A polymerization initiator system for reactive monomers and unsaturated polymers which is contains a Group 4 transition metal or Group 4 transition metal-containing compound, peroxide or diazo compound that is capable of being reduced by the Group 4 transition metal or Group 4 transition metal-containing compound, and pinacol compound is disclosed. Further disclosed are methods for preparing the polymerization initiator and using the polymerization initiator for low temperature curing.