PURPOSE: To obtain the titled polymer having improved thermal stability, by copolymeizing a monomeric mixture containing allyl glycidyl ether with vinyl chloride or a compound copolymerizable therewith in the presence of an oil-soluble polymerization initiator and a dispersing agent in water medium.

CONSTITUTION: A mixture containing (A) allyl glycidyl ether, (B) vinyl chloride or a monomer copolymerizable therewith, e.g. ethylene, is polymerized in the presence of an oil-soluble polymerization initiator and a dispersing agent, e.g. partially saponified polyvinyl acetate, in water medium at a temperature of 30W80°C to give the aimed polymer. As for the oil-soluble polymerization initiator, di-3-methoxybutyl peroxydicarbonate, di-2-methoxyethyl peroxydicarbonate, di-2-ethoxyethyl peroxydicarbonate, etc. may be preferably used.