Copolymerization of linear and cyclic olefins

Copolymers of ethylene with a-olefins, such as the short-chain branched LLDPE (linear low-density polyethylene) impact materials or the EPD (ethylene-propylene-diene copolymer) rubbers represent major percentages of the total polyolefin production, due to their desirable mechanical properties. Solid-state MgCl2-supported Ziegler-Natta catalysts however, have unfavourable reactivity 

Metallocene-based catalysts, on the other hand, have rather comparable reactivity ratios for ethylene and even for the higher ot-olefins. Accordingly, high comonomer incorporation (uniform in each polymer chain and independent of its molar-mass) can be achieved with these single-site catalysts. This is of great advantage with regard to polymer processability and application. 

Single-site metallocene catalysts are also highly reactive vis-a-vis cycloolefins such as cyclobutene, cyclopentene or norbornene. While homopolymers of these cycloolefins have melting temperatures ( 380°C), much too high for technical processability, ethylene-cycloolefin copolymers (COC s) - e.g. ethylene-norbornene copolymers - are amorphous materials with glass transition temperatures, above which they become soft and processable [W. Kaminsky, J. Polym. Sci. A, Polym. Chem., 2004, 42, 3911]. 

Ethylene-norbornene copolymers, which have thermoplastic properties when heated above their glass transition temperatures of ca. 200-250°C, have been commercialized by Ticona GmbH under the trade name TOPAS (Tliermoplas-tic Olefin Polymer of Amorphous Structure). Their properties - exceptional transparency, low double refraction, high stiffnes and hardness, low permeability for moisture and excellent biocompatibility - make these ethylene-norbornene copolymers particularly valuable as engineering polymers, for optical applications and as materials for food and medical packaging. 

Discussion Point DP7i While ethylene and norbornene give essentially alternating, amorphous copolymers, attempts to copolymerize ethylene and cyclohexene give only crystalline polyethylene. Which factors might contribute to these observations Unsaturated norbornenyl chain ends cannot arise by ji-H transfer 

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