Abstract: The present invention relates to an unsaturated polyester resin composition comprising (a) an unsaturated polyester, (b) a vinyl group containing organic compound as reactive diluent and (c) a transition metal compound, wherein the resin composition comprises (d) a compound according to formula (1) as reactive diluent whereby n=0-3; R1 and R2 each individually represent H, C1-C20 alkyl, C3-C20 cycloalkyl, C6-C20 aryl, C7-C20 alkylaryl or C7-C20 arylalkyl; X=O, S or NR3 whereby R3=H, C1-C20 alkyl, C3-C20 cycloalkyl, C6-C20 aryl, C7-C20 alkylaryl, C7-C20 arylalkyl, part of a polymer chain or attached to a polymer chain, and the composition comprises at least one transition metal compound (c) selected from the group consisting of Co, Cu, Mn and Fe compounds; and the resin composition comprises, as vinyl group containing organic compound (b), styrene, a styrene derivative, a vinyl ether, a vinyl amine, a vinyl amide or a mixture of at least two of these compounds