Vinyl-terminated polyesters and vinyl-terminated aliphatic polycarbonates are described which improve the flexibility and elongation at failure of cured samples of unsaturated polyesters or vinyl ester resins both at room temperature and at high temperatures, and without significant loss of surface quality or other mechanical properties. Further, these polymers do not phase separate from uncured polyester resin/styrene solutions, do not significantly increase the initial viscosity of molding compositions in which they have been incorporated, and do not negate the effect of low shrinkage additives during cure.