This study investigates the reaction mechanism of the zwitterionic ring expansion polymerization (ZREP) of monosubstituted ethylene oxide using boron-based Lewis acids as catalysts to produce cyclic polyethers. The research analyzes various parameters, such as the nature of the substituents (electron-withdrawing, EWG or electron-donating, EDG), the type of catalyst, and the solvent, to understand their influence on the reaction's kinetics, thermodynamics, and regioselectivity. Key findings include that EDGs lower the kinetic barrier of the rate-determining step and favor the reaction at the substituted carbon, while EWGs increase this barrier, making the reaction less favorable. The study also shows that all catalysts tested efficiently activate the epoxide monomer with similar barriers for the rate-determining step, and the regioselectivity trends align with the substituent's electronic effects. Steric effects of substituents were found to dominate the competition between monomer addition and cyclization termination, with most catalysts promoting cyclization. Additionally, the solvent had minimal impact on the polymerization mechanism. This computational study provides valuable insights into the synthesis of cyclic polymers and advances the development of catalysts for ZREP of monosubstituted ethylene oxide.