Cationic chain polymerisation

Unlike radical chain polymerisation, initiation in cationic polymerisation uses a true catalyst that is recovered at the end of the polymerisation and is not incorporated at one end of the growing chain. Catalysts for cationic chain polymerisation are molecules able to withdraw electrons, mainly Bronsted (H2SC>4, H3PO4) and Lewis acids (BF3, A1C13, SnCh). The choice of solvent for cationic polymerisation is also important because it plays a major role in the association between cation and counter ion. A too tight association will prevent monomer insertion during the propagation step. However, the use of "stabilized"... 

Figure 25 Overall cationic chain polymerisation mechanism of isobutylene.

Transfer reaction to the monomer, leading to the insertion of an unsaturated end group, is an important reaction in cationic chain polymerisation. As the activation energies of both termination and transfer reactions are higher than that of the propagation step, cationic chain polymerisation can only lead to high molecular masses when undertaken at low temperatures, typically — 100°C. 

Ionic chain polymerisations follow the same basic steps as radical chain polymerisation and are said to be either cationic or anionic depending on the nature of the ion formed in the initiation step. Schemes of the insertion during the propagation step in cationic and anionic chain polymerisations are shown in Figure 24. 

In this discussion the author also explains for the first time why initiating sytems giving complex anions are more effective for polymerisations than those which yield simple anions it is because the latter can and do terminate cationic chains by neutralisation much more effectively than the former. 

The use of different catalytic systems of both ionic [10] and ionic-coordination [13, 14] types in piperylene polymerisation has been proposed. Because of a high reaction rate of chain transfer to monomer (which grows as catalyst acidity increases) and to solvent (which drops as solvent polarity increases) in cationic polymerisation, a polymer with low MW is obtained. Some halides of metals of groups III-V were tested as catalysts of cationic piperylene polymerisation the most suitable were TiCl4 and SnC. The application of SbCl5 and InClj does not ensure an acceptable polymerisation rate and in the case of using AICI3, the insoluble polymer is obtained. 

Chain polymerisation can be classified in four different categories (i) free radical, (ii) anionic, (iii) cationic and (iv) coordination polymerisation. 

In this process, the catalyst produces a cation, usually an ion, which reacts with the n-bond of the electron donating substituted monomer to form the corresponding carbo-cation. Chain propagation is started by the addition of subsequent monomer molecules to this carbo-cation. The chain termination occurs by the transfer of labile atoms or substituents from the catalyst, monomer or other species present in the reaction medium. The whole sequence of the polymerisation reaction is shown in Fig. 1.4. 

In this type of polymerisation an initiating molecule is required so that it can attack a monomer molecule to start the polymerisation. This initiating molecule may be a radical, anion or cation. Chain growth polymerisation is initiated by free-radical, anion or cation proceeded by three steps initiation, propagation and termination. The chemical nature of the substituent group determines the mechanism. 

In the absence of transfer agent, Polymer chains with active, ends which are known as living polymers can be synthesised. In case of cationic polymerisations, termination step is very slow. Ionic reactions are largely affected by solvents used. 

However, the chain termination is much more complicated in case of cationic polymerisation. There cannot be mutual termination in case of cationic polymerisation because the growing chains carry similar charges, and repel each other. Hence there can be three types of termination in case of cationic... 

Isobutene is one of the very small number of aliphatic hydrocarbons which form linear high polymers by cationic catalysis (see Section 5). The reason for this is that only in these few among the lower aliphatic olefins is there found the right balance of those factors which determine the path of a cationic polymerisation. For the formation of linear high polymers it is necessary that the propagation reaction should be much faster than all alternative reactions of the growing end of the chain and for any appreciable numbers of chains to be formed at all, the initiation must be fast. ... 

Once a compound has been shown to polymerise, the most interesting question for me is What is stopping the chains from growing When that question has been answered we must know much about the kinetics of the system and at least a little about its chemistry. Before entering into an account of the reactions which stop chains from growing, it is important to make once again a clear distinction between termination and transfer reactions. There is no reason for not adhering to the radical chemist s definition of termination a reaction in which the chain-carrier is destroyed. In cationic polymerizations there are two main types of termination reaction ... 

The termination reactions are probably more obscure than the other chain-breaking reactions in cationic polymerisation because in most systems they are unimportant compared to the transfer reactions. The only unambiguous evidence for the existence of a termination reaction can come from rate studies of the whole course of the reaction. Several types of behaviour can be distinguished ... 

Reaction with solvent - The solvent influences the course of cationic reactions not only through its dielectric constant, but also because many substances used as solvents are far from inert in these reactions [22, 23]. Although much more experimental material is required before a full treatment of the subject becomes possible, at least one example, the cationic polymerisation of styrene in toluene, is amenable to quantitative discussion. Experiment shows that polymerisation is rapid and complete, the molecular weight is low and the polymer contains para-substituted rings which are almost certainly tolyl endgroups [22]. Theoretically, a polystyryl carbonium ion can react with toluene in six different ways, only two of which (a.l and b. 1 below) can lead to tolyl endgroups in the first case the tolyl group is at the end of the terminated chain, in the second it is the start of a new chain. The alternative reactions can be represented as follows... 

We concluded, therefore, that in sufficiently pure alkyl halide solvents the tert-alkyl tetrahaloaluminates are stable electrolytes and that previous failures to produce them, and the consequent legend of the instability of tcrt-alkyl carbenium ions, arose from the use of inappropriate and insufficiently rigorous experimental techniques. On this basis it seems highly probable that in the polymerised solutions the cations R+ partaking in reaction (viii) were also original ions, i.e., 2 at the end of a live chain and 3 and 4 formed by alumination of a terminal double bond, and not derived ions formed by degradative reactions of monomer or polymer. 

The present paper is an attempt to unravel a rather confused aspect of cationoid polymerisations. This concerns the phenomenon comprised in the term monomer complexation of the growing cation . The idea seems to have occurred for the first time in the work of Fontana and Kidder on the polymerisation of propene by AlBr3 and HBr in w-butane [3]. The kinetics indicated a reaction of zero order with respect to monomer, M to explain this, it was assumed that the growing end of the chain, written as a carbenium ion, Pn+, is complexed with M and that the rate-determining growth step is an isomerisation of this complex ... 

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