Diene elastomers polymer crystallinity

At room temperature, PE is a semi-crystalline plastomer (a plastic which on stretching shows elongation like an elastomer), but on heating crystallites melt and the polymer passes through an elastomeric phase. Similarly, by hindering the crystallisation of PE (that is, by incorporating new chain elements), amorphous curable rubbery materials like ethylene propylene copolymer (EPM), ethylene propylene diene terpolymer (EPDM), ethylene-vinyl acetate copolymer (EVA), chlorinated polyethylene (CM), and chlorosulphonated polyethylene (CSM) can be prepared. 

They are based on various metals. Such as zirconium, complexed with cyclopentadienide anions. This type of compound is called a zirconocene and is used with organoalu-minum to make highly regular polymers. The catalyst has the ability to flip back and forth from making atactic to isotactic polypropylene in the same polymerization. The alternating tacticity of the polymer breaks up the crystallinity of the chains and yields an elastomer. Metallocene catalysts are currently very expensive and cannot yet polymerize dienes such as butadiene, so they have only enjoyed limited commercial success in elastomers. However, this is one of the most intense fields of polymer research and many new product breakthroughs are expected in the near future. 

The occurrence of these reactions is restricted to the amorphous phase. Therefore, the photo-cross-linking process has to be performed at temperatures exceeding the crystalline melting point in the case of highly crystalline polymers such as polyethylene. The cross-linking efficiency can be strongly enhanced by the addition of small amounts of multifunctional compounds such as triallyl cyanurate, TAG (see Chart 7.9), or by the incorporation of special diene moieties into copolymers such as ethylene propylene diene copolymers (EPDM elastomers) [33]. 

Chakraborty S, Sahoo N G, Jana G K and Das C K (2004) Self-reinforcing elastomer composites based on ethylene-propylene-diene monomer rubber and liquid-crystalline polymer, J Appl Polym Sa 93 711-718. 

Coordination catalysts are also used to make elastomers, notably ethylene-propylene-diene (EPDM) rubbers [6]. In this case, homogeneous catalysts are preferred to heterogeneous ones because they generally produce polymers with a more uniform distribution of crystallinity. The unreacted double bonds of the dienes are used during rubber crosslinking reactions. Figure 8.4 shows some typical examples of dienes used for the manufacture of EPDM rubbers. 

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