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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.
The detailed mechanism of the copolymerization of ethylene with norbornene catalyzed by a half-titanocene compound containing phenolic-phosphine ligand has been investigated by using density functional theory. The calculated results show that although the series of titanium compounds have Two kinds of typical structures, but the catalytically active species formed under the action of the cocatalyst are all similar P-Ti bonding cationic species. In the olefin polymerization, the coordination insertion reaction of the olefin monomer easily proceeds from the cation active species Oxygen atoms of the para position from the ethylene and norbornene polymerization steps of the comparison shows that ethylene monomer inserted into the Ti-Me structure of the initial insertion step is much more difficult to insert Ti-Et structure, so the chain initiation step are ethylene However, the insertion of norbornene monomer into Ti-Me structure is much easier than that of ethylene monomer, but it is very difficult to insert continuously because of the steric hindrance of norbornene monomer.In the process of copolymerization, The introduction of NBE monomer can make the Et insertion reaction easier to cross the more difficult insertion Ti-Me structure step, which is the most important reason that the reactivity of NBE and Et copolymerization is far greater than that of catalytic Et homopolymerization.