The cationic ring-opening polymerization of trimethylene urethane (tetrahydro-2H-1,3-oxazin-2-one) in the melt with methyl trifluoromethanesulfonate as initiator results in poly(trimethylene urethane) in yields of approximate to 70%. Under the same reaction conditions 2,2-dimethyltrimethylene urethane (5,5-dimethyltetrahydro-2H-1,3-oxazin-2-one) cannot be polymerized. Poly(2,2-dimethyltrimethylene urethane), however, was obtained via poly condensation polymerization. Both polymers exhibit a uniform microstructure as deduced from NMR spectroscopic analysis. Ring-closing depolymerization in the melt with dibutyltin dimethoxide or titanium tetraisopropoxide at 140 degrees C results the respective monomers in yields of approximate to 90%.