PURPOSE: To obtain a versatile block polymer with industrial ease, by polymerizing different vinyl monomers in sequence in the presence of a specified azo initiator.

CONSTITUTION: Different vinyl monomers [e.g., (meth)acrylic acid, methyl methacrylate, vinyl chloride or styrene] are polymerized in sequence at a polymerization temperature of 40W150°C in the presence of an azo initiator of the formula (wherein R1 is phenyl, alkyl-substituted phenyl, alkyl, derivatives thereof, or cyano), e.g., phenylazotriphenylmethane. When the compound of the formula is used as an initiator, living radical polymerization takes place in a homogeneous system, and a useful block copolymer is synthesized. It is possible to carry out the polymerization in a simple system because the molecular weight can be controlled by regulating an initiator, a monomer amount, and a monomer conversion, so that the industrialization of the production process is extremely easy.