Polymerisation with Homogeneous

In the case of olefin polymerisation in the presence of homogeneous metallocene-based catalysts, the individual polymerisation stages have not been very thoroughly investigated. However, kinetic studies have helped, among others, to define the nature of the active centres and to establish the occurrence of some polymerisation elementary steps in a quantitative way. 

Polymerisation in homogeneous systems has been found to occur very rapidly the time of chain growth is approximately 10—3—10 2 s. With a degree of 

Occasional regioerrors appear significantly to inhibit the polymerisation of a-olefins by methylaluminoxane-activated metallocene catalysts [114, 138, 253— 261], In order to reduce the number of secondary Zr-CH(R)-CH2 species, and therefore to accelerate the polymerisation, advantage has been taken of the chain transfer reaction with hydrogen  

For example, the rates of propylene and 1-butene polymerisation by the rac.-(IndCH2)2ZrCl2—[Al(Me)0]x catalyst increase in the presence of hydrogen roughly by a factor of 10-60 respectively [260]. These polymers were found to be free of misinserted units hydrogenolysis apparently eliminates the slowly inserted 2,1-enchained monomer units and allows the start of a new, fastgrowing polymer chain [scheme (44)] [30], 

Chain transfer reactions in homogeneous olefin polymerisation systems with metallocene catalysts, which terminate individual polymer chains, in some instances can also terminate the polymerisation kinetic chain. For example, chain transfer with the monomer in propylene oligomerisation or polymerisation, which involves a bond metathesis reactions between the Mt-C species of the growing polymer chain and the C H species of methyl [scheme (45)] or vinyl [scheme (46)] groups in the monomer, gives rise to temporally inactive metal allyl or metal-vinyl species respectively [177, 241, 264]  

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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],... 

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],... 

At the end of considerations of olefin polymerisation with homogeneous metallocene catalysts, it should be added that it is necessary to differentiate between the soluble catalyst system itself and the polymerisation system. For... 

Polymerisation with Homogeneous Metallocene Single-site Catalysts... 

Polymerisation with Homogeneous Nickel-complex Catalysts... 

Syndiotactic l,2-poly(4-methyl-l,3-pentadiene) has been formed by polymerisation with homogeneous catalysts, e.g. TiBz4—[Al(Me)0]x and CpTiCl3—[Al (Me)0]x [41,43]. The coordination of the monomer as an s-trans-t/2 ligand rather than an s-cis-r A ligand at the Ti atom has been postulated to be involved in the polymerisation. The s-cis-r A monomer coordination is less favoured for steric reasons in the case of 4-methyl-1,3-pentadiene. A possible scheme for the formation of the 1,2-syndiotactic polymer from this monomer is presented in Figure 5.7 [41,43],... 

Cyclopentadienyl ligands have become extremely important in catalysis for metal such as Ti, Zr, and Hf (Chapter 10) and in academic studies of related elements such as Ta. Ethylene polymerisation with the use of CpiTiCE (alkylated with aluminium alkyl compounds) has been known for many decades, but the intensive interest in derivatives of these compounds started in the early 1980 s following the discovery of MAO (methaluminoxane - see chapter 10) which boosted metallocene catalyst activities by several orders of magnitude. Commercial interest focussed on ethylene copolymers (LLDPE where more homogeneous comonomer incorporation resulted in greatly improved copolymer properties) and in enantiospecific polymerisations for propene, styrene, etc. 

Such examples have shown that the role of the cationic group 4 metal complexes in the coordination polymerisation of ethylene and oc-olefins with homogeneous single-site Ziegler-Natta catalysts must not be limited to those containing cyclopentadienyl-like ligands. 

Many supported highly active catalysts show behaviour similar to case B in Figure 3.13 the polymerisation rate may also start at a maximum value and then decrease more or less rapidly with time. Such kinetic behaviour is also characteristic of some homogeneous catalysts. Other polymerisation systems show no acceleration period but have a polymerisation rate that remains almost constant with time this is a rare case and relates, for instance, to 4-methyl-l-pentene polymerisation with MgCl2-supported catalysts containing phthalate esters as well as to ethylene polymerisation with the Cp2TiCl2—[Al(Mc)0]x catalyst (apart from a short settling period in the latter case) [240],... 

