Stereospecific polymerizations titanium trichloride systems

PP, a semi-crystalline thermoplastic, is made in its homopol5mier form by pol5mierising propylene monomer using stereospecific Ziegler-Natta catalysts. The catalyst system is termed stereospecific because it controls the position of the side (methyl) group in each propylene unit in the polymeric chain. A t3 ical catalyst system may be prepared by combining titanium trichloride with tributyl aluminium or its variants. Most commercial PP is isotactic. 

The accessible product range is also broadened thanks to the exceptional intrinsic stereospecificity and molecular veight control of the catalytic system. High quality and consistency are favoured by easy and smooth control of the polymerization conditions due to the catalyst properties. In addition, future progress can be foreseen as a result of specialized development studies of this type of titanium trichloride catalyst currently under way. 

For instance, in the field of elastomers, alkyllithium catalyst systems are used commercially for producing butadiene homopolymers and copolymers and, to a somewhat lesser extent, polyisoprene. Another class of important, industrial polymerization systems consists of those catalyzed by alkylaluminum compounds and various compounds of transition metals used as cocatalysts. The symposium papers reported several variations of these polymerization systems in which cocatalysts are titanium halides for isoprene or propylene and cobalt salts for butadiene. The stereospecificity and mechanism of polymerization with these monomers were compared using the above cocatalysts as well as vanadium trichloride. Also included is the application of Ziegler-Natta catalysts to the rather novel polymerization of 1,3-pentadiene to polymeric cis-1,4 stereoisomers which have potential interest as elastomers. 

Many ot-olefins were polymerized by the Ziegler-Natta catalysts to yield high polymers and many such polymers were found to be stereospecific and crystalline. Polymerizations of a-olefins of the general structure of CH2 = CH — (CH2) — R, where x is 0-3 and R denotes CH3, CH-(CH3)2, C(CH3)3, or CsHs, can be catalyzed by vanadium trichloride/triethyl aluminum [80]. The conversions are fairly high, though higher crystallinity can be obtained with titanium-based catalysts [81]. Addition of Lewis bases, such as ( 4119)20, (C4H9)3N, or ( 4119)3 , to the catalyst system further increases crystallinity [82]. 

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