Polymerisation catalysts, single-site

Alkoxides and imido are used as anionic ligands in zirconium and titanium catalysts for the polymerisation of alkenes, sometimes as the only anions, but often in combination with cyclopentadienyl ligands. Imides linked to cyclopentadienyl groups form part of the single-site catalyst developed by Dow (Chapter 10) (Figure 1.9, 1). In very different titanium catalysts, namely those used for epoxidation of alkenes, also alkoxide ligands are used (Chapter 14). 

The breakthrough in metallocene catalyst development occurred in the early 1980s when a metallocene catalyst, instead of an aluminium alkyl, was combined with methylaluminoxane (MAO) [8, 9, 10]. This catalyst system boosted the activity of metallocene-based catalyst and produced uniform polyethene with the narrow molar mass distribution typical for single-site catalysts. Efforts to polymerise propene failed, however the product was found to be fully atactic, indicating complete lack of stereospecificity of the catalyst [10]. 

Grubbs group [31, 32] developed another type of Ni-based catalyst. This neutral Ni-catalyst, based on salicylaldimine ligands, is active in ethene polymerisation without any co-activator and originated from the Shell higher olefin process (SHOP). Shortly thereafter another active neutral P,0-chelated nickel catalysts for polymerisation of ethene in emulsion was developed by Soula et al. [33, 34, 35]. The historical development of single site catalysts is represented in Fig. 1. 

In all the low pressure PE processes the polymer is formed through coordination polymerisation. Three basic catalyst types are used chromium oxide, Ziegler-Natta and single-site catalysts. The catalyst type together with the process defines the basic structure and properties of the polyethylene produced. Apart from the MWD and comonomer distribution that a certain catalyst produces in polymerisation in one reactor, two or more cascaded reactors with different polymerisation conditions increase the freedom to tailor... 

It should be mentioned that many of the requirements necessary for the economic production of polyethylene and polypropylene have been achieved. However, catalysts of greater activity and of greater selectivity in the production of polymers and copolymers can be anticipated. This is of prime concern to alkene polymerisation processes in the presence of single-site metallocene catalysts. Such catalysts, undoubtedly of great scientific and commercial importance, have been developed on a large scale within recent years [29,30],... 

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. 

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],... 

Polymerisation with Homogeneous Metallocene Single-site Catalysts... 

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],... 

The described chain migratory insertion mechanism, which operates in olefin polymerisation with metallocene-based single-site catalysts, follows that proposed by Cossee [268,277,278] for olefin polymerisation with heterogeneous catalysts there is, however, no back skip of the polymer chain to the previously occupied position prior to the coordination of the next monomer molecule, but rotation of the chain around the axis of the Mt-CH2 bond takes place (Figure 3.19) [358],... 

Figure 3.20 Formation of the olefin polymerisation single-site catalyst in the reaction of zirconocene, Cp2ZrMe2, with cage i-butylaluminoxane, [Al(r-Bu)0]6...

In the late 1980s and in the 1990s, homogeneous metallocene-based Ziegler Natta and related aluminium-free catalysts as well as other non-metallocene-based homogeneous single-site catalysts, which are active in the syndiospecific polymerisation of styrene, were found also to promote the polymerisation of conjugated dienes [16,38 13],... 

Bonnet, R Hillier, A. C. Collins, A. Dubberly, S. R. Mountford, R Lanthanide mono(borohydride) complexes of diamide-diamine donor ligands Novel single site catalysts for the polymerisation of methyl methacrylate. Dalton Trans. 2005,421 23. 

LLDPE grades based on metallocene (mC) or single-site catalysts (SSC) can show a certain effect of LCB. This is because mC catalysts are able to incorporate vinyl-terminated chains as comonomers. This is considered as a possible way to improve the processing behaviour of these narrow MWD materials. The catalyst system used as well as the polymerisation conditions play an important role in the amount of LCB reached in the final product. Figure 5.12 compares two different mC - LLDPE with and without LCB. 

Weiser, M.S. and Mulhaupt, R. (2006) Living olefin polymerisation and block copolymer synthesis in the presence of a single-site catalyst containing a phenoxyimine Ugand. Macromolecular Symposia, 236,111-116. 

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