Olefin Polymerization with Coordination Catalysts

Olefin polymerization with metallocene catalysts includes a coordination step between the monomer molecule and the catalyst active site, followed by insertion into the metal-alkyl bond between the catalyst center and the growing polymer chain. These steps are repeated many times to form a polymer chain. 

Increasing the Al/M ratio results in higher polymerization activity up to a maximum value, after which a further increase will have Uttle effect on the polymerization rate or may even cause the activity to decrease. Initially the polymerization rate increases with increasing Al/M ratio because the MAO molecules surrounding the active centers help stabiUze them. However, it is supposed that after a given Al/M ratio, higher MAO concentrations may deactivate the active sites. 

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Mathematical models for olefin polymerization with coordination catalysts are usually classified into microscale. 

The kinetic steps shown in Tables 5.2 and 5.3 are widely accepted and are commonly used to develop polyolefin microstructural models, but they do not describe all aspects of olefin polymerization with coordination catalysts ... 

The catalytic cycle is a convenient graphical way to describe the central role played by the active site in the mechanism of polymerization. Changes in the nature of the active site will affect the catalytic mechanism and consequently the activity and the selectivity of the polymerization. Changes in the polymerization reactor conditions, such as temperature and monomer concentration, play a vital role in the catalyst mechanism because they affect the rate constants of each of these steps. Figure 8.13 shows a catalyst cycle for olefin polymerization with coordination catalysts. 

To describe the kinetics of olefin polymerization with heterogeneous catalysts, kinetic models based on adsorption isotherm theories have been proposed [7-10], The most accepted two-step mechanism of ZN polymerization, proposed by Cossee [10-12], includes olefin coordination and migratory insertion of coordinated monomer into a metal-carbon bond of the growing polymer chain. 

Figure 2.19 illustrates Cossee s mechanism for polymerization with coordination catalysts. The active site is depicted as having a coordination vacancy that attracts the electrons in the olefin rr-bond. Coordination is followed by insertion into the polymer chain (R) and the re-establishment of the coordination vacancy for further monomer insertion. This figure also shows an important characteristic of coordination polymerization that makes it very different from free-radical polymerization the monomer is inserted between the carbon-metal bond. As a consequence, the electronic and steric environment surrounding the transition metal has a huge influence on the kinetics of polymerization. This is why... 

Polymerization with coordination catalysts proceeds via two main steps monomer coordination to the active site, and monomer insertion into the growing polymer chain, as illustrated in Figure 8.9. Before insertion, the double bond in the olefin monomer coordinates to the coordination vacancy of the transition metal. After the olefin is inserted into the growing polymer chain, another olefin monomer can coordinate to the vacant site thus the process of insertion is repeated to increase the size of the polymer chain by one monomer unit at a time until chain transfer takes place [17]. In the case of copolymerization, there is a competition between the comonomers to coordinate to the active sites and to be inserted into the growing polymer chains. Different rates of coordination and insertion of comonomers determine the final chemical composition of the copolymer chain. 

Moreover, the molecular catalysts have provided systematic opportunities to study the mechanisms of the initiation, propagation, and termination steps of coordination polymerization and the mechanisms of stereospecific polymerization. This has significantly contributed to advances in the rational design of catalysts for the controlled (co)polymerization of olefinic monomers. Altogether, the development of high performance molecular catalysts has made a dramatic impact on polymer synthesis and catalysis chemistry. There is thus great interest in the development of new molecular catalysts for olefin polymerization with a view to achieving unique catalysis and distinctive polymer synthesis. 

Lithium catalysts are far more stereospecific than the other alkali metal catalysts and are effective even in homogeneous system because of the strong coordinating ability of the lithium. Bawn and Led with (192) have included in their review an excellent discussion on the mechanism of polymerization with lithium catalysts. The key feature is the coordination of the olefinic zr-electrons with vacant s- or -orbitals in the lithium prior to an intramolecular rearrangement involving migration of the carbanion to the more electrophilic carbon of the polarized monomer. 

Based on the proposed model and on the experimental observation that the polymerization rate is proportional to the monomer concentration, it has been concluded that olefin coordination constitutes the rate determining step. However, the variety in preparation and performance of Mg/Ti catalysts for ethylene polymerization is such that it is impossible to reduce it to a single kinetic scheme, even though the active principle may be, in any case, constituted by MgCl2 89-90-91>. Ethylene polymerization with these catalysts has recently been reviewed 89) and will not be dealt with in detail here. However, it is important to underline that Mg/Ti catalysts for... 

