Polyreactive heterofunctional oligoperoxides (HFOPs) were synthesized by radical copolymerization of a peroxidic monomer (PM) with vinyl acetate and butyl acrylate. In this polymerizing system, PM participates mainly in chain propagation reaction. That allows us to use the known statistical terpolymerization composition equation for description of HFOP macrochain formation. At the same time, PM not only effectively terminates growing oligomer chains, but also reduces their total amount. Molecular weight characteristics of HFOPs were studied by gel permeation chromatography. It is proposed that their typical bimodal molecular weight distribution is a result of two competitive mechanisms : a biradical interaction termination and termination with secondary radicals formed as a result of PM transfer. The termination mechanism ratio depends both on composition and conversion level of the monomer mixture. The main regularities of a PM containing system polymerization determined by us allow us to control intentionally HFOP structural and molecular weight characteristics. The possibility of HFOP use for curing and modification of the castable mixtures on the basis of oligodieneurethanes and oligoacrylates has been shown. The use of such oligoperoxides provides improved basic physicochemical properties, an increase of thermoresistance, and fatigue resistance of composite rubbers.