Asymmetric unsaturated spiro orthocarbonates (SOC) that polymerize with an expansion in volume due to the free radical double ring-opening were prepared by cross-couplings of a cyclic thionocarbonate with cyclic tin compounds. Cyclic thionocarbonate was initially generated by the reaction of a diol with thiophosgene. The subsequent coupling of this thionocarbonate intermediate with the cyclic tin product of dibutyltin oxide plus a second diol allowed the corresponding asymmetric SOC's to be prepared in good yields.