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Homogeneous Ziegler–Natta Catalysts

المؤلف:  A. Ravve

المصدر:  Principles of Polymer Chemistry

الجزء والصفحة:  207-208

2026-01-21

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Homogeneous Ziegler–Natta Catalysts

The catalysts that fit into this category are quite diverse chemically. They can be, for instance, reaction products of vanadium tetrachloride with dialkyl aluminum chloride or reaction products of bis-cyclopentadienyl titanium dichloride with aluminum alkyls. Organic titanates also form soluble catalysts upon reactions with certain aluminum alkyls. The homogeneous catalysts can range in efficiency from very high to very low. Usually, a much narrower molecular weight distribution is obtained in polymers formed with these catalysts than with the heterogeneous ones. In addition, syndiotactic poly(α-olefins) form at low temperatures (-78C).

The amount of stereospecific placement, however, is usually not as great as is the isotactic placement with heterogeneous catalysts. Group I–III metal alkyl components are essential to the formation of active centers [239, 240]. Blue complexes of di cyclopentadienyl titanium dichloride with aluminum diethyl chloride or with aluminum ethyl dichloride were some of the early known soluble catalysts [257, 258]:

Many other soluble catalysts were developed over the last 30 or more years. Most of these materials must be prepared and used at low temperatures [259]. The polymerization mechanism is also believed to take place by an initial coordination of the monomers with the transition metals. This is followed by a four-center insertion reaction of the monomer into the transition metal–carbon bond.