Ziegler third component

An important extension of Ziegler-Natta polymerization is the copolymerization of styrene, butadiene and a third component such as dicyclopentadiene or 1, 4-hexadiene to give synthetic rubbers. Vanadyl halides rather than titanium halides are then used as the metal catalyst. 

A non-Ziegler-type catalyst for synthesizing cis-l,4-polybuta-diene was reported by Scott et al. (1964). It contained three components, cobalt (II) chloride, aluminum chloride, and thiophene. The first two components were used as a soluble complex, presumably Co(AlCl4)2, in an aromatic solvent. The third component was added just before contacting the catalyst system with monomer. 

A great range of Ziegler-Natta catalysts has been developed, and all consist of at least two components. Usually, a trialkylaluminum compound is involved, but alkyl aluminum halides, alkyl sodiums, and dialkyl zincs have also been used. The second component is typified by TiCh or TiCls, although other transition metal compounds have been used. Other additives (a third component) have included NaF, amines, and HMPA. These catalysts are not especially well-defined or well characterized chemical entities. They are empirically obtained mixtures that produce desirable results. We return to this issue below. 

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

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