Polymerisation mechanisms catalysts

The progress in the polymerisation of epoxides, which are considered to be very important heterocyclic monomers, concerns both mechanistic studies and technological studies aimed at commercialising certain epoxide polymerisation processes. Although wide studies have dealt with the catalyst structure, only a limited number of catalysts have been structurally determined in the solid state as well as in solution. The epoxide polymerisation mechanism, including stereochemical aspects, has been satisfactorily explained on the molecular... 

No precise information about the olefin polymerisation mechanism has been obtained from kinetic measurements in systems with heterogeneous catalysts analysis of kinetic data has not yet afforded consistent indications either concerning monomer adsorption on the catalyst surface or concerning the existence of two steps, i.e. monomer coordination and insertion of the coordinated monomer, in the polymerisation [scheme (2) in chapter 2], Note that, under suitable conditions, each step can be, in principle, the polymerisation rate determining step [241]. Furthermore, no % complexes have been directly identified in the polymerisation process. Indirect indications, however, may favour particular steps [242]. Actually, no general olefin polymerisation mechanism that may be operating in the presence of Ziegler-Natta catalysts exists, but rather the reaction pathway depends on the type of catalyst, the kind of monomer and the polymerisation conditions. 

In the case of olefin polymerisation with homogeneous Ziegler-Natta catalysts, especially with single-site (metallocene) catalysts, the kinetic analysis may become simpler than in the case of polymerisation with heterogeneous catalysts, and in some instances can serve as a very useful tool for uncovering the true polymerisation mechanism [30,243],... 

Catalysts, Models of Active Sites and the Polymerisation Mechanism... 

Characteristic of these models of active sites is the appearance of the OZn O Zn(0)Et species. Considering the structural properties of the discussed catalysts as well as the polymer chain microstructure and the structure of the end groups of poly(propylene oxide) obtained with the PhOZnOCeHi ( -Bu)OZnEt catalyst, a concerted mechanism of epoxide ring-opening polymerisation with catalysts containing multinuclear species, including those with condensed zinc atoms, has been postulated [65,74] ... 

It may be noted that all of the homosteric catalysts were chiral zinc systems, the composition of which was expressed as [RZnOR ]x[R,OZnOR,]J,(v/y < 1). Antisteric catalysts were chiral zinc- or cadmium-based systems of a common compositional feature expressed as [RZnOR ]x[R OZnOR ]>, or [RCdOR ]x [R OCdOR ]) (x/y > 2). The nature of homosteric and antisteric stereoelection has not yet been elucidated fully at the molecular level because the structure of the operating species and the polymerisation mechanism with these catalysts are not clearly established [52]. 

It is interesting, however, to compare the above mechanisms in terms of the different polymerisation behaviour observed for systems with catalysts containing multinuclear species and mononuclear species. In the case of polymerisations with catalysts possessing mononuclear species, two catalyst molecules must approach one another to form the active site for the propagation [scheme (7)]. Such a requirement is not needed for propagation involving the active sites... 

The possibility of changing the polymerisation mechanism depending on the kind of monomer as well as the catalyst, especially in binary or ternary comonomer systems, is obvious. This may also concern change in the nature of the growing species throughout the propagation of one polymer chain in the presence of both multinuclear catalysts [scheme (33)] and mononuclear catalysts [scheme (34)] ... 

Epoxy-amine systems follow an addition step-growth polymerisation mechanism. The two principal reactions of primary and secondary amines with epoxy oligomers are shown in Reaction scheme 1 [30]. These reactions are catalysed by acids, phenols and alcohols (e.g. impurities in commercial epoxy resins). The presence of water causes a tremendous acceleration, but does not alter the network structure. The hydroxyl groups formed by the amine-epoxy addition steps are also active catalysts, so that the curing reaction usually shows an accelerating effect in its early stage (autocatalysis). 

In attempts to understand more fully the mechanism of Ziegler-Natta polymerisations chemists came to develop what have become known as metallocene catalysts for polymerisation. In due course it was found possible to... 

Today the term anionic polymerisation is used to embrace a variety of mechanisms initiated by anionic catalysts and it is now common to use it for all polymerisations initiated by organometallic compounds (other than those that also involve transition metal compounds). Anionic polymerisation does not necessarily imply the presence of a free anion on the growing polymer chain. 

As indicated by the title, these processes are largely due to the work of Ziegler and coworkers. The type of polymerisation involved is sometimes referred to as co-ordination polymerisation since the mechanism involves a catalyst-monomer co-ordination complex or some other directing force that controls the way in which the monomer approaches the growing chain. The co-ordination catalysts are generally formed by the interaction of the alkyls of Groups I-III metals with halides and other derivatives of transition metals in Groups IV-VIII of the Periodic Table. In a typical process the catalyst is prepared from titanium tetrachloride and aluminium triethyl or some related material. 

The monomer, norbomene (or bicyclo[2.2.l]hept-2-ene), is produced by the Diels-Alder addition of ethylene to cyclopentadiene. The monomer is polymerised by a ring-opening mechanism to give a linear polymer with a repeat unit containing both an in-chain five-membered ring and a double bond. Both cis-and trans- structures are obtainable according to the choice of catalyst used ... 

Ionic polymerisation is subdivided into cationic and anionic mechanisms depending on the charge developed in the growing polymer molecule. Typical catalysts for the former, the cationic polymerisation process, are Lewis acids such as AICI3 or BFj, which often require a co-catalyst, usually a Lewis base, in order to bring about polymerisation. 

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