New ultra high cis polybutadiene technology through a novel neodymium catalyst and its vulcanizate properties

摘要

Ultra high cis polybutadiene is prepared in 1,3-butadiene polymerization by the Nd-catalyst of Nd(neodecanoate)_3(ND)/ AlEt_2Cl/Al(iBu)_3 in the rubber industry. However, the catalytic activity remains low, and the reason is unclear. According to our matrix-assisted-laser-desorption-ionization time-of-flight (MALDI-TOF) mass spectroscopic study, ND is a mixture of various oligomeric and hy-drated compounds. The hydrated oligomeric structure would mainly lower the catalytic activity. In order to design a better Nd-catalyst, Nd(neodecanoate)_3 (neodecanoic acid) (NDH) was designed to satisfy the requisite of anhydrous and monomeric structure. NDH was prepared through ligand-exhange method between neodymium acetate and neodecanoic acid in an organic medium. NDH was identified and characterized with infrared (IR), MALDI-TOF mass, and synchrotron X-ray absorption spectroscopies (XAS), and with computer simulations. Weakly bound neodecanoic acid with the carbonyl peak at ca. 1 670 cm~(-1), and carboxylate anion coordinated to Nd at ca. 1 600 cm~(-1) were observed in the IR study. The molecular ion peak (m/z. =838.7) in MALDI mass and the pronounced absorption edge at 6216 eV in the Nd-Lm XANES (X-ray absorption near edge structure) spectra, respectively, support its monomeric structure. NDH is of monomeric structure satisfying the 8-coordination number in the optimized structure by molecular mechanics. A significant improvement in the catalytic activity for 1,3-butadiene polymerization is achieved through NDH. The Nd-catalyst shows a high activity (2.5 x 10~6 g/Nd mol·h) and produces polybutadiene with 98% cis content. The living active-center of the Nd-catalyst is characterized by XANES (X-ray absorption near edge structure) and UV-visible spectroscopies, and fully optimized by density functional method (B3LYP/CEP-31G), in which neodymium(III) is coordinated with carboxylate, allylic active end, penultimate double bond, and chloride. The bond distances are Nd-C_1 2.586 A, Nd-C_4 2.741 A, Nd-C_5 2.738 A, Nd-Cl 2.651 A, and Nd-O_1 2.167 A, respectively. The "livingness" in the polymerization is also studied using frontier orbital analysis. The HOMO electron density of the Nd catalyst is concentrated on the mostly p orbital of the C_α and C_γ atoms, and the electron density of LUMO conversely is concentrated on the 4f and 5d orbitals of Nd atom. In other words, the C_α atom of the chain-end nucleophilically attacks the C_1 atom (LUMO) of 1,3-butadiene resulting in C—C bond formation while simultaniously Nd electrophilically attacks the C_4 atom (HOMO) of 1,3-butadiene resulting in Nd-C bond formation. The positive atomic charge (+1.09) on Nd and the negative atomic charge (- 0.56) on C_α support the strong ionic character in good agreement of the spectroscopic results. The frontier orbitals of the Ni catalyst also studied, which are different from those of the Li and Nd based catalysts. The HOMO of the Ni catalyst has mostly the d-orbital (Ni) character while the LUMO has the p-orbital (C_α and C_γ atoms) as well as the d-orbital (Ni) character. The π-butenyl coordination at the Ni catalyst is η~1-type but those of the Nd and Li catalysts are η~3. The atomic charge (+0.003) on the Ni atom and the atomic charge (-0.14) on C_α of the Ni catalyst are much lower than those of the Nd catalyst (Nd +1.09, C_α - 0.56).