Low shrinking unsaturated polyester resin composition

In a molding composition having low shrinkability, which comprises

• (a) from 20 to 70% by weight of unsaturated polyester resin;
• (b) from 20 to 70% by weight of a first polymerizable monomer effective for reducing the shrinkability of said unsaturated polyester resin upon hardening thereof;
• and
• (c) an agent for improving the dispersion stability of the mixture of (a) and (b);
• the improvement which comprises:
• said agent (c) consists essentially of from 1 to 20% by weight, based on the weight of said composition, of a block copolymer prepared by polymerizing,
• (i) from 10 to 90 parts by weight of a monomer or monomer mixture consisting essentially of from 70 to 100% by weight of vinyl acetate and from 30 to 0% by weight of a second monomer copolymerizable with vinyl acetate, or
• (ii) from 90 to 10 parts by weight of a monomer or monomer mixture consisting essentially of from 0 to 100% by weight of a styrene-type monomer and 0 to 100% by weight of an acrylate or methacrylate monomer,
• in the presence of a polymeric peroxide as a polymerization initiator, said polymeric peroxide having the formula ##STR10## wherein R.sub.1 is alkylene or substituted alkylene having 1 to 18 carbon atoms, cycloalkylene or substituted cycloalkylene having 3 to 15 carbon atoms, or phenyl or substituted phenylene having 6 to 15 carbon atoms, and R.sub.2 is (1) alkylene or substituted alkylene having 2 to 10 carbon atoms, (2) a group having the formula ##STR11## wherein R.sub.3 is hydrogen or methyl, R.sub.4 is alkylene or substituted alkylene having 2 to 10 carbon atoms and m is an integer of 1 to 13, (3) ##STR12## and n is an integer from 2 to 10 to obtain a first copolymer having peroxy bonds in the molecule, and then effecting a second polymerization of said first copolymer with the other of (i) and (ii) that was not used in the first polymerization, thereby to obtain said block copolymer.