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],... 

Possible pathways for monomer insertion in olefin polymerisation systems with homogeneous single-site metallocene-based catalysts and heterogeneous Zieg-ler-Natta catalysts are shown in Figure 3.17 [122],... 

As regards the polymerisation of higher linear oc-olefins, e.g. 1-butene, with homogeneous vanadium-based syndiospecific catalysts, it rarely occurs and affords only trace amounts of a low molecular weight syndiotactic polymer [394]. 

At the end of considerations of the stereoregulation mechanism in the isospecific polymerisation of a-olefins with homogeneous catalysts obtained from chiral stereorigid metallocenes of class III with C2 molecular symmetry, let us address isospecific propylene polymerisation catalysts, which are methy-laluminoxane-activated non-bridged metallocenes (Table 3.2). 

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]. 

Miller, S. A. and Waymouth, R. M., Stereo- and Enantioselective Polymerisation of Olefins with Homogeneous Ziegler-Natta catalysts , in Ziegler Catalysts, Springer-Verlag, Berlin, 1995, pp. 441-454. 

Although the mechanism of steric control that operates in the isospecific styrene polymerisation system with homogeneous nickel-based [(MeAll)(Cod)Ni]+[PF6] /P(Chx)3 catalyst is not completely clear, it is evident that re-benzyl nickel species play an important role in determining the mode of the styrene insertion. 

Syndiotactic polystyrene was first obtained only recently by Ishihara et al. [5] in polymerisation with a homogeneous catalyst derived from a transition metal compound such as monocyclopentadienyltitanium trichloride and methylalu-minoxane in toluene. Since then, several authors have reported on the synthesis of syndiotactic polystyrene promoted by different catalysts based on metal hydrocarbyls such as benzyl compounds, half-sandwich metallocenes (e.g. monocyclopentadienyl, monopentamethylcyclopentadienyl and monoindenyl metal derivatives), metal alkoxides, metallocenes and some other compounds. These catalysts are commonly derived from titanium or zirconium compounds, either activated with methylaluminoxane or aluminium-free, such as those activated with tris(pentafluorophenyl)boron, and promote the syndiospecific polymerisation of styrene and substituted styrenes [5-10,21,48-70], Representative examples of the syndiospecific polymerisation of styrene using catalysts based on various titanium compounds and methylaluminoxane are shown in Table 4.2 [6,52,53,56,58],... 

It is now realised that almost all catalysts based on Ti(III) or Ti(IV) compounds and methylaluminoxane, soluble in aromatic solvents, could polymerise styrene into a highly syndiotactic polymer. The syndiotacticity measured by 13C NMR spectroscopy can be greater than 98%. Syndiospecific polymerisation of styrene with homogeneous catalysts is characterised by a narrow molecular weight distribution (Mw/Mn can reach a value of 2). 

Styrene enchainment in the syndiospecific polymerisation of styrene with homogeneous catalysts, both containing and not containing a cyclopentadienyl or cyclopentadienyl-like ligand, is through a cis insertion [76,77], This was evidenced by H NMR analysis of copolymers of perdeuterostyrene and... 

In view of these results, it was suggested that syndiospecific and isospecific polymerisations with heterogeneous catalysts containing a chlorine atom in the titanium compound or in the support are promoted by homogeneous species formed in the polymerisation system and by heterogeneous species respectively. 

When supported on AI2O3 as a carrier, half-sandwich metallocenes such as CpTiCl3 and Cp TiCl3 also gave rise to suitable stereospecific catalysts that could even be activated by Al(z -Bu)3 [69]. However, in contrast to the respective homogeneous catalysts (yielding syndiotactic polystyrene), polymerisation with these heterogeneous catalysts afforded isotactic and syndiotactic polystyrenes. 

A particularly high reactivity is exhibited by 4-methyl-1,3-pentadiene in polymerisation with the homogeneous catalysts discussed this monomer is 50-100 times more reactive than butadiene [38,39,43,117],... 

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