Olefins coordination polymerization

An interesting example of the reaction mechanisms with multiple cycles is coordination polymerization. The most important industrial olefin coordination polymerization processes are those where Ti-based (Fig. 5.44) and metallocene catalysts (Fig. 5.45) are used. 

Coordination polymerization Can engineer polymers with specific tacticities based on the catalyst system Can limit branching reactions Polymerization can occur at low pressures and modest temperatures Otherwise non-polymerizable monomers (e.g., propylene) can be polymerized Mainly applicable to olefinic monomers... 

The most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

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. 

Transition metal catalysis plays a key role in the polyolefin industry. The discovery by Ziegler and Natta of the coordination polymerization of ethylene, propylene, and other non-polar a-olefins using titanium-based catalysts, revolutionized the industry. These catalysts, along with titanium- and zirconium-based metallocene systems and aluminum cocatalysts, are still the workhorse in the manufacture of commodity polyolefin materials such as polyethylene and polypropylene [3-6],... 

Polymerization occurs by repeated migratory insertion of olefin into the (Tv-oriented metal-carbon bond by the generally accepted Cossee mechanism [5, 60]. This mechanism is believed to be shared by all transition metal coordination polymerization... 

Precipitation polymerization, also called slurry polymerization, is a variety of solution polymerization where the monomer is soluble but the polymer precipitates as a fine flock. The formation of olefin polymers via coordination polymerization occurs by a slurry process. Here, the catalyst is prepared and polymerization is carried out under pressure and at low temperatures, generally less than 100°C. The polymer forms viscous slurries. Care is taken so that the polymer does not cake up on the sides and stirrer. 

Coordination copolymerization of ethylene with small amounts of an a-olefin such as 1-butene, 1-hexene, or 1-octene results in the equivalent of the branched, low-density polyethylene produced by radical polymerization. The polyethylene, referred to as linear low-density polyethylene (LLDPE), has controlled amounts of ethyl, n-butyl, and n-hexyl branches, respectively. Copolymerization with propene, 4-methyl-1-pentene, and cycloalk-enes is also practiced. There was little effort to commercialize linear low-density polyethylene (LLDPE) until 1978, when gas-phase technology made the economics of the process very competitive with the high-pressure radical polymerization process [James, 1986]. The expansion of this technology was rapid. The utility of the LLDPE process Emits the need to build new high-pressure plants. New capacity for LDPE has usually involved new plants for the low-pressure gas-phase process, which allows the production of HDPE and LLDPE as well as polypropene. The production of LLDPE in the United States in 2001 was about 8 billion pounds, the same as the production of LDPE. Overall, HDPE and LLDPE, produced by coordination polymerization, comprise two-thirds of all polyethylenes. 

A further modification of the active-center model was based on the consideration that the vacancy in the active center is strongly shielded by the polymer chain, mainly by the CH2 and CH3 groups of the second monomer unit. As a result, the vacant site is blocked and inaccessible for olefin coordination. Kissin et al. suggest a polymerization center with two vacancies one shielded and the other open for complexation.344,345 After each insertion step the end of the growing polymer chain flips from side to side and the two vacant sites are alternately available for alkene coordination. 

Ethylene Polymers. Depending on the polymerization conditions, three major types of polyethylene are manufactured low-density polyethylene (LDPE) by free-radical polymerization, linear low-density polyethylene (LLDPE) by copolymerization of ethylene with terminal olefins, and high-density polyethylene (HDPE) by coordination polymerization. The processes yield polymers with different characteristics (molecular weight, molecular weight distribution, melt index, strength, crystallinity, density, processability). 

It is not necessary to incorporate the concept of macrosurfaces nor of olefinic coordination complexes of the metal in order to explain stereospecific polymerization. Simple 4 and 6 membered cyclic transition states account for steric control. 

The first example of a living polyolefin with a uniform chain length was disclosed in 1979 by Doi, Ueki and Keii 47,48) who used the soluble Ziegler-Natta catalyst composed of V(acac)3 (acac = acetylacetonate anion) and A1(C2H5)2C1 for the polymerization of propylene. In this review, we deal with the kinetics and mechanism of living coordination polymerization of a-olefins with soluble Ziegler-Natta catalysts and the synthesis of well-defined block copolymers by the use of living polyolefins. 

