Producing heat stable vinyl chloride polymers at low temperatures in the presence of tetrahydrofuran

In a process for the suspension or dispersion polymerization of a monomeric system containing 80% or more of vinyl chloride and 20% or less of one or more vinylidene monomers containing a terminal CH.sub.2 .dbd.C<

• group selected from vinylidene chloride, vinyl esters, esters of acrylic acid and methacrylic acid, maleate and fumarate esters, ethylene, propylene and butylene in the presence of 0.001 to 1.0 part, based on 100 parts of monomer or monomers, or a monomer-soluble peroxide catalyst, the improvement wherein the polymerization is carried out in an aqueous medium at a temperature in the range of about 0.degree. to 40.degree. C. and in the presence of about 5 parts to about 50 parts by weight, based on 100 parts by weight of the monomer or monomers, of tetrahydrofuran, said polymerization being continued to a point no greater than 80% conversion of monomer or monomers to produce a polymer having a capillary viscosity heat stability in the range of about 15 to about 21 minutes.