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Çë¸ßÊÖ½«ÏÂÃæÁ½¶Î»°·Òë³ÉººÓï¡£·Ç³£Ð»Ð»£¡ The effect of productivity decreasing could be explained by the formation of larger organoaluminate counteranions. They would constitute abarrier to ethylene access to the nickel center. Further studies with variations in the quantities of the nickel complex and alkylaluminum cocatalysts will be made in the future. The polymerization of ethylene by the system 1/alkylaluminum produces branched polymers(see the next section, Polymer Characterization). The behavior of this system has been interpreted by a mechanism comprising ¡°migration¡± and¡°propagation¡± as introduced by Fink et al.18 or ¡°chain walking¡± according to Brookhart et al.19 Figure 1 shows the main features of such a process. The propagation is a sequence of olefin insertion, and the branching formation is originated through an isomerization process. Thissequence of isomerization steps (b-hydride elimination/insertion) without chain transfer, the so-called chain walking, followed by ethylene insertion distinguishes nickel catalysts from the majority of other olefin polymerization catalysts described nowadays. These results have been explained well by theoretical simulations20 and experimentalwork. |
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