Polymerisation reactions reduction activation

We have already come across a few examples in which oxidation-reduction reaction can initiate vinyl polymerisation. Such a polymerisation reaction is known as redox polymerisation. In such reactions the oxidant is generally referred to as initiator and the reductant as activator. 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

The reduction in the propagation rate constant under the effect of aromatic hydrocarbon is due to hydrocarbon and diene monomer competing for coordination at the active centre. The effect of slowing down the polymerisation reaction is due to the fact that aromatic hydrocarbon forms an arene complex with the lanthanide atom. The higher the 7t-electron-donating ability of the aromatic hydrocarbon, the lower the rate constant for polydiene chain propagation. Thus, the rate of polymerisation of dienes decreases in the series benzene > toluene > xylene [11, 14, 26, 28]. 

Neither for olefins nor for heterocyclic monomers do we yet have a sufficiently extensive body of activation energies of the kp-s to make a detailed discussion profitable. It is worth noting, however, that for the cationic (as opposed to the pseudo-cationic) polymerisation of olefins in solvents of DC greater than about 10, it is likely that a reduction of the temperature does not affect the rate except through its effect on k p, since these reactions are mainly carried by free ions only. 

Both the intramolecular and the intermolecular secondary metathesis reactions affect the polymerisation kinetics by decreasing the rate of polymerisation, because a fraction of the active sites that should be available as propagation species are involved in these non-productive metathesis reactions. The kinetics of polymerisation in the presence of metal alkyl-activated and related catalysts shows in some cases a tendency towards retardation, again due to gradual catalyst deactivation [123]. Moreover, several other specific reactions can influence the polymerisation. Among them, the addition of carbene species to an olefinic double bond, resulting in the formation of cyclopropane derivatives [108], and metallacycle decomposition via reductive elimination of cyclopropane [109] deserve attention. 

Finally, the last few years have seen the first examples of the use of molecular-imprinted, polymer-supported catalysts for achieving product selectivity. The imprinted cavities are tailored in such a way that the course of a chemical reaction is directed towards one of the possible products. In the previous section it has already been shown that molecularly imprinted polymers used as microreactors are able to impart to a given reaction a different regio- and stereo-selectivity with respect to the same reaction in solution. Attempts towards an imprinted enantio-selective catalyst were reported by Gamez and co-workers who employed as template monomer an optically active, polymerisable ruthenium complex bearing in its coordination sphere an enantiomerically pure alkoxide [121]. After polymerisation, the alkoxide was split off and the resulting polymer-supported catalyst was used for enantio-selective hydride transfer reductions. The obtained selectivity was higher than for a polymer prepared without the optically active alkoxide but lower than for the same ruthenium complex in solution. 

The mechanism of trimerisation of butadiene by a mixed cobalt(ii) chloride-aluminium triethyl catalyst has been inferred from the natures of the three products characterised. The determination of the enthalpy of dimerisation of aluminium triethyl provides a useful piece of thermochemical data for quantitative discussion of the role and energetics of aluminium triethyl in this type of reaction. Polymerisation of isoprene in the presence of Fe(acac)3-aluminium triethyl-pyridine derivatives mixtures has a negative apparent activation enthalpy, which can be attributed to the instability of the catalytic complex at elevated temperatures. Bis-cyclo-octatetraeneiron(o) is an effective oligomerisation catalyst. The composition of products accessible only by hydrogen migration indicates an oxidative addition-reductive elimination mechanism rather than insertion. 

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