Abstract
The mechanism of stereospecific polymerization of propylene (catalysed by a modified Ziegler-Natta system, TiCl 4/MgCl 2/C 6H 5COOC 2H 5/Al(C 2H 5) 3), has been analysed using the values of triad and pentad tacticity determined using 125 MHz 13C n.m.r. spectroscopy. The well-known single active-site models, such as the Bernoullian, first- and second-Markovian, and enantiomorphic models, were found to be inadequate to describe the observed tacticity. A two-sites model, in which at one site the stereospecific polymerization proceeds in obedience to the Bernoullian model and at the other proceeds under the control of enantiomorphic-site model, is proposed. It was found that the pentad tacticities of both soluble and insoluble (in boiling heptane) fractions of polypropylene agreed well with the two-sites model.
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