Polymerization of 1,4-bis (2-vinyloxyethoxy)benzene (CH2=CH-O-CH2CH2-O-C6H4-O-CH2CH2-O-CH=CH2; 1) was investigated in CH2Cl2 at 0 degrees C with the use of a variety of cationic initiators. SnCl4, SnBr4, AlEtCl2, and BF3OEt2 (strong Lewis acids) and CF3SO3H (a strong protonic acid) yielded crosslinked insoluble polymers immediately after the polymerizations were initiated. The binary initiating systems such as HCl/ZnCl2 and (C6H5O)(2)P(O)OH/ZnCl2 also produced insoluble poly(1)s. At the low initial concentration of ZnCl2, however, the (C6H5O)(2)P(O)OH/ZnCl2 system gave the soluble polymers quantitatively, and gelation occurred only when the reaction mixture was stored for a long time after complete consumption of the monomer. The content of the unreacted pendant vinyl ether groups of the soluble polymers decreased with monomer conversion, and almost all the pendant vinyl ether groups were consumed in the soluble polymer obtained at 100% monomer conversion; this may be ascribed to frequent occurrence of intramolecular crosslinking.