Catalyst Ziegler

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. 

Ziegler-Natta polymerization Stereospecific polymerization of olefines using a Ziegler catalyst. See titanium(IIl) chloride. 

Z-furan [710-25-8] Zidovudine [30516-87-1] Ziegler catalyst Ziegler catalysts... 

Polymerization in a hydrocarbon slurry (usually a light-saturated hydrocarbon) was the first commercial polymerization process to utilize Phillips and Ziegler catalysts. These processes enjoy high popularity because of theit versatihty. 

Molecular Weight Distribution. In industry, the MWD of PE resins is often represented by the value of the melt flow ratio (MER) as defined in Table 2. The MER value of PE is primarilly a function of catalyst type. Phillips catalysts produce PE resins with a broad MWD and their MER usually exceeds 100 Ziegler catalysts provide resins with a MWD of a medium width (MFR = 25-50) and metallocene catalysts produce PE resins with a narrow MWD (MFR = 15-25). IfPE resins with especially broad molecular weight distributions are needed, they can be produced either by using special mixed catalysts or in a series of coimected polymerization reactors operating under different reaction conditions. 

A wide variety of chromium oxide and Ziegler catalysts was developed for this process (61,62). Chromium-based catalysts produce HDPE with a relatively broad MWD other catalysts provide HDPE resins with low molecular weights (high melt indexes) and resins with a narrower MWD (63,64). 

Two modifications of the duidized-bed reactor technology have been developed. In the first, two gas-phase duidized-bed reactors coimected to one another have been used by Mobil Chemical Co. and Union Carbide to manufacture HDPE resins with broad MWD (74,75). In the second development, a combination of two different reactor types, a small slurry loop reactor followed by one or two gas-phase duidized-bed reactors (Sphetilene process), was used by Montedision to accommodate a Ziegler catalyst with a special particle morphology (76,77). This catalyst is able to produce PE resins in the form of dense spheres with a diameter of up to 4—5 mm such resins are ready for shipping without pelletization. 

The Amoco reactor operates at 70—80°C and 2 MPa (300 psi) reactor pressure. The existence of several partially isolated compartments allows a semi-iadependent control of temperature as well as comonomer and hydrogen concentrations within each section, which ia turn offers a substantial control of the molecular weight and MWD of resias. Amoco technology also accommodates a large variety of polymerization catalysts, including Phillips and Ziegler catalysts. 

Most catalysts for solution processes are either completely soluble or pseudo-homogeneous all their catalyst components are introduced into the reactor as Hquids but produce soHd catalysts when combined. The early Du Pont process employed a three-component catalyst consisting of titanium tetrachloride, vanadium oxytrichloride, and triisobutjlalurninum (80,81), whereas Dow used a mixture of titanium tetrachloride and triisobutylalurninum modified with ammonia (86,87). Because processes are intrinsically suitable for the use of soluble catalysts, they were the first to accommodate highly active metallocene catalysts. Other suitable catalyst systems include heterogeneous catalysts (such as chromium-based catalysts) as well as supported and unsupported Ziegler catalysts (88—90). 

Typical heterogeneous Ziegler catalysts operate at temperatures of 70— 100°C and pressures of 0.1—2 MPa (15—300 psi). The polymerization reactions are carried out ia an iaert Hquid medium (eg, hexane, isobutane) or ia the gas phase. Molecular weights of LLDPE resias are coatroUed by usiag hydrogea as a chain-transfer ageat. 

Most Kaminsky catalysts contain only one type of active center. They produce ethylene—a-olefin copolymers with uniform compositional distributions and quite narrow MWDs which, at their limit, can be characterized by M.Jratios of about 2.0 and MFR of about 15. These features of the catalysts determine their first appHcations in the specialty resin area, to be used in the synthesis of either uniformly branched VLDPE resins or completely amorphous PE plastomers. Kaminsky catalysts have been gradually replacing Ziegler catalysts in the manufacture of certain commodity LLDPE products. They also faciUtate the copolymerization of ethylene with cycHc dienes such as cyclopentene and norhornene (33,34). These copolymers are compositionaHy uniform and can be used as LLDPE resins with special properties. Ethylene—norhornene copolymers are resistant to chemicals and heat, have high glass transitions, and very high transparency which makes them suitable for polymer optical fibers (34). 

Eluidized-bed reactors are highly versatile and can accommodate many types of polymerization catalysts. Most of the catalysts used for LLDPE production are heterogeneous Ziegler catalysts, in both supported and unsupported forms. The gas-phase process can also accommodate supported metallocene catalysts that produce compositionaHy uniform LLDPE resins (49—51). 

In the early 1990s, solution processes acquired new importance because of their shorter residence times and abiUty to accommodate metallocene catalysts. Many heterogeneous multicenter Ziegler catalysts produce superior LLDPE resins with a better branching uniformity if the catalyst residence time in a reactor is short. Solution processes usually operate at residence times of around 5—10 min or less and are ideal for this catalyst behavior. Solution processes, both in heavy solvents and in the polymer melt, are inherently suitable to accommodate soluble metallocene catalysts (52). For this reason, these processes were the first to employ metallocene catalysts for LLDPE and VLDPE manufacture. 

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

Few aHyl monomers have been polymerized to useful, weH-characterized products of high molecular weight by ionic methods, eg, by Lewis acid or base catalysts. Polymerization of the 1-alkenes by Ziegler catalysts is an exception. However, addition of acidic substances, at room temperature or upon heating, often gives viscous liquid low mol wt polymers, frequently along with by-products of uncertain stmcture. 

This conceptual link extends to surfaces that are not so obviously similar in stmcture to molecular species. For example, the early Ziegler catalysts for polymerization of propylene were a-TiCl. Today, supported Ti complexes are used instead (26,57). These catalysts are selective for stereospecific polymerization, giving high yields of isotactic polypropylene from propylene. The catalytic sites are beheved to be located at the edges of TiCl crystals. The surface stmctures have been inferred to incorporate anion vacancies that is, sites where CL ions are not present and where TL" ions are exposed (66). These cations exist in octahedral surroundings, The polymerization has been explained by a mechanism whereby the growing polymer chain and an adsorbed propylene bonded cis to it on the surface undergo an insertion reaction (67). In this respect, there is no essential difference between the explanation of the surface catalyzed polymerization and that catalyzed in solution. 

Polyolefins. The most common polyolefin used to prepare composites is polypropylene [9003-07-0] a commodity polymer that has been in commercial production for almost 40 years following its controlled polymerisation by Natta in 1954 (5). Natta used a Ziegler catalyst (6) consisting of titanium tetrachloride and an aluminum alkyl to produce isotactic polypropylene directly from propylene ... 

Low pressure polymerization via ionic catalysts, using Ziegler catalysts (aluminum alkyls and titanium haUdes). 

Low pressure polymerization with Ziegler catalysts supported on inorganic carriers. 

Linear alcohols used for the production of ethoxylates are produced by the oligomerization of ethylene using Ziegler catalysts or by the Oxo reaction using alpha olefins. 

Oligomerization of ethylene using a Ziegler catalyst produces unbranched alpha olefins in the C12-C16 range by an insertion mechanism. A similar reaction using triethylaluminum produces linear alcohols for the production of biodegradable detergents. 

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