Theoretic Studies On Cationic Polymerization Of Epoxide Catalyzed By BF₃

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 dissertation,the cationic ring open-polymerization mechanism of ethylene oxide(EO),oxetane(HO),and tetrahydrofuran(THF)catalyzed by BF₃ are studied by using quantum chemical method.Firstly,quantum chemical method is employed to investigate the mechanism of cationic ring-opening polymerization of EO,oxetane,and THF catalyzed by BF₃-H₂O. By using density functional theory B3LYP/6-311G++(d,p)method,the geometric configurations of each items are optimized.And parameters about the frequency,bond orders,activation energy of each reaction are determined.From the conclusion of quantum calculation,the reaction catalyzed mainly by BF₃-H₂O complex.And the mechanism how solvent decrease the activation energy was found.The mechanism of ring-opening cationic polymerization reaction of Oxetane catalyzed by BF₃ in dichloro methane,without water or promoter,is studied.The effect of dichloro methane in this reaction is analyzed.The activation energy is 56.84 kJ·mol^-1,which is close to 58 kJ·mol^-1reported.For the activation energy of polymerization of THF catalyzed by BF₃-H₂O is too big,the cationic polymerization reaction of THF with epichlorohydrin(ECH)as promoter is studied.For the ring-opening occuring at 3-C and 2-C of ECH,the activation energies are 24.78 kJ·mol^-1and 32.26 kJ·mol^-1.The promoter decrease the activation energy of chain initiation,but it has no effect on chain propagation.This conclusion agrees with literature.In the study of by-product dioxane in polymerization reaction of EO,it is found that the effect of solvent is very small.And so do the catalyst BF₃-H₂O complex and catalyst H⁺.The calculation result explains why the by-product formed when the molecular weight of polymers becomes unchanged. The reactivity ratios for EO and oxetane in their copolymerization are determined. For the reactivity ratio r_E is too big,and r_H is too small,the product of copolymerization could only be copolymer of two blocks(PEO-PHO).The formation of by-product fifteen-ring tripolymer in the polymerization reaction of THF is studied.The activation energy of reaction catalyzed by BF₃-H₂O complex was less than that of H⁺.That is due to the presence of BF₃-H₂O that draws the two atoms nearer to form the bond more easily.