New aspects of cationic polymerization of spiroorthoester: cationic single ring-opening polymerization and equilibrium polymerization

2-methyl-, 2-phenyl-, 2-bromomethyl- and -1,4,6-trioxaspiro[4.6]undecne were found to polymerize under cationic conditions at low temperature to afford the corresponding poly(cyclic orthoester)s [O(CH 2 ) 5 -C(C 2 H 3 O 2 R)-] n through single ring opening of the seven-membered ether rings. The obtained polymers easily depolymerized upon treatment with HCl at room temperature to give the monomers. The polymerizations were suggested to be typical equilibrium polymerizations, to which the Dainton's equation could be applied. Enthalpy and entropy changes of the polymerizations were determined.