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

合成了五价钽、铌芳氧基配合物（2，6-R_2C_6H_3O）MCl_4（M=Ta，Nb; R=H，Me，^iPr，Ph），并系统研究了它们在烷基铝助催化剂R '_3Al（R'= Me，Et，^iBu）存在下对异戊二烯（NBE）开环易位聚合（ROMP）的催化性能。结果表明，钽配合物与^iBu_3Al配合时，对NBE的开环聚合反应具有很高的催化活性，可得到高分子量聚合物（Mn>10^5）和相对较窄的分子量分布（Mw/Mn> 2），表明它们是单活性中心催化剂。随着配体体积的增加，钽催化剂体系的活性降低。与此形成鲜明对比的是，芳氧基铌配合物显示出非常高的活性时，用Me_3Al活化，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。我们还研究了以邻苯二酚、托洛酮、扁柏酚、双酚和联萘酚为配体的五价钽和铌配合物的合成，以及它们在三烷基铝助催化剂存在下对NBE的ROMP反应的催化作用。在这些配合物中，钽配合物也表现出较高的活性时，与^iBu_3Al的活化NBE的ROMP。相比之下，铌的双酚和联萘酚络合物被Me_3Al而不是^iBu_3Al有效活化。用这些配合物得到的聚合物具有高的分子量（Mn>10^5）和相对窄的分子量分布（Mw/Mn> 2），这意味着它们是单活性中心催化剂。这些发现将有助于ROMP催化剂的未来发展。

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.