Study On Structure And Catalytic Mechanism Of Neodymium-based Ternary Catalyst For Synthetic Rubber

In this thesis,ternary catalyst consisted of neodymium carboxylate,aluminum alkyl and chlorine source was studied.A sealed and moistureproof liquid sample cell with adjustable thickness were designed for Nd L_III-edge XAFS measurements.It was found that Nd?vers?₃ molecules formed an oligomer structure in hexane solution with two Nd-O subshells around Nd center.Adding aluminum alkyl to the hexane solution of Nd?vers?₃ partially destroyed the aggregation degree of Nd?vers?₃molecules in hexane solution.Alkyl groups were bonded to Nd center by Nd-C bonding.The Nd-O bonds can be further replaced by high Al:Nd ratio,or by aging process.Adding Al?ⁱBu?₂Cl to the mixture solution intensively restrained the agglomeration of Nd?vers?₃ molecules in hexane solution and the Nd-O bonds can be further replaced by Nd-Cl bonding.All these structural details and their change tendency demonstrate that the decrease of aggregation degree of Nd?vers?₃ molecules in hexane solution can improve the catalytic activity of the ternary neodymium-based catalyst system.The polymerization performance of the ternary catalyst with different aluminum alkyls including AlEt₃,Al?ⁱBu?₃,Al?ⁿOct?₃ and Al?ⁱBu?₂H were also summarized and compared.The alkylaluminum components consist of n-alkyl groups can form four or more Nd-C bonds.The Nd-C bond lengths are positively related to the carbon-chain length of the alkylaluminum,but negatively related to the percent conversion of polymerization.The number average molecular weight and weight average of the synthesized polyisoprene have no obvious correlation with the Nd-C coordination structure,but are dependent on the alkyl carbon-chains in the alkylaluminum cocatalyst.Dialkyl-aluminum hydrides have the smallest molecular weight and the largest molecular weight distribution.Results of computational chemistry suggest that with adding AlEt₃ into the Nd?vers?₃ hexane solution,the coordination structure changes from pure Nd-O bonds to the mixture of Nd-O and Nd-C bonds,which supports the experimental observations.Based on the polymerization process of isoprene catalyzed by Nd-based catalyst,a speculative catalytic mechanism is proposed in this study.