Preparation of Polyethylene by Coordination Mechanism

It is claimed that the branching of polyethylene can be controlled to the extent that the product can even be more branched than conventional low-density polyethylene (1,2—300 branches/1,000 atoms) [18, 19]. The cationic Ni-diimine catalyst shown above (R = H, CH3), with the methylaluminoxane analog, has been found to polymerize ethylene in toluene at room temperature at the rate of 110,000 kg/Ni/h. This is comparable to the metallocene rates. The Pd-based catalysts are less active than their Ni analogs [19]. 

When nickel catalysts are used, the extent of branching is a function of the temperature, ethylene pressure, and catalyst structure. Branching increases as the temperature rises. At higher ethylene 

A patent for the polymerization process of olefins (especially ethylene, a-olefins, cyclopentene, and some fluorlnated olefins) describes the above catalytic systems [20]. The hindered diimines stabilize alkyl Ni(ll) or Pd(n) with cationic complexes. After preparation, the complexes are reduced with methylalu-minoxane and then activated with Lewis acids capable of forming non-coordinating counterions [20]. 

In addition, preparation of catalysts based on iron and cobalt [21] was also reported. These are complexes of bulky pyridine bis-imine ligands with iron or cobalt that are also activated by methylaluminoxane  

The iron-based catalysts are reported to be considerably more active than the cobalt analogs [21]. The yield of linear, narrow molecular weight distribution polyethylene per gram is reported to be very high [21]. 

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