Section 2.3 describes phenomenological models for polymerization kinetics with coordination catalysts. Molar and population balances will be derived using what we like to call the standard model for olefin polymerization kinetics with coordination catalysts. Also how molecular weight averages can be modeled in batch, semibatch and continuous reactors are shown using the method of moments and the method of instantaneous distributions. Unfortunately, the kinetics of olefin polymerization is complicated by several factors that are not included in the standard model some of these effects and possible solutions and model extensions are mentioned briefly at the end of Section 2.2. 

In-situ Polymerization of Olefins with Coordination Catalysts Supported on Clays 59... 

Representative work N. Koga and K. Morokuma,/. Am. Chem. Soc., 115, 6883 (1993). SiH, SiSi, and CH Bond Activation by Coordinatively Unsaturated RhCI(PHj)2. Ab Initio Molecular Orbital Study. F. Maseras, N. Koga, and K. Morokuma,/. Am. Chem. Soc., IIS, 8313 (1993). Ab Initio Molecular Orbital Characterization of the [Os(PRj)j Hj ]- - Complex. H. Kawamura-Kuribayashi, N. Koga, and K. Morokuma, /. Am. Chem. Soc., 114, 8687 (1992). Ab Initio MO and MM Study of Homogeneous Olefin Polymerization with Silylene-Bridged Zirconocene Catalyst and Its Regio- and Stereoselectivity. 

Polymerization Kinetics and Mechanism with Coordination Catalysts 389 Tab. 8.1. Terminal model for blna copolymerization of olefins.M... 

The polymerization of olefins with coordination catalysts is performed in a large variety of polymerization processes and reactor configurations that can be classified broadly into solution, gas-phase, or slurry processes. In solution processes, both the catalyst and the polymer are soluble in the reaction medium. These processes are used to produce most of the commercial EPDM rubbers and some polyethylene resins. Solution processes are performed in autoclave, tubular, and loop reactors. In slurry and gas-phase processes, the polymer is formed around heterogeneous catalyst particles in the way described by the multigrain model. Slurry processes can be subdivided into slurry-diluent and slurry-bulk. In slurry-diluent processes, an inert diluent is used to suspend the polymer particles while gaseous (ethylene and propylene) and liquid (higher a-olefins) monomers are fed into the reactor. On the other hand, only liquid monomer is used in the slurry-bulk pro-... 

Ethylene reacts by addition to many inexpensive reagents such as water, chlorine, hydrogen chloride, and oxygen to produce valuable chemicals. It can be initiated by free radicals or by coordination catalysts to produce polyethylene, the largest-volume thermoplastic polymer. It can also be copolymerized with other olefins producing polymers with improved properties. Eor example, when ethylene is polymerized with propylene, a thermoplastic elastomer is obtained. Eigure 7-1 illustrates the most important chemicals based on ethylene. 

Addition polymerization is employed primarily with substituted or unsuhstituted olefins and conjugated diolefins. Addition polymerization initiators are free radicals, anions, cations, and coordination compounds. In addition polymerization, a chain grows simply hy adding monomer molecules to a propagating chain. The first step is to add a free radical, a cationic or an anionic initiator (I ) to the monomer. For example, in ethylene polymerization (with a special catalyst), the chain grows hy attaching the ethylene units one after another until the polymer terminates. This type of addition produces a linear polymer ... 

In studying two-component polymerization catalysts, beginning with Feldman and Perry (161), a radioactive label was introduced into the growing polymer chain by quenching the polymerization with tritiated alcohols. The use of these quenching agents is based on the concept of the anionic coordination mechanism of olefin polymerization occurring... 

Two possible reasons may be noted by which just the coordinatively insufficient ions of the low oxidation state are necessary to provide the catalytic activity in olefin polymerization. First, the formation of the transition metal-carbon bond in the case of one-component catalysts seems to be realized through the oxidative addition of olefin to the transition metal ion that should possess the ability for a concurrent increase of degree of oxidation and coordination number (177). Second, a strong enough interaction of the monomer with the propagation center resulting in monomer activation is possible by 7r-back-donation of electrons into the antibonding orbitals of olefin that may take place only with the participation of low-valency ions of the transition metal in the formation of intermediate 71-complexes. 

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