Ziegler-Natta catalysts polymerization reactions

On the Mechanism of Olefin Polymerization by Ziegler-Natta Catalysts propagation reaction ... 

The synthesis of a gas-generating composition starts with the synthesis of a poly(vinyltriazole). A substituted triazole salt is added to a free radical brominating reagent, such as A-bromo succinimide, and to a radical initiator to form a brominated triazole. The brominated triazole is then added to triphenylphosphine to form a Wittig salt. Formaldehyde in alkaline medium effects the formation of a vinyltriazole salt, which can be polymerized with AIBN and a catalytic amount of a cationic initiator or a Ziegler-Natta catalyst. The reaction sequence is shown in Figure 9.10. 

EP copolymers are obtained through polymerization promoted by Ziegler-Natta-type catalysts. A Ziegler-Natta catalyst polymerizes olefins by inserting them into a metal-carbon bond. As a consequence, the exact structure of the catalytic site, determined by the interaction between the organometallic complex and the growing polymer chain, has a dramatic influence on the insertion reaction and hence on copolymer characteristics. 

Before the development of the Ziegler-Natta catalyst systems (Section 6.21), polymerization of propene was not a reaction of much value. The reason for this has a stereochemical basis. Consider a section of polypropylene ... 

The study of catalytic polymerization of olefins performed up to the present time is certain to hold a particular influence over the progress of the concepts of the coordination mechanism of heterogeneous catalysis. With such an approach the elementary acts of catalytic reaction are considered to proceed in the coordination sphere of one ion of the transition element and, to a first approximation, the collective features of solids are not taken into account. It is not surprising that polymerization by Ziegler-Natta catalysts is often considered together with the processes of homogeneous catalysis. 

Ziegler-Natta catalyst A stereospecific catalyst for polymerization reactions, consisting of titanium tetrachloride and triethylaluminum. zinc-blende structure A crystal structure in which the cations occupy half the tetrahedral holes in a nearly close packed cubic lattice of anions also known as sphalerite structure. 

In the following review we will focus on two classes of systems dispersed metal particles on oxide supports as used for a large variety of catalytic reactions and a model Ziegler-Natta catalyst for low pressure olefin polymerization. The discussion of the first system will focus on the characterization of the environment of deposited metal atoms. To this end, we will discuss the prospects of metal carbonyls, which may be formed during the reaction of metal deposits with a CO gas phase, as probes for mapping the environment of deposited metal atoms [15-19]. 

By performing excellent model reactions [144], Grubbs and his co-workers demonstrated direct olefin insertion into an M-C bond. Thus, complex 115 was treated with AlEtCl2 to give complex 116, whose decomposition afforded methylcyclopentane. Under the same conditions, the polymerization of ethylene took place. In this way, the insertion of a-olefins into a Ti-C single bond in a model Ziegler-Natta catalyst system was directly observed (Eq. 9). 

Slurry phase (or suspension) process. The uniquedooldng equipment in Figure 23—5 is a loop reactor. This process also takes place in a solvent (in this case, normal hexane, isobutane, or isopentane) so that the mixture can be pumped continuously in a loop while the polymerization is taking place. Feeds (the solvent, comonomer if any, ethylene and Ziegler-Natta catalyst) are pumped into the loop and circulated. Polymerization rakes place continuously at temperatures below the melting point of the polyethylene allowing solid polymer particles to form enough to form slurry. The reaction takes place at 185—212°F and 75—150 psi. A slurry of HOPE in hexane is drawn off continuously or intermittently. 

One recent development in Ziegler Natta catalysts was in producing catalyst particles that expanded as the polymerization reaction occurred. In this polymer the catalyst remains dispersed throughout the polymer, retaining its activity. This led to the development of fluidized bed processes to make polyethylene and polypropylene in which a sphere of polymer formed around each initial catalyst particle, and the polymer remained sohd as the reaction proceeded, rather than requiring a liquid solution. A major class of these catalysts and fluidized bed reactor was developed by Union Carbide and by Shell Oil and called the Unipol process. In this process a very active solid catalyst is introduced into the reactor, and reaction occurs on the catalyst particles, which expand to maintain active sites on the growing polymer sphere. 

A soluble titanium-based modified Ziegler-Natta catalyst [Ti(OR)4-Et3Al, R = n-Bu, isoPr] is employed in the reaction.42 Since similar catalysts may be used for the oligomerization and polymerization of ethylene, the nature and oxidation state of the metal and reaction conditions determine selectivity. Ti4+ was found to be responsible for high dimerization selectivity, whereas polymerization was shown to be catalyzed by Ti3+. According to a proposed mechanism,42,43 this catalyst effects the concerted coupling of two molecules of ethylene to form a metal-lacyclopentane intermediate that decomposes via an intramolecular p-hydrogen transfer ... 

A generally accepted mechanism for the polymerization of ethylene by typical Ziegler-Natta catalysts has been proposed by Cossee (81). More recently a mechanism for transition metal hydrocarbyl catalyzed polymerization of olefins has been discussed (41, 42, 43) which is essentially similar to that shown in Reaction 5. 

Studies of the reactions of propylene and alpha-olefins show that the Ziegler-Natta isotactic polymerizations are between the highly cationic and mildly anionic catalysts. 

Ziegler-Natta catalyst A stereospecific catalyst for polymerization reactions, consisting of titanium tetrachloride and triethylaluminum. 

Catalysts. Iodine and its compounds are very active catalysts for many reactions (133). The principal use is in the production of synthetic rubber via Ziegler-Natta catalysts systems. Also, iodine and certain iodides, eg, titanium tetraiodide [7720-834], are employed for producing stereospecific polymers, such as polybutadiene rubber (134) about 75% of the iodine consumed in catalysts is assumed to be used for polybutadiene and polyisoprene polymerization (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 stabilization of nylon suitable for tire cordage, iodine is used in a system involving copper acetate or borate, and potassium iodide (66) (see Tire cords). 

The coordination polymerization of ethylene and a-olefins with Ziegler-Natta catalysts involves, in general, many elementary reactions, such as initiation (formation of active centers), chain propagation, chain transfers and chain terminations. The length of growing polyolefin chains is limited by the chain-terminating processes, as schematically represented (for ethylene) by 21,49 51)... 

Molecular hydrogen has been known for a long time as an effective chain-transfer agent in the coordination polymerization of ethylene and a-oiefms with Ziegler-Natta catalysts 99-101,50). The mechanism for the reaction of a growing polymer chain with H2 has not been established, The living coordination polymerization system is well suited for an elucidation of the mechanism, since the reaction with H2 can be studied independently of any interference from other chain-terminating processes. 

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