Quantum chemical method was employed to investigate the mechanism of polymerization of ethylene oxide. The electronic structure, charge distribution and bond length of ethylene oxide in ground and excited states and the products of the reaction of ethylene oxide with electrophiles and nucleophiles were investigated theoretically at the B3LYP/6-311G^** level. The mechanism of polymerization of ethylene oxide was discussed on the basis of electronic microstructures. By analysis of their frontier orbitals, the reasonability of the cationic and anionic polymerization mechanism of ethylene oxide was confirmed theoretically. Due to symmetrical properties of frontier orbitals and energy difference of bonding orbitals, the radical polymerization of ethylene oxide can not proceed. However the products of the reaction of ethylene oxide with electrophiles and nucleophiles could react with ethylene oxide, resulting in the possibility of polymerization. The cationic ring-opening polymerization involved the nucleophilic attack of monomer on the oxonium ion, in which instead of ring-opening on monomer, ethylene oxide was activated and ready for addition reaction and side reaction. The anionic ring-opening polymerization proceeded by nucleophilic attack of the propagating anion on monomer, in which ring-opening on ethylene oxide was undertaken. Calculation results satisfactorily explained the experimental facts.

Key words: ethylene oxide, B3L YP, frontier orbital, polymerization mechanism