AbstractA spiro ortho carbonate containing a double bond, 3‐methylene‐1,5,7,11‐tetraoxaspiro‐[5.5]undecane (2) was prepared from 2‐methylene‐1,3‐propanediol. The structure of the monomer was confirmed by its elemental analysis, IR and NMR spectra. The monomer was polymerized with Lewis acids such as boron trifluoride etherate and stannic chloride (tin tetrachloride) as catalysts. The IR and NMR spectra indicated that the polymer has the structure of an alternating copolymer containing ether and carbonate linkages with a pendant double bond (structure8).Monomer2underwent expansion on p