Combined Experimental and Theoretical Approach for Living and Isoselective Propylene Polymerization

The
controlled polymerization of propylene with living catalysts
in an isoselective fashion with high activities is a continuing challenge
for catalysis, with at least one of these properties suffering in
most existing systems. Through experimental and theoretical studies,
the pyridylamidohafnium olefin polymerization catalysts have been
optimized for living behavior, isoselectivity, and activity of propylene
polymerization (Đ = 1.2, [m4] = 91%, TOF = 25 000 h–1). Substitution of the bridgehead position with geminal dimethyl
substituents forces the stereoselective elements of the catalyst into
closer proximity with the active site through a “buttressing
effect” while simultaneously preserving symmetry in the catalyst
after activation and the first monomer insertion into the Hf–Caryl bond of the ligand. Propagation was shown to be favored
over termination through beta-hydrogen transfer to the monomer through
a combination of theoretical modeling and experimental monitoring
of the reaction which showed a linear increase in molecular weight
as a function of polymer yield. Furthermore, through computational
and 13C labeling NMR studies, a different mechanism of
stereocontrol involving a direct ligand–monomer interaction
was confirmed.

采用活性催化剂以等规选择性模式、高活性实现丙烯的可控聚合，始终是催化领域长期存在的一项挑战；现有绝大多数体系中，至少一项核心性能存在不足。通过实验与理论研究，研究人员对吡啶酰胺基铪（pyridylamidohafnium）烯烃聚合催化剂进行了优化，使其在丙烯聚合中具备优异的活性行为、等规选择性与催化活性，其性能参数为：多分散性指数Đ = 1.2、等规四单元组含量[m4] = 91%、转换频率（TOF）= 25 000 h⁻¹。在催化剂的桥头位置引入偕二甲基取代基，可通过“支撑效应（buttressing effect）”使催化剂的立体选择性元件与活性中心更为贴近，同时在催化剂活化以及单体首次插入配体的铪-芳基键后，维持催化剂的对称性。通过理论建模与反应过程的实验监测，研究证实链增长路径相较于β-氢转移至单体的终止路径更具优势；反应过程中，聚合物分子量随收率呈线性增长，验证了这一结论。此外，通过计算化学研究与碳13标记核磁共振（13C labeling NMR）谱学分析，研究证实了一种全新的立体控制机制——配体与单体直接相互作用。