Polyaddition of bifunctional spiro orthoesters (SOEs) with bifunctional acid chlorides tvas examined to develop zero-shrinkage polymerization. The polyaddition afforded the corresponding polyether-esters by repeating the addition reaction accompanying the double ring-opening isomerization of the SOE moiety in a manner similar to the reaction of monofunctional SOEs with acid chlorides. The polyaddition accompanied a slight shrinkage or expansion in volume.