Abstract: Poly(monoperoxycarbonates) of the structure: ##STR1## where R, R.sup.1 and n are defined in the summary of the invention such as 1,1,1-tris(t-butylperoxycarbonyloxymethyl)ethane, intermediates for their preparation as well as processes for their preparation and use are disclosed. The monoperoxycarbonate compounds are useful in initiating the polymerization of ethylenically unsaturated monomers, particularly styrene, curing of unsaturated polyester resins, and in modifying the molecular weight of polymers such as by crosslinking or controlled chain degradation.