Cationic Ring-Opening Polymerization of Tetramethylene Urethane

The cationic ring-opening polymerization of tetramethylene urethane (TeU; systematic name, hexahydro-1,3-oxazepin-2-one, ( 1 )) in the melt at 67 °C with methyl trifluoromethanesulfonate (TfOMe) as the initiator yields poly(tetramethylene urethane) (poly(TeU), ( 6 )) with a regular microstructure. This [5]-polyurethane is a semicrystalline material with a T g of 47 °C and a T m of 209.5 °C. The reaction proceeds via an active chain end mechanism with a protonated cyclic endo iminocarbonate as the active species. The propagation step involves nucleophilic attack of the carbonyl oxygen of the monomer at the endocyclic methylene group adjacent to the oxygen atom of the active species. The propagation rate constant was 4.2 × 10 -4 L·mol -1 ·s -1 . The polymerization of tetramethylene urethane is accompanied by termination reactions; however, transfer reactions and backbiting reactions were not observed. Thermodynamic data support the ring-opening mechanism and the thermal stability (lack of ring-closing depolymerization and/or backbiting reactions).