Three series of self-synthesized poly(vinyl acetate)-based low-profile additives (LPAs), including poly(vinyl acetate), poly(vinyl chloride-co-vinyl acetate), and poly(vinvl chloride-co-vinyl acetate-co-maleic anhydride), with different chemical structures and molecular weights were studied. Their effects on the glass-transition temperatures and mechanical properties for thermoset polymer blends made from styrene, unsaturated polyester, and LPAs were investigated by an integrated approach of the static phase characteristics, cured sample morphology, reaction kinetics, and property measurements. Based on Takayanagi mechanical models, the factors that control the glass-transition temperature in each phase region of the cured samples and the mechanical properties are discussed. (C) 2003 Wiley Periodicals, Inc.