Olefin polymerization ethylene

Ziegler catalysts Complex catalysts prepared by interaction between an organometallic derivative and a transition metal derivative. A typical catalyst is the product of the interaction of TiCU and AIBU3. These catalysts polymerize olefins, particularly ethylene, to polyolefins, the polymerization generally being in a siereoregular manner. 

Density. The density (crystallinity) of catalyticaHy produced PE is primarily determined by the amount of comonomer ( a-olefin) in ethylene copolymer. This amount is easily controlled by varying the relative amounts of ethylene and the comonomer in a polymerization reactor. In contrast, the density of PE produced in free-radical processes is usually controlled by temperature. 

In the early 1950s, Ziegler observed that certain heterogeneous catalysts based on transition metals polymerized ethylene to a linear, high density material at modest pressures and temperatures. Natta showed that these catalysts also could produce highly stereospecific poly-a-olefins, notably isotactic polypropylene, and polydienes. They shared the 1963 Nobel Prize in chemistry for their work. 

Polymerization. Supported catalysts are used extensively in olefin polymerization, primarily to manufacture polyethylene and polypropylene. Because propylene can polymerize in a stereoregular manner to produce an isotactic, or crystalline, polymer as well as an atactic, or amorphous, polymer and ethylene caimot, there are large differences in the catalysts used to manufacture polyethylene and polypropylene (see Olefin polymers). 

In the depropanizer tower the propane and lighter gases are taken overhead to become feed to the ethylene and propylene recovery facilities. Separation is accomplished at a relatively low overhead temperature of -25°F to minimize reboiler fouling by olefin polymerization. 

In essence the active centers for catalytic polymerization of olefins are organometallic complexes of transition metals. For this reason a search for individual organometallic compounds that would possess catalytic activity in olefin polymerization is of great interest. The first attempts to use organometallic compounds of transition metals as catalysts for olefin polymerization were made long ago [e.g. CH3TiCl3 as a catalyst for polymerization of ethylene 116). However, only in recent years as a result of the application of relatively stable organometallic compounds of transition... 

One of the most important reactions in the chemical industry is the cracking (removing hydrogen) of paraffins to form olefins. Olefins are essential starting materials for many common products that we all use, particularly plastics. The simplest olefin is ethylene, which can be polymerized (reacted many times with itself) to give po/yethylene used for bottles, dishware, film, etc. The next higher olefin, propylene, can be polymerized to polypropylene used in the manufacture of rugs, toys, kitchenware, and many other plastic objects. Note that the names of olefins end in -ene, which denotes the presence of a double bond in the compound. 

Small olefins, notably ethylene (ethene), propene, and butene, form the building blocks of the petrochemical industry. These molecules originate among others from the FCC process, but they are also manufactured by the steam cracking of naphtha. A wealth of reactions is based on olefins. As examples, we discuss here the epoxida-tion of ethylene and the partial oxidation of propylene, as well as the polymerization of ethylene and propylene. 

The Phillips Cr/silica catalyst is prepared by impregnating a chromium compound (commonly chromic acid) onto a support material, most commonly a wide-pore silica, and then calcining in oxygen at 923 K. In the industrial process, the formation of the propagation centers takes place by reductive interaction of Cr(VI) with the monomer (ethylene) at about 423 K [4]. This feature makes the Phillips catalyst unique among all the olefin polymerization catalysts, but also the most controversial one [17]. 

Certain half-sandwich phenoxides have been shown to be highly active olefin polymerization catalysts. For example, the zirconium complex (60) polymerizes ethylene with an activity of 1,220 gmmol-1 h-1 bar-1.181 A similar titanium complex (61) displays an activity of 560gmmol ll bar 1 at 60°C.182-189 Comparable activities were also recorded for the copolymerization of ethylene with 1-butene and 1-hexene. 

Other half-sandwich Cr complexes which show good activities for olefin polymerization include those with ether and thioether pendant arms (93) and (94) which show activities of 1,435gmmol-1 h-1 bar-1 and 2,010 gmmol-1 h 1 bar-1 respectively.252 The half-sandwich phosphine complex (95) affords a-olefins arising from chain transfer to aluminum,256,257 while the related boratabenzene chromium(III) complex (96) generates linear PF.258,259 Cationic species have also been investigated, and (97) polymerizes ethylene with an activity of 56 gmmol-1 h-1 bar-1.260-263... 

The first examples of highly active olefin polymerization catalysts based on late transition metals were nickel and palladium complexes containing bulky diimine ligands.310 312 For example, complex (120) was found to polymerize ethylene with an activity of ll,000gmmol h bar A range of PE materials with molecular weights up to 106 and... 

When less bulky ancillary ligands are used /3-hydride elimination leads to the formation of Q-olefins. As a consequence iminopyridine complexes are typically much less active than the diimine catalysts and afford lower-molecular-weight PE.321-324 For example, MAO/(122) polymerizes ethylene to branched oligomers with Mn < 600, and 240 branches per 1,000 carbons.325 Complex (123), is highly active for ethylene polymerization (820gmmol 1 h bar ).326 As with the diimine systems, reduction in the steric bulk of the ligand substituents results in reduced activity and lower-molecular-weight products. 

Several combinatorial approaches to the discovery of transition metal based catalysts for olefin polymerization have been described. In one study Brookhart-type polymer-bound Ni- and Pd-(l,2-diimine) complexes were prepared and used in ethylene polymerization (Scheme 3).60,61 A resin-bound diketone was condensed with 48 commercially available aminoarenes having different steric properties. The library was then split into 48 nickel and 48 palladium complexes by reaction with [NiBr2(dme)] and [PdClMe(COD)], respectively, all 96 pre-catalysts being spatially addressable. 

The course of stereospecific olefin polymerization was studied by using the molecular mechanics programs, MM-2 and Biograph, based on the optimized geometries of the ethylene complex and the transition state [13,203]. Interestingly, the steric interaction at the transition state mainly controls the stereochemistry in polymerization, which proceeds specifically isotactic or syndiotactic depending on the kind of catalyst. 

Non-metallocene complexes, such as aryloxide 31 and amide 138, have also been utilized as catalyst systems for the polymerization of a-olefins. Moreover, the homogeneous olefin polymerization catalysts have been extended to metals other than those in Group 4, as described in Sect. 7. Complexes such as mono(cyclopentadienyl)mono(diene) are in isoelectronic relationship with Group 4 metallocenes and they have been found to initiate the living polymerization of ethylene. These studies will being further progress to the chemistry of homogeneous polymerization catalysts. 

The ethylene polymerization behavior of FI catalysts has been described in previous sections. It is often observed that the cocatalyst that is employed has an influence on the catalytic behavior of a transition metal-based olefin polymerization catalyst. FI catalysts can exhibit unique catalytic behavior depending on the cocatalyst that is used for polymerization. 

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