半茂钛催化乙烯与降冰片烯共聚的理论研究

运用密度泛函方法对含酚-膦配体的半茂钛化合物催化乙烯与降冰片烯共聚合反应的详细机理进行了理论研究。计算结果表明，虽然由于配体的不同，此系列钛化合物具有两种典型结构，但其在助催化剂作用下形成的催化活性种均为相似的P-Ti成键的阳离子物种。在烯烃聚合反应中，烯烃单体的配位插入反应易于从阳离子活性种中氧原子的对位发生。由乙烯及降冰片烯聚合反应各步骤的比较可知，乙烯单体插入Ti-Me结构的初始插入步骤较插入Ti-Et结构困难得多，因而链引发步骤为乙烯均聚的决速步骤。而降冰片烯单体插入Ti-Me结构较之乙烯单体容易得多，但由于降冰片烯单体位阻较大，其连续插入十分困难。在共聚反应过程中，NBE单体的引入可以使得Et插入反应容易越过较难的插入Ti-Me结构步骤，这是NBE与Et共聚反应的反应活性远大于催化Et均聚反应的最主要原因。

Theoretical Study on the mechanism of Ethylene/Norbornene Copolymerization catalyzed by Half-titanocene Complexes

Mechanism of copolymerization of ethylene and norbornene (NBE) catalyzed by half-titanocene complexes bearing o-di(phenyl)phosphanylphenolate ligands have been studied via density functional theory (DFT). Geometry optimization for catalysts and active species indicated that although this series of half-titanocene complexes have two kinds of structures which are four-coordinated and five-coordinated, they all have similar cationic active species structures in the present of cocatalyst. In polymerization, the olefin coordination insertion reaction will initiate from the trans position to the O atom of active species. Energy profiles and reaction-rate calculations results indicated that the insertion of ethylene into the Ti-Me bond process is obviously difficult than the insertion of ethylene into the Ti-Et bond process, lead to the conclusion that the chain initialization process is the rate-determining step of the ethylene homopolymerization. On the contrary, the chain initialization process of norbornene homopolymerization is much more easy, but the insertion of norbornene into the Ti-NBE bond process is very difficult due to the larger steric bulk of norbornene. For the Et/NBE copolymerization, the chain initialization process is relatively easy by the presence of NBE monomer, then the ethylene and fewer norbornene can insert into the polymer chain continuously. That is the main reason for why these half-titanocene complexes displayed excellent ability to ethylene/NBE copolymerization in experimental investigations, while they only exhibited low activities for ethylene homopolymerization.