Other Chain Polymerisation Mechanisms

Although the most important chain reactions are those involving free radicals, they are not the only ones that are possible. The reactive centre at the growing end of a polymer molecule may alternatively be ionic in character or involve co-ordination to metal complexes. 

Ionic polymerisation is subdivided into cationic and anionic mechanisms depending on the charge developed in the growing polymer molecule. Typical catalysts for the former, the cationic polymerisation process, are Lewis acids such as AICI3 or BFj, which often require a co-catalyst, usually a Lewis base, in order to bring about polymerisation. 

It is characteristic of cationic polymerisation that high rates of reaction can be obtained at low temperatures. For example isobutylene [2-methylpropene, 

Termination in such a system occurs in what is essentially a reversal of the initiation reaction, i.e, protonation of the (BF30H) species to yield a polymer containing an unsaturated end group (2.1). 

Anionic polymerisation involves the development of a negative charge on the growing polymer molecule. This is achieved through the use of a catalyst that can readily form anions which themselves react with the vinyl monomer. An example is shown in Reaction 2.14. Here the initiator is sodium and the reaction is carried out in liquid ammonia at -75 °C. 

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In a zip mechanism the affinity of the monomer for the template repeating unit is high (strong ionic interactions may result), the monomer molecules are adsorbed on the template and the adjacent molecules bind themselves to each other (in the case of a step polymerisation) or they add to the active ends of the chain (in the case of a chain polymerisation). 

Sonochemistry is also proving to have important applications with polymeric materials. Substantial work has been accomplished in the sonochemical initiation of polymerisation and in the modification of polymers after synthesis (3,5). The use of sonolysis to create radicals which function as radical initiators has been well explored. Similarly the use of sonochemicaHy prepared radicals and other reactive species to modify the surface properties of polymers is being developed, particularly by G. Price. Other effects of ultrasound on long chain polymers tend to be mechanical cleavage, which produces relatively uniform size distributions of shorter chain lengths. 

Today the term anionic polymerisation is used to embrace a variety of mechanisms initiated by anionic catalysts and it is now common to use it for all polymerisations initiated by organometallic compounds (other than those that also involve transition metal compounds). Anionic polymerisation does not necessarily imply the presence of a free anion on the growing polymer chain. 

As indicated by the title, these processes are largely due to the work of Ziegler and coworkers. The type of polymerisation involved is sometimes referred to as co-ordination polymerisation since the mechanism involves a catalyst-monomer co-ordination complex or some other directing force that controls the way in which the monomer approaches the growing chain. The co-ordination catalysts are generally formed by the interaction of the alkyls of Groups I-III metals with halides and other derivatives of transition metals in Groups IV-VIII of the Periodic Table. In a typical process the catalyst is prepared from titanium tetrachloride and aluminium triethyl or some related material. 

Recently many subtle effects of the ligand structure, concentrations of alkene, and conditions on the polymerisation have been reported to have significant effects on molecular weight, regioselectivity, branching, stereoselectivity or enantioselectivity, incorporation of other monomers, chain transfer, etc. Often these subtle effects can be understood from the mechanism, or they contribute to the understanding of the detailed processes going on. 

PVDF is mainly obtained by radical polymerisation of 1,1-difluoroethylene head to tail is the preferred mode of linking between the monomer units, but according to the polymerisation conditions, head to head or tail to tail links may appear. The inversion percentage, which depends upon the polymerisation temperature (3.5% at 20°C, around 6% at 140°C), can be quantified by F or C NMR spectroscopy [30] or FTIR spectroscopy [31], and affects the crystallinity of the polymer and its physical properties. The latter have been extensively summarised by Lovinger [30]. Upon recrystallisation from the melted state, PVDF features a spherulitic structure with a crystalline phase representing 50% of the whole material [32]. Four different crystalline phases (a, jS, y, S) may be identified, but the a phase is the most common as it is the most stable from a thermodynamic point of view. Its helical structure is composed of two antiparallel chains. The other phases may be obtained, as shown by the conversion diagram (Fig. 7), by applying a mechanical or thermal stress or an electrical polarisation. The / phase owns ferroelectric, piezoelectric and pyroelectric properties. 

The chain length in free radical polymerisations is usually lower than would be expected from the mechanism of termination. The reason for this discrepancy is that the growing polymer chain can transfer the radical to other species, leading to termination of one chain, and thus generating a new radical that will react further. The following transfer mechanisms may occur ... 

Vinyl acetate may be easily polymerised in bulk, solution, emulsion and suspension. At conversions above 30%, chain transfer to polymer or monomer may occur. In the case of both polymer and monomer transfer two mechanisms are possible, one at the tertiary carbon, the other (illustrated in Figure 14.4) at the acetate group. 

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