Polyethylene process polymerization mechanisms

The properties of a polymer depend not only on its gross chemical composition but also on its molecular weight distribution, copolymer composition distribution, branch length distribution, and so on. The same monomer(s) can be converted to widely differing polymers depending on the polymerization mechanism and reactor type. This is an example of product by process, and no single product is best for all applications. Thus, there are several commercial varieties each of polyethylene, polystyrene, and polyvinyl chloride that are made by distinctly different processes. 

Most commercial polyolefins are produced by coordination polymerization catalysts. When compared to free radical processes used to make low-density polyethylene (LDPE), these catalysts work in comparatively gentle conditions, such as lower pressures and temperatures, while providing greater flexibility in controlling the polyolefin molecular structure. An understanding of the polymerization mechanism with coordination catalysts is essential for designing proper systems for the production of polyolefin-clay nanocomposites and wUl be covered in the next section. 

Perhaps one of the greatest contributions of Tref to the understanding of olefin polymerization was the elucidation of the natiue of active sites present on heterogeneous Ziegler-Natta catalysts. The systematic apphcation of Tref to ethylene/1-olefin copolymers made with heterogeneous Ziegler-Natta catalysts in different polymerization processes has shown that aU these resins have a signature bimodal Tref peak that can only be explained by the presence of two or more distinct types of active sites on the catalyst [34]. In contrast, low-density polyethylene (LDPE) made with the free-radical mechanism has a much narrower and unimodal Tref profile (Fig. 18), as expected from a free-radical polymerization mechanism. 

Polyethylene ( CH2—CH2—)x was first produced commercially in 1939. The early process was under high pressure (1,000-3,000 atm) and at temperatures from 80 to 300°C. The polymerization mechanism is via free radical initiators such as benzoyl peroxide... 

For composites with polymerization-modified filler it is typical that the physico-mechanical characteristics should increase symbatically with the quantity of polymer which becomes attached to the filler in the polymerization process. This effect has been observed for polyethylene [293, 321], poly(vinyl chloride) coats [316], and in [336, 337] for kaolin coated with poly(vinyl acetate) and introduced into the copolymer of ethylene and vinyl acetate. 

Linear combination of atomic orbitals (LCAO) method, 16 736 Linear condensation, in silanol polycondensation, 22 557-558 Linear congruential generator (LCG), 26 1002-1003 Linear copolymers, 7 610t Linear density, 19 742 of fibers, 11 166, 182 Linear dielectrics, 11 91 Linear elastic fracture mechanics (LEFM), 1 509-510 16 184 20 350 Linear ethoxylates, 23 537 Linear ethylene copolymers, 20 179-180 Linear-flow reactor (LFR) polymerization process, 23 394, 395, 396 Linear free energy relationship (LFER) methods, 16 753, 754 Linear higher a-olefins, 20 429 Linear internal olefins (LIOs), 17 724 Linear ion traps, 15 662 Linear kinetics, 9 612 Linear low density polyethylene (LLDPE), 10 596 17 724-725 20 179-211 24 267, 268. See also LLDPE entries a-olefin content in, 20 185-186 analytical and test methods for,... 

Finally the ESR spectrum of Nb(7r-allyl)4/alumina was unaffected by the addition of ethylene gas to the ESR sample tube. It is assumed that polyethylene is produced in this process since polymer can be isolated from larger scale reactions under similar conditions. The accepted mechanism for the ethylene growth reaction postulates a steady-state concentration of a a-bonded transition metal-hydrocarbon species which would be expected to modify the ESR spectrum of the supported complex. A possible explanation for the failure to detect a change in the ESR spectrum may be that only a small number of the niobium sites are active for polymerization. Although further experiments are needed to verify this proposition, it is consistent with IR data and radiochemical studies of similar catalyst systems (41, 42, 43). 

The mechanism of the polymerization of this monomer has been studied in far greater detail than any other. It is clear from the outset that a much more complex mechanism is involved than is the case for olefins. A large proportion of the initiator is used to form polymer whose molecular weight is only a few hundreds and the overall molecular weight distribution is so broad as to be rivalled only by those found in polyethylene produced by the high pressure process (19, 39). The initiator disappears almost instantaneously on mixing the reactants (19, 38). Under these conditions, an almost monodisperse polymer would be expected if chain transfer or termination processes are absent. 

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