CATIONIC RING-OPENING POLYMERIZATION OF TRIMETHYLENE URETHANE - A MECHANISTIC STUDY

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), trifluoro methanesulfonic acid (TfOH), and BF3 . OEt(2) as initiators yields pol y(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 poly merization proceeds via an active chain end mechanism with protonated cyclic endo iminocarbonates (5) as active species. The propagation ste p involves a nucleophilic attack of the active species at the monomer followed by a double isomerization process and regeneration of the act ive species. The polymerization kinetics was investigated yielding a p olymerization 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 quan titative difference. On the basis of ion trapping experiments this dif ference is interpreted in terms of deviating initiation mechanisms. It is shown that the polymerization of trimethylene urethane is accompan ied by termination and transfer reactions. A transfer mechanism implyi ng termination in a subsequent step is proposed.