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)... 

In addition to the familiar and extensive role perfluoroaryl boranes and borates fill as co-catalysts for the coordination polymerization of ethylene and a-olefins, these compounds can act as initiators for other polymerization reactions due to their high Lewis acidity. 

Kuran, W., Stereoregulation Mechanism in Coordination Polymerization of Hydrocarbon Monomers. Part I Polymerization of ot-Olefins , Polimery, 39, 570-578 (1997). 

The importance of the electrophilic character of the cation in organo-alkali compounds has been discussed by Morton (793,194) for a variety of reactions. Roha (195) reviewed the polymerization of diolefins with emphasis on the electrophilic metal component of the catalyst. In essence, this review willattempt to treat coordination polymerization with a wide variety of organometallic catalysts in a similar manner irrespective of the initiation and propagation mechanisms. The discussion will be restricted to the polymerization of olefins, vinyl monomers and diolefins, although it is evident that coordinated anionic and cationic mechanisms apply equally well to alkyl metal catalyzed polymerizations of polar monomers such as aldehydes and ketones. 

In the coordinated anionic polymerizations with Group I—III metal alkyls alone, monomer coordination involves overlap of the olefinic jr-electrons with vacant sp3 hybrid orbitals. Since this interaction is very weak it is most effective with easily polarized monomers. In the coordination polymerizations with Ziegler type catalysts, stronger monomer coordination is obtained by overlap of jr-electrons with vacant -orbitals of the transition metal component. The complexes have structures of the type proposed by Dewar (199b) and by Chatt and Duncanson (200) and applied to Ziegler type catalysts by Cossee (201) (Fig. 6). The olefin yr-electrons overlap with the orbital of... 

In coordinated polymerizations with alkyl metal and Ziegler-type catalysts, vacant p- or d-orbitals of a metal component coordinate with the jr-electrons of olefins, diolefins and non-polar monomers. When the polymer chain end is fixed in position and partially stabilized by its metal-containing gegen-ion, repetitive insertion of the polarized and oriented monomer between the chain end and gegen-ion yields stereoregular polymers. Of the various factors which affect polymer stereoregularity, the most important appears to be the gegen-ion structure and its ability to coordinate and orient the monomer. 

Theoretically, it is possible for the process of olefin coordination and insertion to continue as in Ziegler-Natta polymerization (Chapter 52) but with palladium the metal is expelled from the molecule by a p-hydride elimination reaction and the product is an alkene. For the whole process to be catalytic, a palladium(O) complex must be regenerated from the palladium(ll) product of P-hydride elimination. This occurs in the presence of base which removes HX from the palladium(II) species. 

Homogeneous polyaUcene catalysis has progressed to the point where metals not generally associated with coordination polymerization can now be made to promote olefin chain growth and metals long ago associated with polyalkene catalysis have been given new life. 

By far the most important industrial coordination polymerization processes are Ziegler-Natta polymerizations of 1-olefins [107-110], most notably the production of high-density polyethene [111] and stereo-specific olefin polymers and copolymers [108], However, these processes employ solid catalysts, and the complex kinetics on their surfaces have no place in a book on homogeneous reactions. 

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... 

When propylene and higher olefins are polymerized the configuration of the polymer is controlled by the catalyst structure. Catalyst 9-25 contains two equivalent Cl atoms, because the molecule is symmetrical about the Zr-indenyl bonds. Recall that in the active catalyst one Cl will be replaced by the growing polymer and the other by the incoming monomer. The polymer chain could occupy either of the two Cl positions, while the monomer could coordinate at the other position. As a result, both positions are equivalent and this metallocene produces isotactic polypropylene. In structure 9-26, the two Cl sites are not equivalent. At one position, the monomer would find the same environment as in 9-25, but the second... 

Titanium and zirconium tetrabenzyl and the mixed metal—benzyl halides are soluble in hydrocarbon solvents and will polymerize ethylene and a-olefins, the latter to stereo-specific polymers [64], The structures of the true initiators are not known but they are unlikely to be the simple organo-metal compounds. Catalysts of higher activity are obtained when they are used in combination with aluminium alkyls. It is of interest to note that titanium tetra(dimethyl amide) reacts with acrylonitrile to form an active species, which then forms high molecular weight polymer by coordination polymerization [65]. 

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