Vinylidene fluoride-based polymer and its production

PURPOSE:To obtain the subject polymer having such crystal characteristics as to be small in spherulite size without impairing physical properties and processability, etc., by suspension copolymerization of a vinylidene fluoride monomer with specific copolymerizable component(s) by a specified method. CONSTITUTION:In carrying out suspension polymerization of a vinylidene fluoride monomer with 1-5wt.%, based on the total monomers, of copolymerizable component(s) selected from hexafluoropropylene and/or tetrafluoroethylene in the presence of an oil-soluble polymerization initiator in an aqueous medium, the first stage polymerization is made until conversion rate of the vinylidene fluoride reaches 10-50% so as to be 1.3-1.9dl/g in the logarithmic viscosity [eta1] of the polymer formed; subsequently, the second stage polymerization is made under addition of a chain transfer agent (pref. ethyl acetate) to the polymerization system until the logarithmic viscosity [eta2] of the final polymer is lower than that of [eta1] by 0.3-0.7dl/g and reaches 0.9-1.3dl/g.