Ethylene-cyclo-olefin copolymers

Ethylene-cyclo-olefin copolymers have been known since 1954 (DuPont USP2 721 189) but these materials only became of importance in the late 1990s with the development of copolymers of ethylene and 2-norbomene by Hoechst and Mitsui using metallocene technology developed by Hoechst. The product is marketed as Topas by Ticona. By adjustment of the monomer ratios polymers with a wide range of Tg values may be obtained including materials that are of potential interest as thermoplastic elastomers. This section considers only thermoplastic materials, cyclo-olefins of interest as elastomers are considered further in Section 11.10. 

The Ticona materials are prepared by continuous polymerisation in solution using metallocene catalysts and a co-catalyst. The ethylene is dissolved in a solvent which may be the comonomer 2-norbomene itself or another hydrocarbon solvent. The comonomer ratio in the reactor is kept constant by continuous feeding of both monomers. After polymerisation the catalyst is deactivated and separated to give polymers of a low residual ash content and the filtration is followed by several degassing steps with monomers and solvents being recycled. 

Thermoplastics grades have a norbomene content in the range 60-80% with Tg values from 60-180°C, in this range the glass transition being almost linearly related to the norbomene content. The modulus of elasticity increases with norbomene content and for commercial materials is in the range 2600-3200 MPa but density (1.02g/cm), tensile strength 66 MPa and water absorption ( 0.01 %) is little affected by the monomer ratio. 

Polymerisation of conjugated dienes can frequently lead to the formation of linear polymers containing main chain double bonds. Examples of such diene 

Several other elastic materials may be made by copolymerising one of the above monomers with lesser amounts of one or more monomers. Notable amongst these are SBR, a copolymer of butadiene and styrene, and nitrile rubber (NBR), a copolymer of butadiene and acrylonitrile. The natural rubber molecule is structurally a c/i -1,4-polyisoprene so that it is convenient to consider natural rubber in this chapter. Some idea of the relative importance of these materials may be gauged from the data in Table 11.14. 

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Metallocene catalysts have a unique feature of polymerizing cyclo-olefin monomers (i.e., cyclopentene, norbornene) selectively without ring opening, and also enable copolymerization with ethylene [47, 48]. Application of metallocene catalysts to cyclo-olefin copolymer (COC) will be discussed in Section IV. 

Amongst copolymers of PE, there are polyallomers (ethylene-propylene copolymers), copolymers with cyclo-olefins and with vinylic monomers [with vinyl acetate ethylene-vinyl acetate, (EVA), with methacrylic acid, MA, and with vinyl alcohol ethylene - vinyl alcohol, (EVOH)], and chlorinated PE (CPE). CPE, although it exhibits exceptional UV and chemical resistance, gives rise to a high amount of hydrogen chloride gas evolution if combusted. 

While the range of the new metallocene-based polymers includes such specialty polymers as cyclo-olelin copolymers (COC), syndiotactic polystyrene, ethylene/ styrene copolymers, which are stiU in the developmental stage, commercially, the most prominent candidates are the elhylene/a-olefin copolymers such as ethylene/ butylene or hexene copolymers (Exxon s Exact ) or ethylene/l-octene copolymers (Dow s Engage and Affinity ). Depending on the comonomer content, these copolymers have been classified as plastomers or elastomers. At comonomer levels of >25 %, the copolymers exhibit the characteristics of thermoplastic elastomers such as high softness, toughness, flexibility, and resilience and hence been referred to as polyolefin elastomers (POE). CompositionaUy, these POEs usually contain 65 % ethylene and 35 % octene-1, hexene-1, or butene-1 as comonomers. 

Figure 3.1 (a) Distribution of molecular weights for ethylene-propylene copolymers from isospecific metallocenes (b) gel permeation chromatography elution curves for HDPE produced with CpTiPhj and Cp ZrCl mixtures. Cp = cyclo-olefins [23]... 

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