Polyethylene strictly linear

This discovery happened just as we had finished a series of experiments attempting to let our growth reaction proceed up to the addition of 1000 and more molecules of ethylene. A synthesis of such very long aluminum alkyls would have involved a new method of making a strictly linear polyethylene, since the molecular weight of the commercial polyethylene was said to lie in the region of 30,000-50,000. Our attempts had not been successful. We had reached molecular weights of perhaps a few thousand. Obviously also in these experiments a replacement reaction... 

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]. 

ADMET polymerization is performed on a,co-dienes to produce strictly linear polymers with unsaturated polyethylene backbones, as shown in Scheme 2. This step-growth polymerization is a thermally neutral process driven by the release of a small molecule condensate, ethylene [16-20]. Ring-opening metathesis polymerization (ROMP) is widely used to polymerize cyclic olefins and is performed with the same catalysts as in ADMET polymerizations. 

Polyethylene modeling using ADMET step-polymerization began with the production of strictly linear polyethylene by polymerization of 1,9-decadiene followed by exhaustive hydrogenation (Scheme 3) [68,69]. 

Strictly linear polyethylene oligomers are most suitable as catalyst supports. In the case of polyethylenes, a molecular weight of 2,000 is sufficient to get the desired on/off solubility behavior. Ethylene oligomers of this size are easily modified... 

HOPE, LDPE, and LLDPE are the three main types of commercial polyethylenes with a combined global consumption of >80 Mt/year. HDPE is a strictly linear homopolymer while LDPE is a long-branched homopolymer because of the different methods of polymerization. LLDPE, on the other hand, is a linear ethylene copolymer with small amounts of a-olefin comonomers such as butene, hexene, or octene. Traditionally, polyethylenes are classified according to the densities. The density of polyethylene decreases as the branching and/or comonomer content increases. The crystallinity and the properties associated with crystallinity, such as stiffness, strength, and chemical resistance, progressively decrease from HDPE to LDPE/LLDPE to POE grades. 

Polymers may be synthesized as linear, branched, or crosslinked entities, or a mixture containing all three. In many cases, the complete gamut between a strictly linear chain and a densely crosslinked network can be covered systematically. As extreme examples, high-density polyethylene serves as the model for a strictly linear polymer, and the diamond as the most densely crosslinked polymer imaginable. 

Metallocmes (noduce palyediykne that is strictly linear, without side branches, termed LLDPE (linear low-density polyethylene). Other processes tend to produce branches and give a lower quality product. 

Several methods of estimating LCB levels using linear viscoelastic data are described in Section 5.12. A technique based on the shape of the viscosity curve has been proposed for single-site polyethylenes with low levels of LCB. Lai etal [80] found that for strictly linear polyethylenes prepared using single-site catalysts, the Cross equation (Eq. 10.55) gives a reasonably good fit to viscosity data. They further showed that the characteristic time A of the cross equation is proportional to the zero shear viscosity for these materials ... 

Apart from cross-metathesis, ROMP, and RCM, there are other less common metathesis reactions. These are acyclic diene metathesis polymerization (ADMET), ring-opening cross-metathesis (ROCM), ring-rearrangement metathesis (RRM), and ethenolysis. A general ADMET reaction is shown by reaction 7.3.1.7. ADMET reactions are generally performed on a,well-defined and strictly linear polymers with unsaturated polyethylene backbones. Ethenolysis is the cross-metathesis of ethylene with an internal olefin. 

At the end of the 1970s considerable interest developed in what became known as linear low density polyethylenes (LLDPE) which are intermediate in properties and structure to the high pressure and low pressure materials. While strictly speaking these are copolymers it is most convenient to consider them alongside the homopolymers. The LLDPE materials were rapidly accepted by industry particularly in the manufacture of film. The very low density polyethylenes (VLDPE) introduced by Union Carbide in 1985 were closely related. 

However, it is important to note that polymer quantum chemistry is not a ID, solid-state physical science. In strictly ID physics, the systems are periodic in ID and have ID wave functions. In polymer quantum chemistry, the systems and their wave functions are 3D but periodic only in ID. Usual theorems of ID physics are consequently no longer valid [28]. A typical example is that of extrema of the energy bands, which should only occur at the center and the edges of the Brillouin zone in a strictly ID system. For polymers, even in simple cases like the linear zigzag polyethylene chain, some extrema of the energy bands are encountered at arbitrary positions in the first Brillouin zone that are not points of high symmetry (Fig. 36.3). 

The other two examples deal with the flow properties of polymer melts as they are encountered under ordinary processing conditions. Figure 7.2 presents results of measurements of the viscosity of a melt of polyethylene, obtained at steady state for simple shear flows under variation of the shear rate. Data were collected for a series of different temperatures. At low strain rates one finds a constant value for the viscosity coefficient, i.e. a strict proportionality between shear stress and shear rate, but then a decrease sets in. This deviation from linearity is commonly found in polymers and begins even at moderate strain rates. As one observes a decrease in the viscosity, i.e. the ratio between shear stress and shear rate, the effect is usually called shear thinning . 

The first to record the preparation of polyethylene was von Pechmann in 1898 [1], followed shortly thereafter by Bamberger and Tschimer [2]. In both cases polyethylene was prodnced by the decomposition of diazomethane, but the commercial significance of the discovery went unappreciated. Strictly speaking, the decomposition of diazomethane yields polymethylene, the only difference between this and linear polyethylene being that polymethylene molecules can have any number of carbon atoms, whereas polyethylene must have an even number. Friedrich and Marvel, in a 1930 paper [3], reported the unexpected polymerization of ethylene to a non-gaseous product. They did not appreciate the signifi-... 

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