硅胶负载型二茂铬催化剂乙烯聚合机理研究

Mechanistic Study of Ethylene Polymerization over Silica-supported Chromocene Catalyst

硅胶负载二茂铬是一种无需烷基铝助催化剂的传统工业用乙烯聚合催化剂,但是目前其引发乙烯聚合机理尚不明确。本文通过密度泛函理论方法,基于文献提出的金属环状机理、铬氧环机理以及本文首次提出的铬茂环机理对该催化体系的聚合机理进行了系统研究。结果表明,金属环状机理链引发过程反应能垒较低,约为24.9kcal·mol^(-1),与实验结果一致,但是其后续链增长过程却因金属环较大的空间位阻效应而使得能垒达到31kcal·mol^(-1),难以实现较快链增长;铬氧环机理的引发和插入能垒均较高,且Cr与硅胶载体的键合作用也会因乙烯的插入而被破坏,因此难以发生;铬茂环机理因滑移解配位过程需克服的能量较高,导致链引发能垒高达37.3kcal·mol^(-1),与S-9催化剂较快的引发过程矛盾,但其链增长过程需克服的能垒仅为20kcal·mol^(-1)左右,与其较快的链增长速率一致。本论文采用分子模拟的方法,针对S-9催化剂的金属环状机理、铬氧环机理和铬茂环机理等三种可能的引发乙烯聚合机理进行了研究,获得了较为深入的机理认识。

Chromocene supported by silica is a traditional industrial ethylene polymerization catalyst that does not require aluminum alkyl cocatalyst,however,its initiating mechanism for ethylene polymerization remains unclear up to now.The feasibility of metallacycle and oxachromacycle mechanisms suggested in literatures,as well as the chroma-Cp-cycle mechanism which was firstly proposed in this work were systematically investigated by DFT method.It was found that chain initiation could be achieved with a relatively low barrier(ca.24.9kcal/mol)in the metallacycle mechanism,which was consistent with the experiment results.However,because of the large steric effect of the metallacycle,the subsequent propagation would be very slow due to the high insertion barrier(ca.31kcal/mol).For the oxachromacycle mechanism,both the initiation and insertion barriers were very high,and the bonding between Cr and silica support would be impaired by C_(2)H_(4) insertion,which made this mechanism nearly impossible.With respect to the chroma-Cp-cycle mechanism,the energy for cyclopentadienyl slippage to weaken its coordination with Cr center is very high,making the initiation barrier as high as 37.3kcal/mol,which contradicted with the fast initiation nature of S-9 catalyst.Nevertheless,the low barrier(ca.20kcal/mol)for insertion ensured a fast propagation,which was in agreement with experimental observations.In this work,molecular simulation methods are used to study three possible initiation mechanisms including metallacycle mechanism,oxachromacycle mechanism and chroma-Cp-cycle mechanism for S-9 catalyst,which gave a relatively deep understanding on this catalyst under a molecular level.