1-hexene polymerisation

More recently, Landis et al. studied the polymerisation kinetics of 1-hexene with (EBI)ZrMe( t-Me)B(C5F5)3 64 as catalyst in toluene [EBI = rac-C2H4(Ind)2]. Catalyst initiation was defined as the first insertion of monomer into the Zr-Me bond, 65 (Scheme 8.30). Deuterium quenching with MeOD was used to determine the number of catalytically active sites by NMR. The time dependence of the deuterium label in the polymer was taken as a measure of the rate of catalyst initiation. This method also provides information of the type of bonding of the growing polymer chain to zirconium, as n-or sec-alkyl, allyl etc. Hexene polymerisation is comparatively slow, with high regio- and stereoselectivity there was no accumulation of secondary zirconium alkyls as dormant states [96]. 

Since at -40 °C the hexene polymerisation is living, the addition of 10 equivalents of propene to species 66 led to the complete consumption of propene and formation of a Zr-PP-fo-PH block (without further conversion of any unreacted Zr-Me precursor). The identification of the Zr-(propene)j.-(hexene)j, polymer was confirmed by isotopic labelling (l- C-propene, l,T-D-2,3- C-propene) and H,... 

By contrast to the polymerisation of hexene with 64, which can be followed conveniently by variable-temperature NMR, the polymerisation of smaller monomers like ethene and propene illustrate the limitations of spectroscopic methods since with most metallocene catalysts they are too fast. The kinetic behavior of (SBI)ZrMe2/AlBu 3/[CPh3][CN B(C6F5)3 2] at 25 °C was therefore investigated by quenched-flow techniques to estimate the rates of initiation, chain propagation and chain termination [SBI = rac-Me2Si(Ind)2] [97]. The results are summarised here for comparison with the results on 1-hexene polymerisation discussed above. 

Another, highly selective oligomerisation reaction of ethene should be mentioned here, namely the trimerisation of ethene to give 1-hexene. Worldwide it is produced in a 0.5 Mt/y quantity and used as a comonomer for ethene polymerisation. The largest producer is BP with 40 % market share utilizing the Amoco process, formerly the Albemarle (Ethyl Corporation) process. About 25 % is made by Sasol in South Africa where it is distilled from the broad mixture of hydrocarbons obtained via the Fischer-Tropsch process, the conversion of syn-gas to fuel. The third important process has been developed by Phillips. 

F and B NMR spectroscopy. The rate of propene polymerisation with this system was only three times faster than that of 1-hexene. This slow rate contributes to the high regioselectivity of the polymerisation no 2,1-propene misinsertions were detected. H and NMR spectroscopy also provided information about the chain termination mechanism here this occurred by p-H elimination in a first-order process. Polymer chain-end epimerisation, i.e. chirality inversion at the P-carbon of the polymer chain (Scheme 8.31), proceeded via a zirconium tert-alkyl (rather than tt-allyl) intermediate [96c]. 

The properties of mPE can be changed over a wide range by incorporation of co-monomers. The effect of a co-monomer was proved in the co-polymerisation with 1-hexene. By the addition of 60 mol.% 1-hexene to the feed, Mn could be reduced to nearly half the value for homopolyethylene. Also, the density and melting point are reduced steeply by the incorporation of 1-hexene, whereas the polydispersity is not influenced. 

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]. 

If 1-butene or 1-hexene is chosen instead of propylene as the monomer polymerising with the Me2C(MeCp)(Flu)ZrCl2-based catalyst, the polymers obtained become enriched in m diads. This has been suggested to testify to the preference of site isomerisation prior to the coordination of the next monomer molecule with increasing size of the polymerising a-olefin [121]. 

Polymerisation of racemic 4-methyl-1-hexene with the Me2C(Cp)(Flu) ZrCl2— [Al(Me)0]x catalyst was found obviously to produce syndiotactic polymer, which appeared to be a random copolymer of two enantiomers, poly[(7 ,5)-4-methyl-1 -hexene] [436,437],... 

