A matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectroscopy analysis of polythiourethanes obtained by the cationic ring-opening polymerization of a six-membered cyclic thiourethane [3-benzyltetrahydro-1,3-oxazine2-thione (BTOT)] is described. A MALDI-TOF mass spectrum of a polymer obtained by the polymerization of BTOT with boron trifluoride etherate (BF3OEt2) as the initiator in nitrobenzene at 50 degrees C for 24 h followed by an end-capping reaction with diethyldithiocarbarnic acid diethylammonium salt showed a series of well-resolved signals that were assignable to polythiourethanes possessing an amino group at the initiating end and a diethyldithiocarbamate group at the terminating end. In comparison with the MALDI-TOF mass spectra of polymers obtained by polymerization with trifluoromethanesulfonic acid or methyl trifluoromethanesulfonate, the plausible initiating species in the polymerization with BF3OEt2 was estimated to be a proton, which successively eliminated carbonyl sulfide to produce a secondary amine group at the initiating end. The secondary amine group in the obtained telechelic polymer was converted to a tertiary amine group by a reaction with benzyl bromide in the presence of triethylamine, and this was confirmed by MALDI-TOF mass spectroscopy. Furthermore, a telechelic polymer with a pyrrole end group was successfully synthesized by the end-capping reaction of the growing species in the polymerization of BTOT with sodium 1-pyrrolecarbodithioate. (c) 2006 Wiley Periodicals, Inc.