Зиглер-Натта катализаторлары: 1-алкендер полимерлеуі үшін катализдер

Catalyst for synthesis of polymers of 1 alkenes
A Ziegler–Natta catalyst, named after Karl Ziegler and Giulio Natta, is a catalyst used in the synthesis of polymers of 1 alkenes (alpha olefins). Two broad classes of Ziegler–Natta catalysts are employed, distinguished by their solubility:
Heterogeneous supported catalysts based on titanium compounds are used in polymerization reactions in combination with cocatalysts, organoaluminum compounds such as triethylaluminium, Al(C2H5)3. This class of catalyst dominates the industry. Homogeneous catalysts usually based on complexes of the group 4 metals titanium, zirconium or hafnium. They are usually used in combination with a different organoaluminum cocatalyst, methylaluminoxane (or methylalumoxane, MAO). These catalysts traditionally contain metallocenes but also feature multidentate oxygen and nitrogen based ligands. Ziegler–Natta catalysts are used to polymerize terminal alkenes (ethylene and alkenes with the vinyl double bond):
n CH2=CHR → −[CH2−CHR]n−;

The 1963 Nobel Prize in Chemistry was awarded to German Karl Ziegler, for his discovery of first titanium based catalysts, and Italian Giulio Natta, for using them to prepare stereoregular polymers from propylene. Ziegler–Natta catalysts have been used in the commercial manufacture of various polyolefins since 1956. As of 2010, the total volume of plastics, elastomers, and rubbers produced from alkenes with these and related (especially Phillips) catalysts worldwide exceeds 100 million tonnes. Together, these polymers represent the largest volume commodity plastics as well as the largest volume commodity chemicals in the world. In the early 1950s workers at Phillips Petroleum discovered that chromium catalysts are highly effective for the low temperature polymerization of ethylene, which launched major industrial technologies culminating in the Phillips catalyst. A few years later, Ziegler discovered that a combination of titanium tetrachloride (TiCl4) and diethylaluminium chloride (Al(C2H5)2Cl) gave comparable activities for the production of polyethylene. Natta used crystalline α TiCl3 in combination with Al(C2H5)3 to produce first isotactic polypropylene. Usually Ziegler catalysts refer to titanium based systems for conversions of ethylene and Ziegler–Natta catalysts refer to systems for conversions of propylene. Also, in the 1960s, BASF developed a gas phase, mechanically stirred polymerization process for making polypropylene. In that process, the particle bed in the reactor was either not fluidized or not fully fluidized. In 1968, the first gas phase fluidized bed polymerization process, the Unipol process, was commercialized by Union Carbide to produce polyethylene. In the mid 1980s, the Unipol process was further extended to produce polypropylene. In the 1970s, magnesium chloride (MgCl2) was discovered to greatly enhance the activity of the titanium based catalysts. These catalysts were so active that the removal of unwanted amorphous polymer and residual titanium from the product (so called deashing) was no longer necessary, enabling the commercialization of linear low density polyethylene (LLDPE) resins and allowed the development of fully amorphous copolymers. The fluidized bed process remains one of the two most widely used processes for producing polypropylene.

Natta first used polymerization catalysts based on titanium chlorides to polymerize propylene and other 1 alkenes. He discovered that these polymers are crystalline materials and ascribed their crystallinity to a special feature of the polymer structure called stereoregularity. The concept of stereoregularity in polymer chains is illustrated in the picture on the left with polypropylene. Stereoregular poly(1 alkene) can be isotactic or syndiotactic depending on the relative orientation of the alkyl groups in polymer chains consisting of units −[CH2−CHR]−, like the CH3 groups in the figure. In the isotactic polymers, all stereogenic centers CHR share the same configuration. The stereogenic centers in syndiotactic polymers alternate their relative configuration. A polymer that lacks any regular arrangement in the position of its alkyl substituents (R) is called atactic. Both isotactic and syndiotactic polypropylene are crystalline, whereas atactic polypropylene, which can also be prepared with special Ziegler–Natta catalysts, is amorphous. The stereoregularity of the polymer is determined by the catalyst used to prepare it.

These catalysts are metallocenes together with a cocatalyst, typically MAO, −[O−Al(CH3)]n−. The idealized metallocene catalysts have the composition Cp2MCl2 (M = Ti, Zr, Hf) such as titanocene dichloride. Typically, the organic ligands are derivatives of cyclopentadienyl. In some complexes, the two cyclopentadiene (Cp) rings are linked with bridges, like −CH2−CH2− or >SiPh2. Depending on the type of their cyclopentadienyl ligands, for example by using an ansa bridge, metallocene catalysts can produce either isotactic or syndiotactic polymers of propylene and other 1 alkenes.

Ziegler–Natta catalysts of the third class, non metallocene catalysts, use a variety of complexes of various metals, ranging from scandium to lanthanoid and actinoid metals, and a large variety of ligands containing oxygen (O2), nitrogen (N2), phosphorus (P), and sulfur (S). The complexes are activated using MAO, as is done for metallocene catalysts. Most Ziegler–Natta catalysts and all the alkylaluminium cocatalysts are unstable in air, and the alkylaluminium compounds are pyrophoric. The catalysts, therefore, are always prepared and handled under an inert atmosphere.

The structure of active centers in Ziegler–Natta catalysts is well established only for metallocene catalysts. An idealized and simplified metallocene complex Cp2ZrCl2 represents a typical precatalyst. It is unreactive toward alkenes. The dihalide reacts with MAO and is transformed into a metallocenium ion Cp2CH3, which is ion paired to some derivative(s) of MAO. A polymer molecule grows by numerous insertion reactions of C=C bonds of 1 alkene molecules into the Zr–C bond in the ion:

Many thousands of alkene insertion reactions occur at each active center resulting in the formation of long polymer chains attached to the center. The Cossee–Arlman mechanism describes the growth of stereospecific polymers. This mechanism states that the polymer grows through alkene coordination at a vacant site at the titanium atom, which is followed by insertion of the C=C bond into the Ti−C bond at the active center.