Halides organic halide polymerisation

If one wants to use only a metal halide as the initiator for an alkene polymerisation, an analysis similar to ours can be made for the metal halide alone. In addition, there is a 1 2 equilibrium for the organic halide [35, 36] and a molecular aggregate <-> single molecule equilibrium associated with the metal halide. Thus, a solution of a carbocation salt is more exactly described by the series of linked equilibria summarised in Scheme 5. 

The first step in catalytic reactions of the Heck type is the oxidative addition of the organic halide to Pd(0) species to form an intermediate organopalladium halide constituting the Pd(II) species. This is followed by insertion of the olefinic bond and subsequent /khydrogen elimination [scheme (30)]. The catalyst is recycled by the reaction of the Pd(II)-hydride species with a base [scheme (31)]. It is worth noting here that palladium species, L2(X)Pd—ArCH=CH2, do not propagate the chain growth polymerisation of the CH2=CHArX monomer via its olefinic bond in the discussed process. 

The general mechanism in atom transfer radical polymerisation is depicted in Scheme 8.11. The main difference to conventional radical polymerisation is in the presence of a metal complex. Free radicals are generated from reaction between the initiator (such as an organic halide) and the metal species which further controls the reaction by reversibly transforming the free radicals into a dormant species.1"6 However, it ought to be pointed out that in ATRP contrary to, for example, Ziegler-Natta-type catalysts, the polymerisation does not take place at the metal centre. 

N-Vinylcarbazole has been polymerised by a whole series of organic halides by Otsu and coworkers In view of the extreme sensitivity of this monomer to traces of... 

Subsequently it was found that in solvents of low dielectric constant (hydrocarbons and carbon tetrachloride) neither polymerisations [2] nor alkylations [3] and isomerisations [4] could be effected by a metal halide alone, but that a third substance, called the co-catalyst, is required. The most common, and indeed ubiquitous co-catalyst is water other substances proved to act in this capacity are alcohols [5], organic acids [6] and nitro-compounds [7]. In each case the catalysis can be ascribed to a complex protonic acid PXnA H+ ... 

The opposite of the stabilisation of an ester is its activation. If we include in the concept ester the alkyl halides, their Friedel-Crafts reactions provide familiar examples of this phenomenon. An unusual example especially relevant to our present considerations is provided by some results made available to me in advance of publication by Giusti and Andruzzi. Their results [38] on the polymerisation of styrene by iodine and hydrogen iodide can be interpreted in terms of an organic iodide derived from styrene, probably 1-phenylethyl iodide, being activated by the co-ordination of one or two molecules of iodine. This process appears to polarise the C—I bond to such an extent that the normally stable ester becomes activated to a chain-propagating species and induces a pseudocationic polymerisation ... 

The lack of isomerisation indicates that a non-ionic process is involved. The reaction does not continue to give a polymerisation of ethylene, because in contrast to the tertiary C-Cl bond which is broken, the primary C-Cl bond which is formed is too strong to be activated sufficiently by the A1C13. For the polymerisations involving an organic (monomer) halide and a metal halide, such as ZnBr2, it seems reasonable to represent the insertion also as occurring via a 6-membered cyclic transition state (II) ... 

Kennedy and collaborators have studied the polymerisation of olefinic monomers induced by catalytic combinations consistir of mixtures of organic chlorides and aluminium alkyls. Although the authors claimed that these mixtures gave rise to carbenium salts capable of initiating viiq l polymermtion, we believe that the process leading to active species does not necessarily imply the formation of transient carbenium ions derived from the catalyst pair. For this reason, these systems are not discussed in this section but rather in Chapter IV together with other aluminium halide initiated polymerisations. 

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