Polymerisation with Heterogeneous Ziegler-Natta

1 Polymerisation with Heterogeneous Ziegler-Natta Catalysts 

A number of models of the active centres in Ziegler Natta catalysts have been postulated. The diversity of these models arises from the multitude of products found to be formed or believed to be formed in the reaction of the catalyst precursor with the activator [e.g. schemes (4) to (8) and (12)]. The proposed active centres fall into either of two general categories those containing monometallic species with the central transition metal atom (e.g. Ti), and those containing bimetallic species with the central transition metal atom linked via bridges with the metal atom originating from the activator (e.g. Al). 

It should be emphasised that strong support has been provided for this mechanism of 7-olefin polymerisation, involving sites located on lateral a-TiCb faces, from molecular orbital calculations [268]. Analogous model sites, but located on layers in relief with respect to lateral faces of layered TiCfi structures [279-282], as well as for catalysts with the MgCT/TiCU precursor [279], have also been considered in molecular mechanics studies. 

Variations in monometallic and bimetallic mechanisms have been proposed. The monometallic mechanisms seem to be inherently simpler than bimetallic mechanisms except for requiring a migration step. On the other hand, bimetallic mechanisms that consider the presence of the activator in the polymerisation system can be more convincing in some instances. 

One has to realise the extreme chemical complexity of catalysts such as MgCl2/internal Lewis base/TiCU AlEt3/external Lewis base. Moreover, the 

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Termination of the olefin polymerisation with heterogeneous Ziegler-Natta catalysts by the addition of carbon monoxide to the system is often used in the laboratory to determine the active centres of the catalyst. 

For the majority of olefin polymerisations with heterogeneous Ziegler Natta catalysts, the polymerisation rates, Rv, are proportional to the concentrations of procatalyst (MtX ) and monomer (M), but do not depend on the concentration of alkylaluminium activator (A) as long as a threshold concentration is maintained [37] ... 

The mechanism that is commonly considered to operate in the polymerisation of ethylene and a-olefins in the presence of group 4 metallocene-based catalysts is that devised by Cossee [268, 276, 277] for propylene polymerisation with heterogeneous Ziegler-Natta catalysts, though modifications invoking effects such as a-agostic hydrogen interactions with the metal centre have been proposed [343,344]. 

Styrene polymerisation with heterogeneous Ziegler Natta catalysts activated by alkylaluminium compounds generally produces a mixture of isotactic and non-stereoregular polymer. For example, polystyrene produced with the... 

Table 4.1 Relative reactivity of vinylaromatic monomers in coordination polymerisation with heterogeneous Ziegler-Natta catalyst ...

Y. V. Kissia, Jsospecifiic Polymerisation ofiOlefiins with Heterogeneous Ziegler-Natta Catalysts, Spriager-Vedag, New York, 1985. 

In the case of olefin polymerisation with homogeneous Ziegler-Natta catalysts, especially with single-site (metallocene) catalysts, the kinetic analysis may become simpler than in the case of polymerisation with heterogeneous catalysts, and in some instances can serve as a very useful tool for uncovering the true polymerisation mechanism [30,243],... 

On the other hand, the polymerisation of trans-1 -(2 H)-propene with heterogeneous Ziegler-Natta catalysts leads to the formation of the respective polymer of threo-diisotactic structure [scheme (49)], which proves the m-insertion of the monomer [26,275] ... 

What are the advantages of half-sandwich metallocene-based catalysts as compared with heterogeneous Ziegler-Natta catalysts in styrene polymerisation What are the possible consequences of this for developing industrial processes ... 

Heterogeneous Ziegler Natta catalysts were found [446,495,496] to polymerise a, ffl-diolefins, yielding polymers with predominantly occurring c/.v-fl-mcth-ylene-3-cyclopentyl) units. The cis-trans diastereoselectivity, which reflects the catalyst structure, can hardly be controlled with heterogeneous catalysts, but it is strongly influenced by substituents at the cyclopentadienyl ligands in... 

Functionalised a-olefins capable of undergoing insertion polymerisation with Ziegler-Natta catalysts are, in principle, monomers in which the heteroatom (X) does not electronically interact with the double bond to be polymerised in such monomers, the heteroatom is separated from the double bond CH2=CH-(CH2)x X [326,384,518,522-528], Monomers with the heteroatom directly bound to the double bond, i.e. those of the CH2=CH-X type, may also undergo polymerization, but when the heteroatom is silicon or tin (X= Si, Sn) [522-526], Representative examples of the insertion polymerisation of functionalised a-olefins and their copolymerisation with ethylene and a-olefins in the presence of heterogeneous Ziegler-Natta catalysts are shown in Table 3.7 [2,241,326,384,518,522-528],... 

Subsequent investigations revealed that, in principle, styrene undergoes isospecific polymerisation in the presence of heterogeneous Ziegler Natta catalysts [1-4], Although polystyrene of isotactic structure was also prepared with the use of homogeneous nickel-based coordination catalysts, it appeared to be of low molecular weight [22,23]. 

In contrast to heterogeneous Ziegler-Natta catalysts, homogeneous catalysts based on biscyclopentadienyl derivatives of group 4 transition metals, which contain cationic metallocene species of formally d° 14-electronic structure, hardly promote the polymerisation of conjugated dienes, since the diene can act as a donor of four electrons rather than of two electrons as in monoolefin polymerisation (let us recall that the polymerisation of conjugated dienes is catalysed by half-sandwich metallocene-based catalysts). However, it has been reported [162] that statistical copolymers of ethylene and butadiene were obtained with the Cp2ZrCl2— [Al(Me)0]x catalyst. 

It is known [14,15] that, on the surface of the heterogeneous Ziegler-Natta catalysts promoting the isotactic polymerisation of propene, active centres are also present which may give rise to the formation of significant amounts of V-rich sequences (syndiotactic or syndiotactoid ). As a matter of fact, syndiotactic polypropene was isolated for the first time as an impurity from samples of isotactic polypropene prepared in the presence of catalyst systems such as, for example, a- or y -TiClj in combination with A1(C2H5)2F or LiC4H9 [16]. 

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