Polymerisation ionic

A number of important addition polymers are produced by ionic mechanisms. Although the process involves initiation, propagation and termination stages the growing unit is an ion rather than a radical. 

The process of anionic polymerisation was first used some 60 or more years ago in the sodium-catalysed production of polybutadiene (Buna Rubers). Typical catalysts include alkali metals, alkali metal alkyls and sodium naphthalene, and these may be used for opening either a double bond or a ring structure to bring about polymerisation. Although the process is not of major importance with the production of plastics materials, it is very important in the production of synthetic rubbers. In addition the method has certain special features that make it of particular interest. 

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. 

Anionic polymerisation is more likely to proceed when there are electron-withdrawing substituents present in the monomer (e.g.—CN,—NO2 and phenyl). In principle initiation may take place either by addition of an anion to the monomer, viz  

In the absence of impurities there is frequently no termination step in anionic polymerisations. Hence the monomer will continue to grow until all the monomer is consumed. Under certain conditions addition of further monomer, even after an interval of several weeks, will cause the dormant polymerisation process to proceed. The process is known as living polymerisation and the products as living polymers. Of particular interest is the fact that the follow-up monomer may be of a different species and this enables block copolymers to be produced. This technique is important with certain types of thermoplastic elastomer and some rather specialised styrene-based plastics. 

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The details of the commercial preparation of acetal homo- and copolymers are discussed later. One aspect of the polymerisation so pervades the chemistry of the resulting polymers that familiarity with it is a prerequisite for understanding the chemistry of the polymers, the often subde differences between homo- and copolymers, and the difficulties which had to be overcome to make the polymers commercially useful. The ionic polymerisations of formaldehyde and trioxane are equiUbrium reactions. Unless suitable measures are taken, polymer will begin to revert to monomeric formaldehyde at processing temperatures by depolymerisation (called unsipping) which begins at chain ends. 

H. Yuki and co-workers, in O. Vogl and J. Eumkawan, eds., Ionic Polymerisation, Marcel Dekker, Inc., New York, 1976. 

M. Szwarc and M. Van Beylen, Ionic Polymerisation and Pining Polymers, Chapman and HaU, New York, 1993. 

Acrylate esters can be polymerised in a variety of ways. Among these is ionic polymerisation, which although possible (6—9), has not found industrial apphcation, and practically all commercial acryUc elastomers are produced by free-radical polymerisation. Of the four methods available, ie, bulk, solution, suspension, and emulsion polymerisation, only aqueous suspension and emulsion polymerisation are used to produce the ACMs present in the market. Bulk polymerisation of acrylate monomers is hasardous because it does not allow efficient heat exchange, requited by the extremely exothermic reaction. 

As a result of the work of Ziegler in Germany, Natta in Italy and Pease and Roedel in the United States, the process of co-ordination polymerisation, a process related to ionic polymerisation, became of significance in the late 1950s. This process is today used in the commercial manufacture of polypropylene and polyethylene and has also been used in the laboratory for the manufacture of many novel polymers. In principle the catalyst system used governs the way in which a monomer and a growing chain approach each other and because of this it is possible to produce stereoregular polymers. 

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. 

The ionic polymerisation of styrene is as dangerous. Interlaminar compounds of sodium or potassium with graphite catalyse the polymerisation of styrene. This method can usually be controlled. Nevertheless, it gives rise to detonations. It was assumed that in these cases the lamellar structure of graphite is destroyed and the metallic particles dispersed. 

Interlaminar compounds of sodium or potassium in graphite will ionically polymerise styrene (and other monomers) smoothly. The occasional explosions experienced were probably due to rapid collapse of the layer structure and release of very finely divided metal. 

Figure 24 Schemes of propagation steps in ionic polymerisations.

With the exception of LDPE, polyolefins like other polyethylenes and polypropylene, which represent the largest amount of vinyl-type polymers produced in the world, are neither synthesized by radical nor by classical ionic polymerisation processes. Different types of polymerisation catalysts are in use for these purposes. The Cr-based Phillips catalyst, Ziegler-Natta type catalysts, metallocene or other more recently discovered catalysts, including late transition metal catalysts, are all characterized by their propagation step where the olefin monomer inserts into a carbon-transition metal link. ... 

Many ionic polymerisation routes (both cationic and anionic) are also used to make industrial polymers. The end group functionality typically depends on the initiator used to make the polymer. 

The methodology employed, however, is applicable to many other free radical polymers generated from vinyl monomers (such as, e.g., polystyrene). It should be noted that this methodology is also equally applicable to many polymers generated by condensation and ionic polymerisation routes. 

A process related to ionic polymerisation where the catalyst system complexes of aluminium alkyls and titanium halides governs the way in which a monomer and a growing chain approach each other. 

Ionic polymerisation is an addition polymerisation and is of commercial importance. 

History of ionic polymerisation is very old and dates back to as early as 1789 AD when resinification of terpentine by concentrated sulphuric acid was first reported. 

In 1936, Lewis acids were used as catalysts for polymerisation. Grignard reagents were used for polymerisation in 1945. Since 1956, there are innumerable paper on different types of ionic polymerisations. 

Ionic polymerisation forms high Molecular weight products and reactions can be easily carried out at room temperature or low temperature. Ionic polymerisations are chain reactions and are analogous to radical chain reactions. They also involve initiation and propagation steps. 

Ionic polymerisation can be categorised into two classes, (i) Cationic polymerisation and (ii) Anionic polymerisation, depending upon the nature of ions used for the initiation of polymerisation. The cationic polymerisation refers to the process in which a positive ion is used for initiation. When a negative ion is used for initiation of polymerisation, then process is referred to as anionic polymerisation. 

Suggestions Concerning the Ionic Polymerisation of Vinyl Ethers (1950)... 

Other evidence indicating true ionic polymerisation was found in some reactions in which, because of the relatively low molar ratio [styrene]/[HC104], the styrene sequestered by complexing with the ester was a considerable fraction of the total [styrene], and therefore the colour developed while there was still an appreciable concentration of monomer. In these experiments the reaction curves showed a marked kink, corresponding to a spectacular increase in rate, just at the instant of color formation, and the previously very slow polymerisation was brought to completion in a few seconds from this time. 

Suggestions Concerning the Ionic Polymerisation of Vinyl Ethers, S.D. Hamann, P.H. Plesch and H.A. Skinner, Scientific Proceedings of the Royal Dublin Society, 1950, 25 (N.S.) 141-144. 

M. SzwARC, M. VAN Beylen, Ionic Polymerisation and Living Polymers Chapman and Hall, New York, USA, 1993. 

Fuoss (40) has improved Bjerrum s original treatment (37) of this situation and, although a number of other sophistications have been introduced, his formulation (41) is the one most used today. In fact rather fortuitously the relatively low dielectric constants of solvents employed in organic chemical reactions, particularly ionic polymerisations, are ideal media for the application of these theories. The analysis carried out by Fuoss leads not surprisingly to an equation... 

Cyclopentadiene undergoes ionic polymerisation by Friedel-Crafts catalysts32. Its polymerisation by y-rays is markedly suppressed by ammonia or amines but much less by diphenylpicrylhydrazyl or oxygen33. This again points to the ionic rather than the free radical nature of the radiation-induced polymerisation. The quenching effect of ammonia was postulated as due to reactions of the type... 

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