Certain new derivatives of 2,6,7-trioxabicyclo[2,2,2]octane are found to undergo cationic ring-opening polymerization with positive expansion in volume or with no substantial change in volume, in contrast with known polymerizable compounds such as epoxides which present considerable shrinkage in volume on polymerization. These new bicyclic orthoester compounds can be prepared in a facile way known per se.