Branching Branched metallocene polyethylene, values

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. 

The zero-shear viscosity t]o of linear polymers scales exponentially with molecular weight [102] above the critical chain length Me, but LCB polymers repeatedly deviate from this dependency. In comparison to linear polymers of similar M, polymers with low levels of LCB exhibit enhanced zero-shear viscosity values and, in a qualitative sense, C-NMR-based LCB content often [85, 92, 93], but not always [100], correlates well with the viscosity increase. For long-chain branched LDPE, the t]q in comparison to linear polyethylene of similar is lower [103, 104]. A zero-shear viscosity t]o value higher than that of the corresponding linear polymers of similar M , is reported to occur at an LCB content of 0.2 LCB/10,000 C but the increase becomes more pronounced as the LCB content grows [85, 91, 92, 105,106]. This feature of low amounts of LCB has also been utilized to explore the extent of metallocene LCB [13, 85, 106, 107]. 

Figure9.21 Storage modulus versus frequency data [57] for a series of metallocene (constrained geometry catalyst) polyethylenes with progressively increasing levels of long-chain branching, as given in Table 9.1, compared to the predictions of the hierarchical model at 160 C.The parameter values used in the hierarchical model are M = 1167, G 5 = 2.0 10 Pa, Tg = 3.5 -10 s with a = 4/3.These values were obtained from fits of model predictions to rheological data for model hydrogenated 1,4-poly butadiene combs, except that was adjusted to account for a difference in temperature. From Park and Larson [59].

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