Lanthanide catalysts diene polymerisation

The distinctive feature of lanthanide and in particular Ziegler-Natta catalysts is that they allow one to synthesise polydienes with a high content of c/s-1,4-units. In most cases, lanthanides are used in the form of blends and concentrates. About 50% of lanthanides consumed worldwide are used for the production of catalysts for various chemical processes. Using lanthanide catalysts in the manufacture of synthetic rubbers can increase the number of these processes. A large number of studies have been devoted to polymerisation of dienes with lanthanide catalytic systems. Many of these studies are concerned with the problems related to the mechanism that controls the microstructure of polydienes. Although not all aspects of stereoregulation have been clarified, many problems have been solved and possible explanations offered for some of the others [1-4]. 

The polymerisation of dienes occurs when the lanthanide catalyst contains an organometallic component (usually, trialkylaluminum or dialkylaluminum hydride). Systems LnHal3 nL-AlR2lTal do not reveal catalytic activity. Moreover, dialkylaluminum... 

The main parameters of diene polymerisation with lanthanide-based catalytic systems are similar to those of polymerisation with ion-coordinated catalysts on the basis of d-metals. This can be seen from the following facts polymerisation of dienes has an anionic coordinated character [18] at polymerisation temperatures from 20 to 25 C, the reaction is of first order with respect to the monomer and catalyst (this property is independent of the natures of catalyst and hydrocarbon solvent, the only exception to this rule being the system considered in work [18]) for most of catalysts studied [18, 21, 26, 28, 41] and, the apparent activation energy of the reaction of polymerisation of dienes is of the order of 33.5 kj/mol [20, 41]. For lanthanide catalysts, the concentration of active centres is somewhat higher than for conventional Ziegler-Natta catalysts, e.g., for neodimium-based catalysts their content varies from 6-10% [12, 41, 42, 50] to 15-20% [54-57]. 

It is generally accepted that, in the polymerisation of dienes on lanthanide catalysts, the growing chain is attached to the transition metal by an 7t-allyl bond and that the chain growth occurs by incorporation of the monomer via the metal-carbon o-bond. In the case of neodymium catalysts, the delocalised 7t-allyl type structure of the terminal unit has been observed by spectroscopic methods [8, 26, 28, 58-60]. The results reported in these papers show that the relative contents of cis-l,A- and tri2ns-1,4-units in polydienes depend on the type of solvent used, the polymerisation temperature, structure of diene monomer, and the composition of lanthanide-based catalysts. These data can be interpreted in terms of the concept of isomerisation equilibrium between anti- and syn-forms of n-allyl terminal unit. One of the arguments in favour of the existence of this isomerisation... 

Abstract Over the past decade significant advances have been made in the fields of polymerisation, oligomerisation and telomerisation with metal-NHC catalysts. Complexes from across the transition series, as well as lanthanide examples, have been employed as catalysts for these reactions. Recent developments in the use of metal-NHC complexes in a-olefin polymerisation and oligomerisation, CO/olefm copolymerisation, atom-transfer radical polymerisation (ATRP) and diene telomerisation are discnssed in subsequent sections. 

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