新型Ni催化剂催化乙烯聚合的中性机理

用密度泛函理论中的B3LYP LANL2MB方法 ,研究了Ni 水杨亚胺催化剂催化乙烯聚合的中性反应机理并和阳离子活性中心的催化反应机理进行了比较 .计算结果表明 ,整个中性催化机理类似于阳离子催化机理 ,但是也有不同 .两种机理都是从带空位的活性催化剂开始 ,乙烯以垂直于催化剂平面的方式占据空位 ,为了有利于甲基的迁移 ,乙烯向甲基的方向旋转 90° ,形成四元环过渡态 .插入反应发生后 ,Ni和 β C之间形成一种氢桥键 ,协助新空位的形成 ,实现链的增长 .乙烯与中性活性中心的相互作用远远强于乙烯与阳离子活性中心的相互作用 .中性催化机理较阳离子催化机理容易引发 .阳离子催化的过渡态所需的活化能比中性催化所需的活化能低 ,表明阳离子反应机理比中性反应机理容易进行 ,甲基在中性催化过渡态中的迁移明显不同于在酸性液化过渡态中的迁移 .β agostic相互作用在中性催化反应机理中 ,在主导烷基给合物中Ni所带的电荷方面 ,起着关键性的作用 .

THE NEUTRAL MECHANISM OF THE NOVEL NICKEL CATALYST FOR ETHYLENE POLYMERIZATION

Some Ni(Ⅱ) salicyladimine complexes are neutral single component polymerization catalysts which can tolerate many functional groups. They can catalyze polymerization of olefins even olefins with polar groups. The catalytic polymerization mechanism was modeled by the reaction of ethylene with (NH CH CH CH O)NiCH 3R.The neutral reaction mechanism and the cationic reaction mechanism of ethylene polymerization were studied based on DFT method at B3LYP/LANL2MB level.The result shows that the insertion is initiated by the complexation of the incoming ethylene with the electron-deficient metal center to form a π-complex.This complexation assists the insertion of the ethylene into the metal-carbon bond via a four member cyclic transition state.After insertion,a product with γ-agostic interaction is formed.Finally,a product is formed with the β-agostic interaction species resembling the starting material,so a new vacant coordination site is available for the next ethylene to coordinate and propagate the chain.Moreover,the calculation results indicate that the neutral reaction mechanism is similar with the cationic reaction mechanism.Compared with the cationic reaction mechanism,the neutral reaction mechanism is easier to initialize.By comparing,it is shown that the interaction of ethylene with the cationic active center is stronger than that with the neutral active center.The activation energy for the cationic mechanism is lower than that for the neutral mechanism.It implies that the process of the cationic reaction mechanism is easier than that of the neutral mechanism.The β-agostic action plays an important role in the neutral catalytic reaction mechanism,and it dominants the Ni charges in the alkyl complexes.