Ziegler-Natta reaction

We have considered the addition of three radicals, Cl, H- and CH3, as models for bases. In the Ziegler-Natta reaction, Lewis bases involved are esters of aromatic acids and hindered amines. Bases are supposed either to interact with the support (MgCl2) or to coordinate with Ti [40, 41]. 

The interest in the dimers is double. Dimerisation is a first model for the surface of the solids, increasing the coordination of the atoms without changing the oxidation states. Dimers placed on support (epitactically on the lateral surfaces of MgCl2 crystals or on the comers or edges of crystallites) have also supposed to be the reactive sites for the Ziegler-Natta reaction [40, 41]. 

The subsequent polymerization mechanism should be analogous to that accepted for Ziegler-Natta reactions shown at the top of the following page. 

In a broad range of Ziegler-Natta reaction conditions, the rate of polymerization is a first-order function of monomer concentration, except at very low concentrations [26]. However, the rate of metallocene-catalyzed polymerization is of a higher order of reaction. This is possibly caused by localized heating at high monomer concentrations [27]. 

Coates, G.W. and Waymouth, R.M. (1995) Transition metals in polymer synthesis Ziegler-Natta reaction, in Abel, E.W., Stone, F.G.A., Wilkinson, G., Hegedus, L. (eds.). Comprehensive Organometallic Chemistry II, vol. 12, Pergamon Press, New York, pp. 1193-1208. 

Coates GW, Waymouth RM. 12.1—Transition metals in polymer synthesis Ziegler— Natta reaction. In Wilkinson EWAGAS, editor. Comprehensive organometallic chemistry II. Oxford Elsevier 1995. p. 1193—208. 

Before the development of the Ziegler-Natta catalyst systems (Section 6 21) polymer ization of propene was not a reaction of much value The reason for this has a stereo chemical basis Consider a section of polypropylene... 

When propene is polymerized under free radical conditions the polypropylene that results IS atactic Catalysts of the Ziegler-Natta type however permit the preparation of either isotactic or syndiotactic polypropylene We see here an example of how proper choice of experimental conditions can affect the stereochemical course of a chemical reaction to the extent that entirely new materials with unique properties result... 

Among other possible reactions, these free radicals can initiate ordinary free-radical polymerization. The Ziegler-Natta systems are thus seen to encompass several mechanisms for the initiation of polymerization. Neither ionic nor free-radical mechanisms account for stereoregularity, however, so we must look further for the mechanism whereby the Ziegler-Natta systems produce this interesting effect. 

In the commercial process for the production of polypropylene by Ziegler-Natta catalysts, hydrogen is added to terminate the reaction, so neither of these reactions is pertinent to this process. 

Catalysts. Iodine and its compounds ate very active catalysts for many reactions (133). The principal use is in the production of synthetic mbber via Ziegler-Natta catalysts systems. Also, iodine and certain iodides, eg, titanium tetraiodide [7720-83-4], are employed for producing stereospecific polymers, such as polybutadiene mbber (134) about 75% of the iodine consumed in catalysts is assumed to be used for polybutadiene and polyisoprene polymeri2a tion (66) (see RUBBER CHEMICALS). Hydrogen iodide is used as a catalyst in the manufacture of acetic acid from methanol (66). A 99% yield as acetic acid has been reported. In the heat stabiH2ation of nylon suitable for tire cordage, iodine is used in a system involving copper acetate or borate, and potassium iodide (66) (see Tire cords). 

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

AH higher a-olefins, in the presence of Ziegler-Natta catalysts, can easily copolymerise both with other a-olefins and with ethylene (51,59). In these reactions, higher a-olefins are all less reactive than ethylene and propylene (41). Their reactivities in the copolymerisation reactions depend on the sise and the branching degree of their alkyl groups (51) (see Olefin polya rs, linear low density polyethylene). 

Similar to IFP s Dimersol process, the Alphabutol process uses a Ziegler-Natta type soluble catalyst based on a titanium complex, with triethyl aluminum as a co-catalyst. This soluble catalyst system avoids the isomerization of 1-butene to 2-butene and thus eliminates the need for removing the isomers from the 1-butene. The process is composed of four sections reaction, co-catalyst injection, catalyst removal, and distillation. Reaction takes place at 50—55°C and 2.4—2.8 MPa (350—400 psig) for 5—6 h. The catalyst is continuously fed to the reactor ethylene conversion is about 80—85% per pass with a selectivity to 1-butene of 93%. The catalyst is removed by vaporizing Hquid withdrawn from the reactor in two steps classical exchanger and thin-film evaporator. The purity of the butene produced with this technology is 99.90%. IFP has Hcensed this technology in areas where there is no local supply of 1-butene from other sources, such as Saudi Arabia and the Far East. 

Polypropylene. One of the most important appHcations of propylene is as a monomer for the production of polypropylene. Propylene is polymerized by Ziegler-Natta coordination catalysts (92,93). Polymerization is carried out either in the Hquid phase where the polymer forms a slurry of particles, or in the gas phase where the polymer forms dry soHd particles. Propylene polymerization is an exothermic reaction (94). 

The primary use of TiCl is as a catalyst for the polymerisa tion of hydrocarbons (125—129). In particular, the Ziegler-Natta catalysts used to produce stereoregular polymers of several olefins and dienes, eg, polypropylene, are based on a-TiCl and A1(C2H3)3. The mechanism of this reaction has been described (130). SuppHers of titanium trichloride iaclude Akso America and Phillips Petroleum ia the United States, and Mitsubishi ia Japan. 

Carbometalation, an important reaction of RTi(TV) compounds ia which RTi adds to a C=C or CM2 multiple bond and results ia a net R—H addition, is iavolved ia Ziegler-Natta polymerisation as follows ... 

In situ preparation of polymer blends of 1,4-polybutadiene with polystyrene, or poly(l-butene) has been achieved by using the heterogeneous Ziegler-Natta type catalyst (C2H )2A1C1—Ti(OC4H )4 in the host polymers (217). Homogeneous catalysts can also be used to catalyze these reactions (218). 

Alternatively, the intermediate acetaldehyde (qv) for this process was obtained from ethylene by the Wacker process (9). A small amount of -butyl alcohol is produced in the United States by the Ziegler-Natta chain growth reaction from ethylene [74-85-1] (10). 

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