reacting a radically polymerizable monomer with a cationically polymerizable monomer in the presence of a radical initiator.

CONSTITUTION: This polymer containing a radical polymerization segment and a cationic polymerization segment is obtained by simultaneously polymerizing (C) a radically polymerizable monomer and (D) a cationically polymerizable monomer by using (A) a radical initiator and (B) an electron acceptor simultaneously or in a time lag. In the case of the polymerization in a time lag, the components C is polymerized first and then the component D is polymerized. Preferably a styrenic monomer such as styrene, p-methoxystyrene or α- methylstyrene is preferably used as the component C, butyl vinyl ether, cyclohexene oxide or caprolactam as the component D, azobisisobutyronitrile as the component A and an onium salt (especially diphenyliodonium hexafluorophosphate) as the component B.