Synthesis of the asymmetrically substituted 2-methylene-7-phenyl-1,4,6,9-tetraoxaspiro[4.4]nonane monomer in four steps and its polymerization are described. The spiromonomer is liquid at room temperature. Photoinitiated cationic polymerization of 2-methylene-7-phenyl-1,4,6,9-tetraoxaspiro[4.4]nonane at ambient temperature in bulk was investigated, using a cationic photoinitiator system based on (eta(5)-2;4-cyclopentadiene-1-yl)[1,2,3,4,5,6-eta](1-(methylethyl)benzene)iron(I) hexafluorophosphate (Irgacure 261). Upon irradiation, within 10 mon quantitative double ring-opening polymerization to a structurally well-defined poly(alkylene-carbonate-ketone) with molecular weight average M(w) of 27.000 (GPC) occurred. No elimination, vinylidene polymerization, or single ring-opening polymerization was detected, as commonly observed in the case of free-radical initiation of similar spiroorthocarbonates, spiroorthoesters, or methylenedioxolanes were detected. Despite quantitative double ring opening, no volume expansion occurred during polymerization. A volume shrinkage of 4.1% was found. General structural prerequisites for expanding and pseudoexpanding monomer systems are discussed.