Polymerisation with Soluble Vanadium-based

3 Polymerisation with Soluble Vanadium-based Ziegier-Natta Catalysts 

Let us recall also that vanadium-based soluble Ziegier-Natta catalysts have found widespread industrial application for the manufacture of elastomeric ethylene/propylene copolymers and ethylene/propylene/diene terpolymers [319-322]. The most commonly used vanadium-based catalysts for random ethylene/propylene copolymerisation are those prepared from VCI4, VOCI3, V(Acac)3, VO(OEt)Cl2, VO(OEt)2Cl or VO(OEt)3 as precursors and AlEt3, AlEt2Cl or Al(z-Bu)2Cl as activators, with an Al/V molar ratio not exceeding 3 1 [37, 72], 

According to most models proposed for polymerisation with vanadium-based homogeneous Ziegier-Natta catalysts such as VCI4—A1R2C1, the active site may involve pentacoordinated three-valent vanadium species with three Cl atoms, the secondary carbon atom of the last monomer unit inserted (predominantly by 2,1 enchainment of the a-olefin) and the coordinating olefin [323-327]. However, a model of the active site that assumes a hexacoordinated V(III) species with four Cl atoms, the carbon atom of the last chain unit and the coordinating olefin has also been proposed [328], 

in this connection, that it is incorrect to ascrise the formation of small amounts of syndiotactic polypropylene fraction in the presence of heterogeneous Ziegier-Natta catalysts to the operation of a mechanism assuming a lack of back skip of the chain in the last stage of insertion in the presence of these 

4 Polymerisation with Homogeneous Metallocene Single-site Catalysts 

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Syndiotactic polymers of higher a-olefins such as 1-butene and 4-methyl-1-pentene are produced by homogeneous metallocene-based catalysts [117, 429, 430], In contrast to polymerisation with metallocene-based catalysts, higher a-olefins are much less reactive in polymerisation with soluble vanadium-based catalysts, and already in the case of 1-butene polymerisation only yield trace amounts of low molecular weight syndiotactic polymer [394]. 

The Mw/Mn ratio is usually equal to 5-10 for polyethylene [49,64,66,67, 123,244-247], A much lower polydispersity is displayed by polymers obtained in polymerisation with homogeneous metallocene catalysts the Mw/Mn ratio usually does not significantly exceed a value of 2 [22,95,101,112,138,140], By polymerising propylene with soluble vanadium-based Ziegler-Natta catalysts at low temperature, a very narrow molecular weight distribution of the polypropylene has been found (the Mw/Mn ratio usually reaches values of 1.15-1.25) and a linear increase in its Mn with time has been observed, indicating a noticeable living character of the polymerisation [75,76,241],... 

The syndiospecific polymerisation of propylene with soluble vanadium-based Ziegler Natta catalysts is not completely regiospecific [389 392], i.e. the monomer unit enchainment is not entirely head-to-tail. In addition to syndiotactic stereoblocks, the polymer also contains sterically irregular stereoblocks. The whole polymerisation can be thus described as a copolymerisation with four head-to-tail and tail-to-tail stages [2,379]. 

Although low-temperature syndiospecific polymerisation of propylene with soluble Ziegler-Natta catalysts, based on soluble vanadium compounds and dialkylaluminium chlorides as activators, was first carried out successfully as... 

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