1-hexene copolymerisation

Fig. 2 Comonomer response of siloxy-substituted metallocene catalyst in copolymerisation of ethene with 1-hexene. The catalysts are (1) rac- Et(Ind)2ZrCl2 (2) rac- Et(2-ferf-BuSiMe2OInd)2 ZrCl2 (3) rac- Et(3-ferf-BuSiMe2OInd)2 ZrCl2 (4) meso- Et(2-tert-BuSiMe2OInd)2 ZrCl2 (5) meso- Et(3-terf-BuSiMe2OInd)2 ZrCl2...

Figure 2 presents examples of some siloxy-substituted metallocene catalysts (2-5) and, for comparison, an ordinary bis(indenyl) catalyst (1). The relative reactivity in ethene/1-hexene was investigated [44, 59, 60, 61, 62] in order to predict their copolymerisation ability compared with the conventional bis(indenyl) catalyst (1). 

The comonomer response (compare rac-catalysts 2 and 3 with the corresponding meso-catalysts 4 and 5 in Fig. 2) can be improved by using meso-isomers of C2 symmetric metallocenes [60]. In copolymerisation of ethene with 1-hexene the reactivity ratios of siloxy-substituted meso-isomers, catalysts 4 and 5, are comparable with those of Me2Si(2-Me-Benz(e)Ind)2ZrCl2 [52], which is considered to be a highly efficient copolymerisation catalyst. 

Coordination catalysts allowed for the first time the copolymerisation of ethylene with other olefins such as 1-butene, 1-hexene or 1-octene, which, by introducing side branches, reduces the crystallinity and allows a linear low-density polyethylene to be produced at comparatively low pressures [136], Figure 2.3 shows schematic structures for the three polyethylenes, with the main features exaggerated for emphasis [46]. 

Extensive efforts have also been made to develop olefin polymerisation catalysts based on metallocenes with only one ligand of the cyclopentadienyl type. Ethylene-,dimethylsilylene- or tetramethyldisilylene-bridged mono(l-tetra -methylcyclopentadienyl), mono(l-indenyl) or mono(9-fluorenyl)-amidotita-nium complexes, such as dimethylsilylene(l-tetramethylcyclopentadienyl)(t-butyl)amidotitanium dichloride [Me2Si(Me4Cp)N(/-Bu)TiCl2] (Figure 3.10), have recently attracted both industrial and scientific interest as precursors for methylaluminoxane-activated catalysts, which polymerise ethylene and copolymerise ethylene with 1-butene, 1-hexene and 1-octene [30,105,148-152]. 

Also, the copolymerisation of ethylene and a-olefins can be readily performed using Phillips catalysts. The copolymerisation of ethylene with 1-butene or 1-hexene is the basis of the large expanding linear low-density polyethylene market [28,37,43,237]. 

Random ethylene copolymers with small amounts (4-10 wt-%) of 7-olefins, e.g. 1-butene, 1-hexene, 1-octene and 4-methyl- 1-pentene, are referred to as linear low-density polyethylene, which is a commercially relevant class of polyolefins. Such copolymers are prepared by essentially the same catalysts used for the synthesis of high-density polyethylene [241]. Small amounts of a-olefin units incorporated in an ethylene copolymer have the effect of producing side chains at points where the 7-olefin is inserted into the linear polyethylene backbone. Thus, the copolymerisation produces short alkyl branches, which disrupt the crystallinity of high-density polyethylene and lower the density of the polymer so that it simulates many of the properties of low-density polyethylene manufactured by high-pressure radical polymerisation of ethylene [448] (Figure 2.3). 

Ethylene has also been trimerized selectively to 1-hexene, used as a monomer for copolymerisation with ethylene (Chapter 7). 

Ethylene may be copolymerised with vinyl acetate to make ethyl-vinyl acetate, offering high seal integrity and clarity for frozen food applications where a high degree of toughness is required. Ethylene copolymers with other olefins such as propylene, 1-hexene and 1-octene allow a range of properties to be achieved. Linear low density polyethylene (LLDPE) has a... 

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