The effects of reactive poly(methyl methacrylate) (PMMA) and poly(vinyl acetate)-block-PMMA as low-profile additives (LPAs) on the volume shrinkage characteristics and internal pigmentability for low-shrink unsaturated polyester (UP) resins during curing at 110 degrees C were investigated. These reactive LPAs, which contained peroxide linkages in their backbones, were synthesized by suspension polymerization with polymeric peroxides as initiators. Depending on the LPA composition and molecular weight, the reactive LPAs led to a considerable volume reduction or even to a volume expansion after the curing of styrene (ST)/UP/LPA ternary systems; this was attributed mainly to the expansion effects of the LPAs on the ST-crosslinked polyester microgel structures caused by the reduction in the cyclization reaction of the UP resin during curing as well as to the repulsive forces between the chain segments of UP and LPAs within the microgel structures. The experimental results were explained by an integrated approach of measurements for the static phase characteristics of the ST/UP/LPA system, reaction kinetics, cured sample morphology, and microvoid formation with differential scanning calorimetry, scanning electron microscopy, optical microscopy, and image analysis. With the aid of the Takayanagi mechanical model, the factors leading to both a good volume shrinkage control and acceptable internal pigment-ability for the molded parts were also explored. (c) 2005 Wiley Periodicals, Inc.