Oligomerisation and polymerisation

The two main versions of the ions-at-any-price rival theories, namely the large concentration of invisible ions view and the minute concentration of conventional carbenium ions view are both shown to be incompatible with all the facts. It is emphasised that many industrial cationoid oligomerisations and polymerisations are pseudo-cationic. 

Figure 9.7. Example of a new catalyst for ethene oligomerisation and polymerisation...

Cyclophane synthesis necessarily involves the closure of medium- to large-sized rings and is therefore always problematic because of competing oligomerisation and polymerisation reactions. It is beyond the scope of this work to present a detailed treatment of the numerous procedures for preparing cyclophanes, but an excellent overview may be found in the key reference and literature. Some common techniques and particular tricks in this advanced form of organic synthesis are worth referring to, however, and a few important reactions in cyclophane (and macrocyclic) synthesis are listed below ... 

A -Heterocyclic Carbene Complexes in Polymerisation, Oligomerisation and Telomerisation Reactions... 

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. 

Sc (ri -C5Me4)Si(Me)2NBu (PMe3)(p.-H)]2 was a catalyst for the oligomerisation of propene, but-l-ene or pent-l-ene and polymerisation of Using stoichiometric... 

Chain transfer reactions in homogeneous olefin polymerisation systems with metallocene catalysts, which terminate individual polymer chains, in some instances can also terminate the polymerisation kinetic chain. For example, chain transfer with the monomer in propylene oligomerisation or polymerisation, which involves a bond metathesis reactions between the Mt-C species of the growing polymer chain and the C H species of methyl [scheme (45)] or vinyl [scheme (46)] groups in the monomer, gives rise to temporally inactive metal allyl or metal-vinyl species respectively [177, 241, 264] ... 

In the last decade, very significant developments have been made in the fields of polymerisation (oligomerisation and telomerisation) using NHC complexes with metals. However, we want to pay special attention to those processes only catalysed by NHCs without the need of any metal. In this field, Hedrick et al. disclosed in 2002 a transesterification-ring opening polymerisation (ROP) in which imidazolinylidene carbenes catalyse the synthesis of biodegradable polyesters from cyclic esters (Seheme 2.24). More recently, Liu et al. have shown how a different imizadolium carbene is even able to promote the polymerisation of two different monomers in a controlled fashion. NHC-catalysed processes other than ROP have been described, such as the oxa-Michael addition polymerisation of acrylates recently reported by Matsuoka et al. ... 

Piperylene oligomers (cationic oligomerisation, process terminating, and polymerisate washing) [7]. 

The reaction between acrylonitrile and formaldehyde (as paraformaldehyde or tri-oxane), under strong acid catalysis (usually sulphuric) and most often in presence of catalytic quantities of acetic anyhydride, to produce triacrylohexahydrotriazine, is inclined to violent exotherm after an induction period. The runaway can be uncontrollable on sub-molar scale. It may be due to acrylate polymerisation or to increasing reactivity of the formaldehyde equivalent due to progressive de-oligomerisation. Procedures claimed to prevent the risk have been described in the literature but do not seem reliable. 

In a closer examination of the formation of oligomers, Ferris and co-workers found that the reaction is favoured by relatively high salt concentrations (e.g. 1M NaCl), while the presence of divalent cations is not necessary. The formation of RNA oligomers was found to be temperature sensitive the yields decrease when the temperature is raised from 4°C to 50° C. The addition of meteorite material (from 3 meteorites) does not catalyse the polymerisation reaction only galena (PbS) can do this. The authors thus assume that RNA oligomers could have been formed on the young Earth in solutions of alkali metal salts in the presence of montmorillonite and a pH value of 7-9 (Miyakawa et al., 2006). Ferris (2006) has provided a short but up-to-date survey of montmorillonite-catalysed RNA oligomerisation. 

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