In situ X-ray absorption fine structure study on the polymerization of isoprene assisted by Nd-based ternary catalysts

Abstract

Nd-based ternary catalysts are widely used for the polymerization of isoprene in rubber industry, however structure information during the polymerization of isoprene as well as the catalytic mechanism are still ambiguous. In this paper, the in situ XAFS technique was used to extract the local atomic structure around the Nd center of the catalyst consisting of Nd(vers)₃, AlEt₃, and Al(ⁱBu)₂Cl. Theoretical calculations were used to verify the structure changes in the alkylation process. Four catalysts with different alkylaluminums {AlEt₃, Al(ⁿOct)₃, Al(ⁱBu)₃, and Al(ⁱBu)₂H} were, respectively, used for the polymerization of isoprene. The results demonstrate that the substitution of Nd–C for Nd–O bonds is reasonable in the alkylation step. The structural evolutions in the alkylation and chlorination steps, as well as in the polymerization processes of isoprene are all similar for these Nd-based ternary catalysts with different alkylaluminum components. In the polymerization process of isoprene, there are only about 2 Nd–Cl bonds at 2.85 Å and 1–2 Nd–C bonds at 2.42 Å around the Nd centers which are in a monomer state. The Nd–C bonds can be attributed to the contribution of isoprene. This highly-opened structure of the Nd active centers activates greatly the catalytic activity of the Nd-based ternary catalysts. A conjectural catalytic mechanism for the polymerization of isoprene has been deciphered on the basis of the in situ XAFS results. This study is expected to be helpful for the improvement and application of the Nd-based ternary catalysts.