Oligomerisation cationic

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. 

Since the proton affinity P(Rn=) of the branched olefin Rn= is certainly greater than P(C2H4), the reaction would be endothermic a similar argument applies to the transfer of carbonium ions. This may account for the low yields observed even in the heterogeneous cationic oligomerisation of ethylene. The most likely transfer reactions are hydride ion and methide ion transfers between oligomer ions and molecules (see below), leading to conjunct polymerisation [12]. 

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. 

Identification by Syntheses of a Product from the Cationic Oligomerisation of Stilbene in Toluene, D.S. Brackman and P.H. Plesch, Chemistry and Industry, 1955, 255. 

Carbocations such as t-butyl cation which are sufficiently electrophilic to react efficiently with styrene and linear dienes are themselves highly unstable and cannot be isolated as stable salts. They tend to oligomerise or react readily even with the most stable anions. Of course they can be generated in situ in a polymerisation reaction (1,95),... 

Lewis acid-free catalysts for acyclic diene metathesis obviate the formation of carbocations, which in turn completely eliminates competing reactions, mostly involving cationic oligomerisation via 1-alkene bonds. Thus, metathesis polycondensation occurs quantitatively to yield high molecular weight poly(l-alke-nylenejs with vinyl end groups and ethylene as a byproduct. 

The first example for biphasic oligomerisation of olefins in ionic liquids was published in 1990, reporting on the dimerisation of propene by nickel(II) catalysts in chloroaluminate ionic liquids of the general formula [cation]Clx-(AlCl3)y with either [C4Ciim]+, [C4py]+ or [(C4)4P]+ as cation.[10] It was found that in basic ionic liquids, y < 0.5, no catalysis took place. Excess chloride, which is present in such basic chloroaluminates, poisons the catalyst and it was shown that nickel compounds of the type NiCkCPRok... 

Despite its fame as the first initiator used in cationic polymerisation, sulphuric acid has not frequently been used for fundamental studies in this field. Ite low vapour pressure hinders proper purification procedures even in high vacuum, and its limited solubflity in the solvents currently used for these investigations is another serious drawback. While alkenes are only oligomerised by this acid, aromatic monomers can give high polymers under suitable conditions. Our present discussion will be limited to the only system investigated in depth, i.e. the polymerisation of stjrene. 

The idea that extensive plasma-gas phase oligomerisation is occurring by means of ion-molecule reactions is neither new nor particularly controversial however, attempting to link the cations thus formed to the deposit growth has proven controversial, and is by no means mainstream thinking. 

Ziegler catalysts studied for the oligomerisation of a-olefins tend to be based on modified first-generation catalysts, i.e. triethylaluminium/titanium tetrachloride. The aluminium/fitanium ratio has a marked effect on product properties, at less than 0.8 1, liquids were produced, whereas at ratios above 1 1, waxy products were obtained [6, 7]. This effect is believed to relate to changes in the catalysis mechanism, from cationic to anionic with higher proportions of aluminium. 

The electrophilic catalysts used for cationic piperylene oligomerisation display different activity which stipulates in principle, the ability of upgrading the commercial production of synthetic drying oil at the stage of cationic piperylene oligomerisation by selecting a catalytic system with corresponding activity and selectivity. 

Table 3.1 Cationic piperylene oligomerisation in the presence of some electrophilic catalysts ... 

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

Use of high pressure through supercritical carbon dioxide (CO2) has been reported to facilitate cationic oligomerisation of soybean oil with boron trifluoride (BF3) etherate as a catalyst, giving products suitable as lubricating agents and hydraulic fluids [6]. 

Naked [Fe4] ions in the gas phase were shown by mass spectrometry to oligomerise ethyne to benzene via the cationic intermediate [Fe(C2H2)m] (m = 1-4)256. r was reported that the ethene ligand in [Fe(CO)2 P(OMe)3 2(Tl2-C2H4)] could be substituted by diphenylacetylene to form [Fe(CX))2 P(OMe)3 2(il -C2Ph2)]. The X-ray crystal structures of the alkene and alkyne complexes were determined257. Reaction of [CpFe P(OMe)3)2(NCMe)]+[PF6] with [PhCXH]... 

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