Long-chain branched metallocene polyethylenes

The long-chain branched metallocene polyethylenes made with the constrained geometry catalysts described in Chapter 3 pose their own special problem, because the level of branching, 0.01 to 0.1 LCB per 1,000 carbon atoms, is too low to be detected by means of GPC-MALLS. For these materials the factor j8 in Eq. 2.104 has been established to be 0.5 by use of C-13 NMR [57], and this makes it possible to use GPC-LALLS-DV to determine the distribution of radii of gyration and the number of branches per 1000 carbon atoms. Then using the results... 

These monocyclopentadienyl amidotitanium complexes, which are classified as constrained-geometry catalysts, are capable of producing low-density polyethylene (ethylene copolymers with C4, C(, or Cg 1-alkenes) that also contain long-chain branches, in contrast to strictly linear low-density polyethylene (ethylene copolymers with C4, C(, or Cg 1-alkenes) produced by bent metallocene-based catalysts [30,105,148,149]. 

In addition to the metallocenes described previously, so-called halfsandwich compounds or constrained-geometry catalysts (Fig. 3) such as dimethylsilyl-t-butylamido cyclopentadienyl titanium dichloride are used. These catalysts are excellent for producing polyethylenes with long-chain branching and can incorporate high amounts of comonomers such as 1-octene... 

Fig. 8 Loss angle as a function of frequency at 150°C obtained for high-density, metallocene catalyzed polyethylene samples of increasing long-chain branching. (From Ref.. )...

S.J. Park and R.G. Larson. Modeling the linear viscoelastic properties of metallocene-catalyzed high density polyethylenes with long-chain branching. J. Rheol. 2005, 49, 523-536. 

Blends of HOPE with long-chain branched polyethylenes (HBPE) prepared from metallocene catalysts have been studied by DSC and their crystal stmcmres interpreted in terms of phase behavior. The HBPE contained long-chain branches and short branches formed form octane comonomer. HBPE with 7.5-12.0% octane exhibited phase separation, whereas HBPE with 2% octane were found to be miscible with HOPE over the whole composition range. Eong branches were few and did not contribute to the immiscibility (16). 

Yan, D., W. J. Wang, and S. Zhu. 1999. Effect of long chain branching on rheological properties of metallocene polyethylene. Polymer 40 1737-1744. 

The incorporation of the macromer was investigated and calculated by NMR measurements. The maximum incorporation rate was 0.52 mol%. This means that about 59.7 wt% of the polymer is composed of macromer units and, on average, every 400th carbon atom of the backbone chain is branched. The melting point of the long-chain branched polyethylene decreases from 136 to 12 PC, and the zero shear-rate viscosity increases from 142 to 280 Pa s. Such long-chain branched copolymer can be produced much more easily by metallocene/MAO catalysts than by Ziegler-Natta catalysts. 

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]. 

The ability to incorporate long-chain branches appears reasonably common among metallocene catalysts. Experimental results for LCB formation with both CGC and conventional metallocene-catalyzed polymerizations are in line with an in situ copolymerization mechanism. For copolymerization of vinyl-terminated polyethylene molecules to occur, the first requirement is the presence of termination mechanisms producing vinyl-terminated macromonomers. Secondly, the catalyst must able to incorporate these macromonomers into a growing chain. Macromers probably do not move from one active site to another, but instead insertion of macromer to another chain takes place at same site where it was formed in a intramolecular incorporation manner. 

The level of long-chain branching by metallocene catalysts, however, is low and this complicates the characterization. Metallocene-catalyzed LCB polyethylene thus consists of a mixture of linear and LCB chains, where the LCB structures make up only a small fraction of the total number of chains. 

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