Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis revealed that the precision control (or the living nature) of the cationic polymerization of vinyl ethers with SnCl4 or TiCl4 critically depends on the Lewis acid concentration and temperature. Specifically, at an extremely low Lewis acid concentration, for example, the polymerization with the HCl-vinyl ether adduct (an initiator) is living at -78 degreesC in CH2Cl2 solvent, whereas side reactions occurred at a higher concentration of SnCl4 or at a higher temperature, -15 degreesC. This was more pronounced with SnCl4 than with TiCl4, which was due to a stronger Lewis acidity of SnCl4 as suggested by NMR analysis of the model reactions. (C) 2001 John Wiley & Sons, Inc.