Copolymers of vinyl acetate and acid monomers were synthesized by emulsion polymerization and applied as shrinkage control additives in the polymerization of unsaturated polyester (UP) and styrene at low temperatures. The presence of acid groups on the copolymer chain changes the selectivity of the cobalt promoter, and in turn, the relative reaction rate in the thermoplastic-rich and the UP-rich phase during polymerization. The kinetic results from differential scanning calorimeter showed that the copolymer containing stronger carboxyl groups tended to attract more cobalt promoter and led to a higher reaction rate in the thermoplastic-rich phase. The dilatometry results confirmed that the increased reaction rate in the thermoplastic-rich phase led to an earlier formation of microvoid and, consequently, less volume shrinkage of the resin during polymerization.