Coordination catalyst types

2 Catalysts for olefin polymerization 2.2.1 Coordination catalyst types 

Type Physical state Examples Polymer type 

Ziegler-Natta Heterogeneous Homogeneous TiCIs, TiCU/MgCh VCI4, VOCI3 Non-uniform Uniform 

Metallocene Homogeneous Heterogeneous Cp2 ZrCl2 Cp2 ZrCl2/Si02 Uniform Uniform 

Late-transition metal Homogeneous Ni, Pd, Co, Fe with diimine and other ligands Uniform 

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This chapter describes the coordination polymerization of acyclic and cyclic vinylic monomers, conjugated dienes, and polar vinylic monomers with the most important catalytic systems known in this area. A chronological classitication for the development of the main coordination catalyst types is outlined, as well as polymerization kinetics and mechanisms and applications of polymers obtained through different metallic complexes. 

Linear polyethylene (high density) was introduced in the late 1950s, with the development of coordination catalysts. Chlorosulfonation of these base resins gave products that were superior to the eadier, low density types in both chemical resistance and mechanical properties and with distinct advantages in mbber processibiUty (6,7). 

CSM products may be divided into three groups depending on the type of precursor resin low density (LDPE), high density (HDPE), and linear low density (LLDPE). LDPE is made by a high pressure free-radical process, while HDPE and LLDPE are made via low pressure, metal coordination catalyst processes (12) (see Olefin polymers). 

Name the two principal types of coordination catalyst. Describe their general characteristics. 

We make polyethylene resins using two basic types of chain growth reaction free radical polymerization and coordination catalysis. We use free radical polymerization to make low density polyethylene, ethylene-vinyl ester copolymers, and the ethylene-acrylic acid copolymer precursors for ethylene ionomers. We employ coordination catalysts to make high density polyethylene, linear low density polyethylene, and very low density polyethylene. 

The way by which all the factors involved influence the course of a reaction varies from case to case, and prediction is largely empirical. For catalytic processes, the actual species acting as catalyst is often unknown because coordination number, type of ligands, stereochemistry of the complex, and formal charge are difficult to establish in the reaction medium. Often many species are present, and the most active may be the one having the lowest coordination number and being present in a concentration so low that it cannot be detected spectroscopically. Only kinetic studies can provide evidence for such species. 

These are a special type of coordination catalysts, made up two components. The two components are generally referred to as the catalyst and the cocatalyst. 

This type of alkoxylation chemistry cannot be performed with conventional alkali metal hydroxide catalysts because the hydroxide will saponify the triglyceride ester groups under typical alkoxylation reaction conditions. Similar competitive hydrolysis occurs with alternative catalysts such as triflic acid or other Brpnsted acid/base catalysis. Efficient alkoxylation in the absence of significant side reactions requires a coordination catalyst such as the DMC catalyst zinc hexacyano-cobaltate. DMC catalysts have been under development for years [147-150], but have recently begun to gain more commercial implementation. The use of the DMC catalyst in combination with castor oil as an initiator has led to at least two lines of commercial products for the flexible foam market. Lupranol Balance 50 (BASF) and Multranol R-3524 and R-3525 (Bayer) are used for flexible slabstock foams and are produced by the direct alkoxylation of castor oil. 

Beside the structure of the monomer, also the type of catalyst used should play an important role in favouring the synthesis of either prevailingly (R) and (S) separable polymers or random (R) (S) copolymers from racemic monomers until now all the separable polymers have been produced by heterogeneous coordination catalysts. 

In all the low pressure PE processes the polymer is formed through coordination polymerisation. Three basic catalyst types are used chromium oxide, Ziegler-Natta and single-site catalysts. The catalyst type together with the process defines the basic structure and properties of the polyethylene produced. Apart from the MWD and comonomer distribution that a certain catalyst produces in polymerisation in one reactor, two or more cascaded reactors with different polymerisation conditions increase the freedom to tailor... 

