Soluble Vanadium-based Catalysts

Homogeneous Ziegler-Natta catalysts of the classic type, capable of olefin polymerisation, are usually derived from reactions of a hydrocarbon soluble transition metal compound with an alkylmetal compound in a hydrocarbon solution (heptane, hexane, toluene). However, the polymer formed in the presence of such soluble Ziegler-Natta catalysts is very often precipitated during polymerisation. 

The only important industrial applications of such soluble catalysts are of those prepared from VC14, VOCl3, V(Acac)3, VO(OEt)Cl2, VO(OEt)2Cl, VO(OEt)3 or VO(OBu)3 as precursors and AlEt3, AlEt2Cl or AI(/-Buj2CI as activators, in heptane solution, by which ethylene/propylene copolymers and ethylene/propylene/non-conjugated diene terpolymers are produced [72]. The AW molar ratio in these catalysts does not usually exceed a value of 3 1. 

Nevertheless, many vanadium-based catalysts and polymerisation systems comprising them have received much academic attention in the hope that they might provide models for heterogeneous catalysts and polymerisation systems, since the problems connected with surface properties and particle size were believed to have been overcome. It must be noted, however, that homogeneous vanadium-based catalysts appeared to be more complex than was thought. There is no decisive evidence on the structure of catalytic sites formed by reaction between the procatalyst and activator. 

It may be interesting, in connection with the ethylene/propylene copolymers mentioned above, to present here some homogeneous Ziegler-Natta catalysts formed by soluble complexes of titanium and magnesium chlorides with alkyl phosphates as catalyst precursors and alkylaluminium compounds as activators (TiCl4)x.(MgCl2)r [0=P(0Bu)3]3-A1(/-Bu)3 and Cl3TiOMgCl-[0 = P(0Bu)3]3- A1(z -Bu)3 (Al/Ti molar ratio of ca 10 1). These catalysts have been used for random ethylene/propylene copolymerisation [73], 

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Table 1, Stereoregularities and molecular weights of polypropylenes prepared with different soluble vanadium-based catalysts at —78 °C... 

The syndiotactic polypropylenes prepared with soluble vanadium-based catalysts usually contain some irregular linkages of propylene units arranged in head-to-head (Eq. 31) and tail-to-tail (Eq. 32) sequences. Doi95) has shown that the syndiotactic triad fraction [rr] of polypropylene decreases with an increase of the amount of the irregular linkages of propylene units [F01 + F10] (see Figure 16). 

Fig. 16. Relation between the syndiotactic triad fraction [rr] and the heterotactic dyad fraction of irregular propylene unit sequences [F01 + F10] in polypropylenes obtained at —78 °C with various soluble vanadium-based catalysts (from Ref. 95 )...

Block copolymers of propylene with ethylene have been produced in commercial polymerization processes using heterogeneous Ziegler-Natta catalysts. In all processes the block copolymers are produced in small concentrations, and the major products are homopolymers. Well-defined block copolymers free of homopolymer impurities can be prepared with catalysts exhibiting a living polymerization character. In this section we deal with the synthesis of well-defined block copolymers using the living polypropylene which has been prepared with soluble vanadium-based catalysts. 

At the same time, syndiotactic polypropylene was also isolated by Natta et al. and characterised [145]. Syndiotactic polypropylene which was obtained by low-temperature polymerisation using soluble vanadium-based catalysts [146] could not be, however, commercialised, although it had a blend of interesting usable properties. 

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]. 

There are also known soluble vanadium-based catalysts. Those obtained from soluble compounds such as V(Acac)3 or VO(OR)3 as precursors being activated... 

Fig. 8b. The decay in the activity of a soluble vanadium-based catalyst in ethylene polymerization. [Vd. di (isopropyl salicylate)-AlEt2Cl.

One of the models proposed to explain the stereospecificity for soluble vanadium-based catalysts postulates that it is the minimization of steric effects in the four-center transition state for monomer insertion (see Eq. (2.83)) which is responsible for the stereospecificity of the polymerization (Zambelli and Tosi, 1974 Corradini et al., 1989). Thus, it is considered that the tra 5 -configuration minimizes steric effects in the transition state and this leads to a syndiotactic configuration of the polymer chain as shown below. In general, the kinetics... 

Doi, Y. Tokuhiro, N. Soga, K. Polymerization of diolefins by a soluble vanadium-based catalyst. Kobunshi Ronbunshu 1989, 46, 215-222 Chem. Abstr. 1989, 111, 78655. 

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