Radical polar cross-over

The sheer size and value of the polyethylene industry ensure that there is continued research, progress, and development in catalysis, for their potential commercial impact. Although this whole subject is not within the scope of this chapter, we mention a couple of aspects of the progress, which offer the potential to impact this industry. In 1995, DuPont introduced work, carried out with them at the University of North Carolina—via the largest patent applicafion ever in the USA. They disclosed what are described as post-metallocene catalysts. These are transition and late transition metal complexes with di-imine ligands, which form part of the DuPont Versipol technology. Such catalysts create highly branched to exceptionally linear ethylene homopolymers and linear alpha-olefins. Late transition metals offer not only the potential for the incorporation of polar comonomers, which until now has only been possible in LDPE reactors, but also their controlled sequence distribution, compared to the random composition of free radical LDPE copolymers. Such copolymers account for over 1 million tons per annum [20]. Versipol has so far only been cross-licensed and used commercially by DuPont Dow Elastomers (a former joint venture, now dissolved) in an EPDM plant. 

Cross termination n. In free radical copolymerization, termination by reaction of two radicals terminated by monomer units of the opposite type, i.e., termination, by combination or disproportionation with rate constant /cab Crosstermination is often favored over termination by reaction between two like radicals due to polar effects. 

Equation (7.52) can also be derived from Eq. (7.43) by eliminating radical concentrations with the help of two steady-state assumptions written as Eqs. (7.44) and (7.47) and then using the definitions of ri and r2- Equation (7.52) represents a one-parameter model for the copolymerization rate (Melville et al., 1947) containing the parameter 0. Statistically, ip is expected to equal unity. However, the values of ip obtained in practice by inserting experimental copolymerization rates into Eq. (7.52) are frequently greater than unity. (For styrene-methyl methacrylate, for example, (p is 15, while for styrene-butyl acrylate ip is 150.) These deviations are ascribed to polar effects that favor cross-termination over homotennination. 

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