Development of novel highly active ROMP catalysts based on Group 5 transition metals

制备了具有芳基配体的五价钽、铌配合物（2,6- r_2c_6h_3o）MCl_4 (M=Ta, Nb； R=H, Me, ^iPr, Ph)，并在烷基铝助催化剂R'_3Al （R'=Me, Et, ^iBu）存在下系统研究了它们对降冰片烯（NBE）开环复分解聚合（ROMP）的催化行为。发现钽配合物与^iBu_3Al结合，对NBE的ROMP表现出非常高的活性，得到高分子量聚合物（M_n bbb10 ^5）和相对较窄的分子量分布（M_w/M_n ~ 2），这表明它们是单位点催化剂。随着配体体积的增大，钽催化剂体系的活性降低。与之形成鲜明对比的是，芳氧铌配合物在Me_3Al的活化下表现出非常高的活性，从而得到高分子量的聚合物。随着配体体积的增加，铌催化剂体系的活性增加。我们还研究了五价钽和铌配合物的合成，这些配合物具有儿茶酚酸、三酚酸、红木酸、双酚酸和双酚酸配体，以及它们在三烷基铝作为助催化剂的情况下对NBE的ROMP的催化作用。在这些配合物中，钽配合物在被^iBu_3Al活化后对NBE的ROMP也表现出很高的活性。相比之下，me3al比^ ibu3al更能有效地活化铌的双酚和双酚配合物。这些配合物得到的聚合物具有较高的分子量（M_n bbb10 ^5）和相对较窄的分子量分布（M_w/M_n ~ 2），表明它们是单位点催化剂。这些发现将有助于未来ROMP催化剂的发展。

Pentavalent tantalum and niobium complexes with aryloxy ligands, (2,6-R_2C_6H_3O)MCl_4 (M=Ta, Nb; R=H, Me, ^iPr, Ph), were prepared, and their catalytic behaviors for the ring-opening metathesis polymerization (ROMP) of norbornene (NBE) were systematically studied in the presence of alkylaluminum cocatalysts, R'_3Al (R'=Me, Et, ^iBu). Tantalum complexes were found to show very high activity for the ROMP of NBE in combination with ^iBu_3Al to give high-molecular-weight polymers (M_n>10^5) and relatively narrow molecular weight distributions (M_w/M_n〜2), implying that these are single-site catalysts. With increasing the bulkiness of the ligand, the tantalum catalyst systems became less active. In sharp contrast, the aryloxyniobium complexes were revealed to exhibit very high activity upon activation with Me_3Al to give high-molecular weight polymers. With increasing the bulkiness of the ligand, the activity of the niobium catalyst systems increased. We also studied synthesis of pentavalent tantalum and niobium complexes having catecholato, tropolonato, hinokitiolato, biphenolato, and binaphtolato ligands, and their catalyses for the ROMP of NBE in the presence of trialkylaluminum as a cocatalyst. Among those complexes, the tantalum complexes also showed high activity upon activation with ^iBu_3Al for ROMP of NBE. In contrast, the biphenolato and binaphtholato complexes of niobium were effectively activated with Me_3Al rather than with ^iBu_3Al. The polymers obtained with those complexes had high molecular weight (M_n>10^5) and relatively narrow molecular weight distributions (M_w/M_n〜2) implying that these are single-site catalysts. These findings will contribute to future development of ROMP catalysts.