PURPOSE: To obtain the titled copolymer having high fluidity in processing and excellent physical properties such as rubber elasticity, tensile strength, etc., by charging a propylene (co)polymer, an aqueous medium and an oil-soluble polymerization initiator in a polymerization reactor, and polymerizing vinyl chloride in the reactor keeping its concentration at a specific level.

CONSTITUTION: (A) A propylene polymer or a propylene-α-olefin copolymer containing small amount of α-olefin, (B) an oil-soluble polymerization initiator (e.g. α-cumyl peroxyneodecanoate), (C) pure water, and if necessary (D) ≤0.15% dispersant (e.g. methyl cellulose) or ≤15% inert organic solvent (e.g. methanol), based on water, are used as raw materials. Required amounts of the above raw materials are charged in an autoclave, and the system is substituted with nitrogen gas. The system is evacuated, vinyl chloride monomer is introduced into the system so as to keep the partial pressure of vinyl chloride at 0.4W0.99 times the saturated vapor pressure of the monomer at the polymerization temperature, and is polymerized at 30W60°C under gentle agitation.