论文部分内容阅读
钒系烯烃聚合催化剂在工业上有着不可替代的位置,它可用于制备高活性窄分布的聚合物、乙烯与α-烯烃共聚物和间规聚丙烯等。但由于实验手段难以确定钒催化剂活性物种的结构,进一步对催化机理的确认及催化剂结构的改进十分困难。本文运用密度泛函方法对水杨醛亚胺钒配合物催化乙烯聚合的活性物种结构进行了理论研究。对多种活性物种模型的比较研究结果表明,对此催化反应最有利的活性物种为中性双金属物种a1, a1结构中包含两个连接铝原子与钒中心的氯桥结构。研究同时表明,助催化剂AlEt2Cl的存在不仅加速了钒配合物前体的烷基化反应,同时其对活性物种a1结构中氯桥的形成至关重要。最后还研究了该催化体系的链终止反应机理。“,”The use of vanadium‐based catalysts allows the preparation of high molecular mass polymers with uniform molecular mass distributions, polypropylene and ethylene/α‐olefin copolymers with highα‐olefin incorporation. However, the design of ligand systems with vanadium catalysts would face difficulties, because it is difficult to experimentally determine the structures of the active species of vanadium catalysts. In this paper, possible structural candidates for the active species in ethylene polymerization catalyzed by the salicylaldiminato vanadium complex combined with AlEt2Cl were investigated using density functional theory. By comparing theoretical simulation results with pre‐vious experimental investigations, especially regarding the crucial role of the diethyaluminum chlo‐ride (AlEt2Cl) cocatalyst, it was concluded that a neutral bimetallic species containing two Al–Cl–V bridging bonds is the most favorable structure model for the active vanadium species. A notable effect of Al co‐catalysts was clarified in the theoretical investigation. During the formation of the active species, AlEt2Cl act as an assistant for the alkylation and alkyl abstract processes of precur‐sors. More importantly, AlEt2Cl is necessary for the formation of the bis(chlorine‐bridged) structure in the active species, which showed a notable effect on the structural stability of the active species and its catalytic activity. Additionally, we investigated the chain termination mechanism in this system.