One problem of 2,co-polymerisation is that the molecular weight of the product, poly[2,co-(a-olefin)], is always very low (Mw k6x 103), whereas the molecular weight distribution is small (Mw/Mn ss 1.6). It is possible, however, to increase the molecular weight of the polymer, e.g. the molecular weight of poly[2,6-(l-hexene)], to Mw 90 x 103 by increasing the reaction pressure to 1400 MPa. The reason for this is the possible kinetic pressure effect in the case of 1-hexene in which the insertion but not the isomerisation is the rate determining step for 2,co-polymerisation [183]. 

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). 

The cyclopolymerisation of 1,5-hexadiene leads to polymers of substantially higher molecular weights than the polymerisation of 1-hexene with the same catalysts [498], undoubtedly owing to some steric hindrance of the atom transfer that usually terminates the growth of a polymer chain [30],... 

Preferred olefins in the polymerisation are one or more of ethylene, propylene, 1-butene, 2-butene, 1-hexene, 1-octene, 1-pentene, 1-tetradecene, norbornene and cyclopentene, with ethylene, propylene and cyclopentene. Other monomers that may be used with these catalysts (when it is a Pd(II) complex) to form copolymers with olefins and selected cycloolefins are carbon monoxide (CO) and vinyl ketones of the general formula H2C=CHC(0)R. Carbon monoxide forms alternating copolymers with the various olefins and cycloolefins. 

LLDPE is actually a copolymer of ethylene, with butene, octene or hexene. It is finding increasing use due to economies of polymerisation and film strength. It provides the strongest heat seal of the PEs, has more extensibility and a capability of being downgauged. Other ethylene polymers include the following. 

As will be described in Chapter 6, titanium-based complexes show excellent catalytic activity in polymerisation processes. Oligomerisation of ethylene to afford short-chain linear a-olefins such as 1-butene, 1-hexene and 1-octene (Scheme 5.4) has been investigated as they are used as comonomers for the synthesis of linear low-density polyethylene. In this regard, oligomerisation of ethylene catalysed by titanium complexes has been commercialised for many years. 

Since the mechanism of ethene polymerisation commonly suggested for transition-metal catalysts involves the formation of metallacycle, development of this type of catalyst was performed in order to elucidate the type of intermediate involved in the reaction. Binuclear Cr(ii) metallacycles showed little production of 1-hexene when reacted with ethene at room temperature, even after reacting for over 24 h. However, mononuclear Cr(m) metallacycles were considered possible intermediates as these complexes are able to trimerise ethene. Thus, Monillas et al. concluded this to be the likely intermediate involved in the reaction. Nonetheless, the use of the Cr(i) dinitrogen complex yields the desired product, 1-hexene, in a mechanism... 

Fig. 2.1. Schematic illustration of polyethylene molecular structure of various density ranges (Elias 1992). Top LDPE, radical polymerisation yields a number of (long) side ehains. Bottom HDPE, catalytic polymerisation gives rise to linear ehains with a small number of short branches. Both drawings in the middle illustrate LLDPEs produced by catalytic polymerisation with a-olefines. Small amounts of bntene-1, hexene-1 or octene-1 co-monomers lead to etlyl, butyl or hexyl side chains. Polymerisation in the gaseous phase produces chains arranged in a block-shaped fashion and distributed at various frequencies along the chaia Solution phase polymerisation provides a statistical random distribution along the whole chain...

Indeed, simple monomers, snch as styrene, ethylene, propylene, hexene, vinyl chloride, acrylonitrile and caprolactam, nsnally do occur in the corresponding polymers. In addition to unreacted monomer, any non-polymerisable impnrities in the original monomer feed to the polymerisation conld occur in the final prodnct. Thns, styrene monomers can contain low concentrations of numerous saturated and nnsatnrated hydrocarbons, ethyl benzene being particnlarly prevelent and these, particnlarly the saturated compounds which do not polymerise, will occur in the finished polymer and have implications in the nse of the polymer food packaging. It is not nnknown for compounds as toxic as benzene to occnr at very low concentrations, nsnally less than 10 parts per million in styrene monomer, and this could, therefore, also occur in the polymer. For foodgrades of polystyrene, the monomer content is nsnally nowadays limited to 0.2% maximum. Acrylonitrile monomer may be fonnd in amonnts up to 0.1 % in finished polymer, whilst negligible amounts of monomer are fonnd in polyamide and polymethyl-1-pentene. With thermosets, phenol and formaldelyde are likely to be found even in the most carefully manufactured grades. 

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