Ziegler- Natta coordination polymerization

The use of ionic polymerizations for the preparation of highly stereoregular polymers is restricted to specific monomers, in particular polar monomers. Generally this method is not appropriate for non-polar monomers because they require stronger coordination than can be achieved with the counter-ions used in ionic polymerizations. 

Ziegler-Natta catalysts that are soluble in the solvent in which they are prepared (i.e. homogeneous) are of limited use because in general they do not provide stereochemical control. Nevertheless there are some notable exceptions. For example, syndiotactic polypropylene can be prepared at low temperatures (e.g. -78°C) using soluble catalysts based upon vanadium compounds (e.g. VCU + AlEt ). In addition, homogeneous catalysts prepared from benzyl derivatives of Ti and Zr have yielded isotactic polypropylene but are of low activity. 

2 Propagation monomer insertion at group /-/// metal-carbon bonds 

A number of mechanisms have been proposed for propagation by insertion of monomer at groups I-III metal-carbon bonds after initial polarization of the monomer by coordination to the transition metal. Since both metals are involved, these are often termed bimetallic mechanisms. An example is the mechanism proposed by Natta in which the active site is an electron-deficient bridge complex formed by reaction between a surface Ti atom and AIR3. Propagation may be represented by 

3 Propagation monomer insertion at transition metal-carbon bonds 

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V. Cationic Aspects of Alkyl Metal and Ziegler-Natta Coordination Polymerizations. 539... 

A model including all the presently known processes occurring in Ziegler— Natta coordination polymerizations has been published by Bohm [35], It is a special case of the Rideal mechanism [36], and it can be applied to both... 

Cp2TiMe2 is activated by various borate salts to give catalytic systems for the polymerization of ethylene, propylene, and styrene. A conventional Ziegler-Natta coordination polymerization mechanism is proposed for ethylene and propylene polymerization, while a carbocationic polymerization mechanism has been suggested for styrene.1497... 

Homogeneous Catalysis for Ziegler-Natta Coordination Polymerization (Section 29.6B) Initiation of Anionic Polymerization of Alkenes (Section 29.6D)... 

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). 

A Iky nes have been polymerized using ionic and radical initiators, but the polymer molecular weights are low. High molecular weights are obtained by using Ziegler-Natta coordination catalysts (Sec. 8-4d-2) [Chien et al., 1980]. The polymers are of considerable interest in terms of their potential as (semi)conducting materials. 

The centres of coordination polymerizations should be able to form only a weak and reversible coordination bond with the monomer. Compounds irreversibly solvating the coordination centres (under the given conditions) act as catalytic poisons. Thus monomers with unscreened polar substituents and most heterocycles cannot be polymerized on Ziegler-Natta coordination centres (see Chap. 3 Sect. 4.1). 

For substitution of monodentate 77-hydrocarbon ligands (ethylene, acetylene) a priori both mechanisms are possible. In this case an ability to change the coordination number in the transition state will be decisive. It is probable that square-planar complexes react by an associative mechanism with an increase in coordination number in the transition state. For the octahedral complexes, intermediates with lower coordination number are preferable (D-type mechanism). There is as yet no evidence for a transition state involving a-bonded ethylene or acetylene. However, both molecules are capable of inserting into transition metal-carbon u-bonds 10). It is quite probable that such an insertion mechanism operates in the Ziegler-Natta ethylene polymerization 11). 

The mechanism of the Ziegler-Natta-catalyzed polymerization of a substituted ethylene is shown in Figure 28.1. The monomer forms a tt complex (Section 6.5) with titanium at an open coordination site (i.e., a site available to accept electrons) and the coordinated alkene is inserted between the titanium and the growing polymer, thereby extending the polymer chain. Because a new coordination site opens up during insertion of the monomer, the process can be repeated over and over. 

What metals form catalysts in Ziegler-Natta coordinated anionic polymerizations. 

Post Ziegler and Natta Coordination Polymerization of Olefins... 

There are many reaction mechanisms for vinyl addition polymerizations. In approximate order of importance they are free radical polymerization, coordination metal catalysis (Ziegler-Natta), anionic polymerization, cationic polymerization, and group transfer polymerization. Regardless of specific mechanism, these polymerizations tend to be fast, essentially irreversible, highly exothermic and approximately first order with respect to monomer concentration. 

In conclusion, from our point of view, the growing interest in the synthesis of novel metal transition or lanthanide based catalyst systems could offer novel opportunities for further improvements in the nanocompounding process by in-situ polymerization via Ziegler-Natta coordination catalysis. 

Typical chain polymerizations are those of monomers containing the carbon-carbon double bond, e.g., ethylene, isobutylene, isoprene, styrene and acrylonitrile. Polymerization is initiated by radical, cationic, anionic or Ziegler-Natta (coordination) initiators. All monomers except 1-alkylethylenes, 1,1-dialkylethylenes and vinyl ethers undergo radical polymerization. Ionic chain polymerizations are much more selective than radical polymerizations. Cationic initiation is limited to monomers containing electron-donating substituents, e.g., alkoxy, 1,1-dialkyl, phenyl and vinyl. Anionic initiation is limited to monomers with electron-withdrawing substituents, e.g., CN, COOR, phenyl and vinyl. 1-Alkylethylenes such as propylene are polymerized only by... 

Olefins can insert into metal-alkyl bonds as well as into metal-hydride bonds provided that a vacant coordination site is available on the metal center. This reaction can thus be repeated even when the alkyl ligand lengthens, which makes the proposed mechanism a Ziegler-Natta olefin polymerization. This type of polymerization is detailed in Chap. 15.1 ... 

With such structural information available complex mechanistic proposals can be tested and rather subtle questions can be answered. An illustration is furnished by the investigation of the mechanism of the isotactic polymerization of propylene with Ziegler-Natta coordination catalysts of the type TiCl3 Al(C2Hs)2Cl by determining the type of error occasionally introduced. ... 

Coordination polymerization of isoprene using Ziegler-Natta catalyst systems (Section 6 21) gives a material similar in properties to natural rubber as does polymerization of 1 3 butadiene Poly(1 3 buta diene) is produced in about two thirds the quantity of SBR each year It too finds its principal use in tires... 

Section 14 15 Coordination polymerization of ethylene and propene has the biggest eco nomic impact of any organic chemical process Ziegler-Natta polymer ization IS carried out using catalysts derived from transition metals such as titanium and zirconium tt Bonded and ct bonded organometallic com pounds are intermediates m coordination polymerization... 

At present it is not possible to determine which of these mechanisms or their variations most accurately represents the behavior of Ziegler-Natta catalysts. In view of the number of variables in these catalyzed polymerizations, both mechanisms may be valid, each for different specific systems. In the following example the termination step of coordination polymerizations is considered. 

The stereospecific polymerization of alkenes is catalyzed by coordination compounds such as Ziegler-Natta catalysts, which are heterogeneous TiCl —AI alkyl complexes. Cobalt carbonyl is a catalyst for the polymerization of monoepoxides several rhodium and iridium coordination compounds... 

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