phenyl]thiophenium hydroxide inner salt was synthesized and then polymerized in bulk and solution over the temperature range 65-140 °C. Bulk polymerization gave linear polymer in 88.3-90.6% yield. The polymer was identified as poly-[oxytetramethylenethio[3-(2-hydroxyethoxy)-l,4-phenylene]] by elemental analysis, IR, and NMR. Sizeexclusion chromatography showed the number-average molecular weight to be in the range 51 300-67 800, depending on the reaction temperature. These are the highest polymer molecular weights ever reported for a zwitterion polymerization. Cyclic oligomers from dimer to hexamer were identified by mass spectroscopy and NMR as minor products. The polymer yields were no higher than 19.4% in solution polymerization unless the TFA salt of the zwitterion was present as an initiator. The yields increased to 62% in the presence of TFA salt. The polymer molecular weights varied from 4420 to 9730, depending on concentration, temperature, and the presence of TFA salt. The predominant polymer end groups were identified by NMR as vinyl and phenol for bulk polymerization and alcohol and phenol for solution polymerization. Our previously proposed mechanism for this type of zwitterion polymerization is discussed in terms of the present study.