Telomerisation mechanism

Consider a telomer being formed from a cyclopentenyl polymer growing under the pairwise mechanism (Scheme 12.14) with growth being curtailed by cross-metathesis under two extreme conditions (i) with only pent-2-ene present (C4 C5 C6 = 0 100 0) and (ii) with a fully equilibrated mixture of acyclic monoalkenes (C4 C5 C6 = 1 2 1). Under condition (i), one would expect the formation of only hierarchical telomers (n = 1,2,3,4,5, etc.) of the type (C2)-[(cyc-C5) ]-(C3) as the pent-2-ene is split into a C2 and a C3 unit across the growing cyclo polyene. In contrast, under condition (ii), one would expect each hierarchical telomer to be formed in a 1 2 1 ratio of (C2)-[(cyc-C5)n]-(C2) (C2)-[(cyc-C5) ]-(C3) (C3)-[(cyc-Q)n]-(C3)> depending on whether there is cross-metathesis with C4, C5 or C6 (ratio = 1 2 1). The outcome will thus depend on how quickly the pent-2-ene is equilibrated by homo-metathesis to yield the C4, C5 and C6 mixture. Analysis of the rate of pent-2-ene homo-metathesis showed that it was not fast. Indeed, it proceeded at approximately the same rate as the telomerisation reaction. One would thus expect the telomer product early in the reaction to be essentially pure (C2)-[(cyc-C5) ]-(C3) species. Then, as C4 and C6 increase in concentration relative to C5, formation of the (C2)-[(cyc-C5) ]-(C2) and (C3)-[(cyc-C5) ]-(C3) telomers should increase proportionally. This was not found to be the case. 

Loftus [22], Recently, we have reviewed such a reaction [23, 24] in which the mechanism and kinetics of radical and redox telomerisations are described. 

Below, the initiation step and mechanisms involved in telomerisation are described, followed by the various processes used, the telogens, the monomers and their respective reactivities. The following part deals with the pseudoliving radical telomerisation of fluorinated monomers and finally different applications of fluorinated telomers are given. 

Telomerisation can be initiated from various processes photochemical (in the presence of UV, X or y rays), in the presence of radical initiator or redox catalysts, or thermally. For each case, different mechanisms have been proposed. Below are explained the initiation processes and some examples are listed in Table 1. 

As noted in both radical and redox mechanisms of telomerisation of fluoro-alkenes, termination reactions always occur by recombination and not by disproportionation [29]. 

The mechanism is rather similar to that of the radical telomerisation except for the initiation step in which the radicals are produced from the telogen on heating as follows ... 

Scheme 1. Mechanism of telomerisation of fluoroalkene F2C = CRR with RpOF (in order to generalise the formulae, the expression C2F2RR has been prefered to CF2CRR or CRR CF2 since the nature of R and R (which can be either electron-withdrawing or electron-donating groups) influences the sense of addition of RfO to a preferential site of the fluoroolefins).

A series of various alcohols were also investigated in the photochemical induced telomerisation of HFP at room temperature and led mainly to monoadducts composed of both isomers. This can be explained according to the mechanism proposed by Haszeldine et al. [25] presented in Sect. 2.1. Such a monoadduct is now a commercially available fluoroalcohol produced by several companies (e.g. Hoechst). 

Even if the reactivity of monomers is described in Sect. 5, it is worth comparing those of CF3CH = CF2 and CF3-CF=CH2 about CF3I. Only this latter olefin leads to telomers, useful for hydraulic fluids, lubricants and for heat transfer media [293 ]. Similarly, C2F5CF = CF2 has been telomerised thermally by Paciorek et al. [294] who suggested a valuable mechanism. 

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