Ethylene Ziegler polymerization

The report of Bestian Clauss that methyltitanium trichloride will undergo a slow growth reaction with ethylene in polar inert solvents at low temperatures (6) is considerably important because of its relation to the Ziegler polymerization reaction. 

Apel , is obtained by Ziegler polymerization. It is available in film and resin form. The material exhibits a high density and a bulky structure. It is amorphous, optically isotropic and non polar (70). Topas is copolymerized from norbornene and ethylene using a metallocene catalyst. 

The straight-chain alcohol used for this ester is known commercially as Alfol 610 and is produced by the Ziegler polymerization of ethylene. The composition of the alcohol is 20% Ce. 35% Cs. and 44% C10. 

It is known that the polymerization of ethylene by trialkyl aluminum is not a rapid reaction at normal pressures and temperatures. Ziegler, Gellert, Holzkamp, Wilke, Duck and Kroll (72) have found that ethylene was polymerized to higher trialkylaluminums only at elevated temperatures and pressures. Anionic hydride transfer commonly occured under these conditions. However, the addition of a transition metal halide such as titanium tetrachloride, the classical Ziegler catalyst, polymerized ethylene rapidly under mild conditions. 

Since 1961, the industrial importance of the hydroformylation reaction has been threatened by newer processes (19) such as the Ziegler polymerization of ethylene, the Wacker process, and the direct oxidation of petroleum (153). The industrial aspects of the Oxo reaction were reviewed in 1965 when the world production capacity for Oxo products was estimated at 0.5 million tons per year (39). 

Derivation Benzene is alkylated with dodecene, to which it attaches itself in any secondary position the resulting dodecylbenzene is sulfonated with sulfuric acid and neutralized with caustic soda. For ABS (branched-chain alkyl) the dodecene is usually a propylene tetramer, made by catalytic polymerization of propylene. For LAS (straight-chain alkyl), the dodecene may be removed from kerosene or crudes by molecular sieve, may be formed by Ziegler polymerization of ethylene, or by cracking wax paraffins to a-olefins. 

Some Ziegler catalysts, especially those in which ethylene is polymerized in the presence of TiCls and AlEtaCl, can behave as if they were quite stable. Steady polymerization takes place over a long period at a nearly constant rate until the polymer concentration becomes so large... 

A major distinction between heterogeneous Ziegler—Natta polymerization and single-center polymerization is the facility with which comonomers are incorporated into the chain. Adding steric encumbrances to the catalyst has been shown to enhance this effect. With the parent biscyclopentadienylzir-conocene dichloride, ethylene polymerizes 25 times faster than propylene, with the ethylene tetrahdy-roindenyl complex ethylene is polymerized 10 times faster than propylene, and with the 2-methyl-4-aryl complexes such as 3 or 9, ethylene and propylene are polymerized with comparable rates. 

Scheme 3-9 Ziegler polymerization of ethylene with Ti/Ai cataiysts Exercise 3.6...

Coordination polymerization of olefins was first proposed in 1956 for the unusual, at that time, low-pressure polymerization of ethylene and polymerization of propylene with the transition metal catalysts discovered by Ziegler in 1953, and for the ferric chloride catalyzed ring-opening polymerization of propylene oxide to crystalline polymer reported by Pruitt et al. in a Dow patent. Polymerization carried out in the presence of a coordination catalyst is referred to as coordination polymerization . This term is used when each polymerization step involves the complexation of the monomer before its enchainment at the active site of the catalyst [9]. 

Linear low-density polyethylene is a copolymer in which ethylene is polymerized under Ziegler-Natta conditions in the presence of a smaller quantity of a second alkene such as 1-hexene. What structural feature characterizes the resulting polymer ... 

Ziegler polymerizations occur mostly in olefins and dienes (Table 19-1), although not every catalyst system is equally effective. As a rule of thumb, one can say that all Ziegler catalysts that polymerize a-olefins (CH2=CHR) also polymerize ethylene. The reverse of this rule, however, does not hold. Catalysts with transition elements of group VIII do indeed polymerize dienes, but not a-olefins whereas compounds with elements of groups IV-VI initiate polymerization in both a-olefins and dienes. 

The Ziegler polymerization is carried out in solution (hydrocarbons, heavy oil) at 60-75°C, i.e., below the melting point of poly(ethylene), and at pressures of 1-10 bar, with yields near 100%. In 1 h about 200 liters of ethylene per liter of solution will be converted to polymer. However, the economic advantage of the low-pressure process is canceled by the necessity of removing the remaining catalyst, which is effected by the destruction of the catalyst with ethanol. 

Supported metal complex catalysts for alkene polymerization. Supported chromium complexes on silica have been used for many years in the Phillips process for ethylene polymerization, and promoters are not required. Like these supported complexes, the classical TiClj Ziegler polymerization catalysts have also long been viewed as presenting surface catalytic sites that are well described as molecular analogues. 

K. Ziegler, a German chemist, observed an Aufbaureaktion (growth reaction) when ethylene is polymerized in the presence of triethylaluminum [1,18] (see Eq. 2.24). 

Metallocene catalysts for the polymerizations of olefins have been known since early 1957 when Natta and co-workers first reacted triethyl aluminum (AlEts) and bis(cyclopentadienyl) titanium dichloride ( -C5H5)2TiCl2 to form a complex that polymerized ethylene. The structure of this complex was described and the polymerization results reported. With ethylene they reported to have made 7 g of crystalline polyethylene in about 8 h at 95°C with 40 atm ethylene pressure in n-heptane. Later in the same year, Breslow and co-workers repeated Natta s experiments. They found that the blue complex described by Natta was a somewhat poor catalyst (in agreement with Natta s findings), but discovered that small amoimts of oxygen in the ethylene boosted polymerization activity. When compared to the heterogeneous Ziegler-Natta catalyst system, these metallocene catalysts were poor with respect to polymerization activity. They were used essentially for mechanistic studies because of their simplicity and ease of structure elucidation. 

The method just outlined was used, at the earlier stages of these kinds of study [101], for the elucidation of the mechanism of polymerization which was, at that time, an unsolved puzzle. The chemical problem can be stated briefly in the following way when ethylene is polymerized using Ziegler-Natta catalysts a linear polymer is formed. It was known that the catalysts acts on the double bond of the ethylene unit and the polymethylene chain can grow on both sides of the ethylene unit once the double bond has been opened. The opening, however, can occur in cis or trans configuration. 

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