The Polymerisation of Alkenyl Monomers

Sawamoto and Higashimura recently used a similar apparatus to follow tire protonation of p-methoxystyrene by HSO3CH3 and by HSO3CF3 in 1,2-dichloroethane at room temperature. These authors assumed bimolecular protonation, without actually carrying out a detailed kinetic study of the system, and reported a second-order rate constant of 0.6 and about 5 x 10 M s respectively, an increase reflecting the large difference in strength between the two acids. These results must however await kinetic confirmation. 

It is surprising to note the paucity of research in this domain, considering its paramount importance in cationic polymerisation. One can only hope that the example given by the excellent work of Kunitake and Takarabe will stimulate other authors to take up similar studies. 

This section is devoted to a systematic analysis of publications concerned in a direct or indirect way with the mechanism of initiation in the cationic polymerisation of alkenyl monomers by Br nsted acids. Included in it are many examples of failures, i.e., systems in which no polymerisation was observed. Such experiments are important because the lack of production of active species can give considerable information a-bout the alternative processes taking place vdien the two reactants are mixed. We have also included all the information we could gather about the state and properties of the initiators in the media relevant to the general context of this review. 

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Biswas and Maity recently reviewed the polymerisation of alkenyl monomers by molecular sieves. The cationic nature of these systems seems well established, but the mechanism of initiation and the possible role of residual water as cocatalyst need further investigation. 

In media of low dielectric constant it is well established that staimic chloride needs a cocatalyst to initiate the cationic polymerisation of alkenyl monomers. This point has been clearly proved for carbon tetrachloride and benzene solutions. In the present section the role of water as cocatalyst in these processes will be examined, but given the substantial amount of literature available on this topic, we will concentrate on important kinetic and mechanistic aspects, relevant to our new interpretation. We remind the reader that much of the older work has been thorou y reviewed in Plesch s second book ... 

The presence of free Lewis acid in these types of systems tends on the contrary to favour the polymerisation of the olefin, or its isomerisation. This situation has been exploited in a large number of investigations in which carboxylic acids have been used as cocatalysts for the cationic polymerisation of alkenyl monomers. The majority of these reports are qualitative in that they do not examine the kinetics and mechanism of these cocatalytic processes. But a few excellent studies have been carried out dealing with these fundamental aspects, and these are reviewed below. 

The use of ethers as cocatalysts for the cationic polymerisation of alkenyl monomers induced by Lewis acids has received little systematic attention and the mechanism through which these compounds operate is not well understood. The complex diethyl-ether-boron fluoride has been extensively used as a very convenient cationic initiator, but mostly for preparative purposes. As in the case of alcohols and water, ethers are known to act as inhibitors or retarders in the cationic polymerisation of olefins, if used obove cocatalytic levels, because they are more nucleophilic than most rr-donor monomers. Imoto and Aoki showed that diethyl ether, tetrahydrofuran, -chloro-diethyl ether and diethyl thioether are inhibitors for the polymerisation of styrene-by the complex BF3 EtjO in benzene at 30 °C, at a concentration lower than that of the catalyst, but high enough (0.5 x 10 M) to quench the active species formation for a time. Their action was temporary in that the quenching reaction consumed them, and therefore induction periods were observed, but the DP s of the polystyrenes were independent of the presence of such compounds, as expected from a classical temporary inhibition. 

The preparation, characterisation and reactivity of acylium ions and acyl complexes and esters have been extensively investigated, partfcularly in the last two decades. Their use as initiators in the cationic polymerisation of alkenyl monomers is however a more recent extension of these studies, except for some old exploratory work. 

Within the scope of this review we shall only consider those compounds possessing one or more alkenyl functions susceptible to activation by electrc hilic attack. Included in this family is a vast array of monomers varying in basicity from ethylene, which is so resistant to protonation that the ethyl carbenium ion has hitherto eluded observations even under the most drastic conditions (see below), and which in fact is equally resistant to cationic polymerisation, to N-vinylcarbazole, whose susceptibility to this type of activation is so pronounced that it can be polymerised by almost any acidic initiator, however weak. We shall also deal with olefins which, because of steric hindrance, can only dimerise (e.g., 1,1-diphenylethylene) or cannot go beyond the stage of protonated or esterified monomeric species (e.g., 1,1-diphenylpropene). The interest of such model compounds is obvious they allow clean and detailed studies to be conducted on the kinetics and mechanism of the initiation steps and on the properties of the resulting products which simulate the active species in cationic polymerisation. The achievements and shortcomings of the latter studies will be discussed below. 

The interaction of an acid with an alkenyl monomer can generate ionic chain carriers, but also covalent products with varying degrees of polarity. It has been shown that in certain systems these ester molecules can propagate the growth of a polymer chain, while in others they are inactive. Another source of covalent species in cationic polymerisation is the collapse (recombination) of the ionic pair or the X displacement from the anion to the carbocation discussed in the previous section. 

The actual observation of ester molecules in a polymerising system involving alkenyl monomers should be a priority among the tasks of future investigations in this field. Extremely interesting results have been obtained in similar studies vrith heterocyclic monomers (see and refs, therein), particularly with tetrahydrofliran. 

The mode of attack of stable carbenium ions on to alkenyl monomers has been proved in some cases to proceed via direct electrophilic addition. However, for certain monomers, e.g., styrene and a-methylstyrene, the mechanism of this reaction has not yet been established unambiguously. We believe that studies on model compounds such as substituted styrenes incapable of polymerising because of steric impediment could provide an answer to this important question. 

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