The development of organometallic initiators, both of the lithium type and of the transition-metal coordination type, occurred rapidly in the decade following the late 1950s. The lithium initiators were developed without the fanfare of coordination-type initiators. This situation developed because of the remarkable ability of the coordination catalysts to induce stereospecific polymerization of a-olefins. 

Polymerisations and copolymerisations of heterounsaturated and heterocyclic monomers in the presence of coordination catalysts constitute a distinct group of coordination polymerisation processes. Considering the nature of the coordination bond of the a type between the monomer heteroatom (beyond carbon monoxide [60]) and the metal atom, the complexes formed differ essentially from the re complexes of unsaturated hydrocarbon monomers with transition metals. 

Since olefin insertion into the metal carbon bond has been established to be of the cis type, it has been considered to proceed by a concerted mechanism involving the formation of a four-membered transition state. However, various models of active centres and of the insertion mechanism have been proposed for olefin polymerisation systems with coordination catalysts. 

Knowledge of the coordination polymerisation of olefins would not be complete without consideration of the types of process used in industry for polyolefin manufacture. Problems encountered in production influence developments in the area of catalysis in olefin polymerisation, an improvement in a catalyst being defined as leading to a reduction in the cost of making the polymer or giving better product properties. Therefore, the principal types of polyolefin production involving coordination catalysts of various types are dealt with briefly. Since modern polyolefin production processes offer a versatile range of polymers, the main commercially available olefin polymerisation products and their typical uses are also considered. 

However, no method of polymerisation known before 1954 allowed one to obtain polymers with a high regularity of structure from the most common conjugated dienes. A true breakthrough in the development of conjugated diene rubbers took place after the discovery of stereospecific polymerisation with transition metal-based coordination catalysts. From the late 1950s, a rapid development of industrial production of solution types of polybutadiene by means of polymerisation with Ziegler-Natta catalysts was observed. 

Both isomers are polymerised by some coordination catalysts, whereas with others only the (E) isomer is polymerised. Of the various possible stereoregular poly(l,3-pentadiene)s, only five have been prepared so far cis- 1,4-isotactic [11,12], trans- 1,4-isotactic [13], cA-l,4-syndiotactic [14,15], 1,2-cA-syndiotactic [16] and 1,2-traro-syndiotactic [17] polymers. In addition to these stereoregular polymers, amorphous polymers of various types, consisting of different monomeric units in variable proportions, have been obtained [7]. 

In general, copolymerisation of conjugated dienes with coordination catalysts is difficult because of the strong selectivity of the catalysts towards monomers of the particular type. The use of suitable catalysts, however, makes it possible to obtain copolymers of conjugated dienes with various monomers. Obtained copolymers can be random or alternating, depending on the kind of comonomers and catalysts. 

Give reasons why pristine polyacetylenes formed with coordination catalysts of various types at low temperature are of the cis configuration. 

Azacyclic monomers containing an endocyclic nitrogen atom which are most commonly subjected to polymerisation in the presence of coordination catalysts include a-aminoacid /V-carboxyanhydrides. However, other monomers of this type, such as morpholine-2,5-dione, have also been subjected to coordination polymerisation. 

It should be noted that, in the presence of zinc-based coordination catalysts of the EtZnX type, the cyclotrimerisation of aliphatic as well as aromatic isocyanates occurred exclusively [269]. 

In 1955 investigators from the Firestone Tire and Rubber Company and the B. F. Goodrich Company announced the synthesis of polyisoprene with over 90 percent cis-1,4 structure. The work at Firestone was based on lithium metal catalysts, whereas the work at Goodrich was the result of using Ziegler-Natta type coordination catalysts.22,23... 

Since the discovery of coordination catalysts based on various metal alkoxides (Fe, Zn, Al), such catalysts have proved of great value for the preparation of high-molecular-weight polymers. Many studies have been devoted to coordination polymerization, but the mechanism has not yet been fully elucidated. Depending on the type of the catalyst and the metal employed, we can speak of coordinate cationic and coordinate anionic processes. 

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