Quantum Chemical Study On Cationic Ring-opening Polymerization Of Epoxide

With the rapid development of science and technology, there has been a growing interest in the polyethers. Study on the ring-opening polymerization mechanism of epoxide would have instructional bearing for producing high quality polyethers. In this disseration, the cationic ring open-polymerization mechanism of THF, butylene oxide and ethylene oxide were studied on the basis of electronic microstructures by using quantum chemicl method.Firstly, quantum chemical method was employed to investigate the mechanism of cationic ring-opening polymerization of tetrahydrofuran (THF) and butylene oxide.By using density functional theory B3LYP/6-311G++(d,p)method, the geometric configuration was optimized for tetrahydrofuran,butylene oxide, secondary oxonium ion and tertiary oxonium ion. And parameters about the electronic structure, bondlength, energy of frontier orbital of these species were determined. The mechanism of ring-opening cationic polymerization reaction of tetrahydrofuran and butylene oxide was analyzed on the basis of electronic microstructures by using frontier orbitaltheory. The transition states of the reaction between THF, butylene oxide and secondary oxonium ion were acquired. By using frequency analysis and inner reactioncoordinate analysis the rationality of these transition states were affirmed. Thepotential energy surface of these reactions were plotted, and the energy barrier of the reaction between THF and secondary oxonium ion was determined to be 182.0.116 kJ/mol. As to butylene oxide and secondary oxonium ion the barrier was determined to be 92.672kJ/mol.The reaction between oxonium ion and water in the polymerization of THF was investigated too. It have been observed that oxonium ion reacted with water more easily than with THF. So water could be used as ceasing reagent to control the molecular weight of the polymer.In this thesis, the mechanism of endogenetic ring forming reaction in the cationic polymerization of THF, butylene oxide and ethylene oxide was investigated. It have been observed that in the cationic polymerization of ethylene oxide, the endogenetic ring forming reaction reacted more easily than in the cationic polymerization of THF. In the cationic polymerization of ethylene oxide the mechanism of the endogenetic ring forming reaction was S_n1, while in the cationic polymerization of THF and butylene oxide, the mechanism was S_n2.The reactivity ratios for THF and butylene oxide in their copolymerization were determined by using transition theory. Respectively the reactivity ratio r_THF-and r_BO were determined to be 0.35 and 1.94. From the conclusion of quantum calculation, butylene oxide was more active than THF.Calculation results could explain the experimental facts satisfactorily.