The cationic ring-opening polymerization of trimethylene urethane (TU; systematic name, tetrahydro-2H-1,3-oxazin-2-one, (I)) in the melt at 100 degrees C with methyl trifluoromethanesulfonate (TfOMe), trifluoromethanesulfonic acid (TfOH), and BF3 . OEt(2) as initiators yields poly(trimethylene urethane) (poly(TU), (2)) with a uniform microstructure. The reaction mechanism is examined for TfOMe and TfOH as initiators. According to NMR spectroscopic results, with both initiators the polymerization proceeds via an active chain end mechanism with protonated cyclic endo iminocarbonates (5) as active species. The propagation step involves a nucleophilic attack of the active species at the monomer followed by a double isomerization process and regeneration of the active species. The polymerization kinetics was investigated yielding a polymerization rate constant of 4.2 x 10(-4) L . mol(-1). s(-1) for the TfOMe-initiated process. Comparison of the kinetic data of the TfOMe- and the TfOH-initiated polymerizations reveals a qualitative and quantitative difference. On the basis of ion trapping experiments this difference is interpreted in terms of deviating initiation mechanisms. It is shown that the polymerization of trimethylene urethane is accompanied by termination and transfer reactions. A transfer mechanism implying termination in a subsequent step is proposed.