Anion polymerisation

Homopolymer. Formaldehyde polymerises by both anionic and cationic mechanisms. Strong acids are needed to initiate cationic polymerisation. Anionic polymerisation, which can be initiated by relatively weak bases (eg, pyridine), can be represented by the following equations Initiation... 

A. D. Broske and co-workers, in T. E. Hogen-Esch and J. Smid, eds., Kecent Advances in Anionic Polymerisation, Elsevier, New York, 1987, p. 363. 

M. Motton, Anionic Polymerisation Principles and Practice, Academic Press, Inc., New York, 1983. 

M. Szwarc, Eiving Polymers andMechanisms of Anionic Polymerisation, Spriager-Vedag, Berlin, 1983. 

The requirements for a polymerisation to be truly living are that the propagating chain ends must not terminate during polymerisation. If the initiation, propagation, and termination steps are sequential, ie, all of the chains are initiated and then propagate at the same time without any termination, then monodisperse (ie, = 1.0) polymer is produced. In general, anionic polymerisation is the only mechanism that yields truly living styrene... 

Anionic polymerisation techniques aie one of many ways to synthesise a special class of block copolymers, lefeiied to as star block copolymers (eq. 25) (33). Specifically, a "living" SB block is coupled with a silyl haUde coupling agent. The term living polymerisation refers to a chain polymerisation that proceeds in the absence of termination or transfer reactions. 

R. MUkovich, in J. E. McGrath, A., Anionic Polymerisation Kinetics, Mechanisms, and Synthesis, ACS Symposium Series, No. 166, American Chemical Society, Washington, D.C., 1981, p. 41. 

The process of anionic polymerisation was first used some 60 or more years ago in the sodium-catalysed production of polybutadiene (Buna Rubbers). 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 eause 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. 

A further feature of anionic polymerisation is that, under very carefully controlled eonditions, it may be possible to produee a polymer sample which is virtually monodisperse, i.e. the molecules are all of the same size. This is in contrast to free-radical polymerisations which, because of the randomness of both chain initiation and termination, yield polymers with a wide molecular size distribution, i.e. they are said to be polydisperse. In order to produce monodisperse polymers it is necessary that the following requirements be met ... 

Yet another feature of anionic polymerisation is the possibility of coupling chains together at their living ends . Where the coupling agent is bifunctional... 

Tbe system may be used for homopolymers and for block copolymers. Some commercial SBS triblock thermoplastic rubbers and the closely related K-resins produced by Phillips are of this type. Anionic polymerisation methods are of current interest in the preparation of certain diene rubbers. 

The styrene-diene triblocks, the main subject of this section, are made by sequential anionic polymerisation (see Chapter 2). In a typical system cc-butyl-lithium is used to initiate styrene polymerisation in a solvent such as cyclohexane. This is a specific reaction of the type... 

In dry air and in the presence of polymerisation inhibitors methyl and ethyl 2-cyanoacrylates have a storage life of many months. Whilst they may be polymerised by free-radical methods, anionic polymerisation is of greater significance. A very weak base, such as water, can bring about rapid polymerisation and in practice a trace of moisture on a substrate is enough to allow polymerisation to occur within a few seconds of closing the joint and excluding the air. (As with many acrylic monomers air can inhibit or severely retard polymerisation). 

Closely related to these but thermoplastic rather than rubber-like in character are the K-resins developed hy Phillips. These resins comprise star-shaped butadiene-styrene block copolymers containing about 75% styrene and, like SBS thermoplastic elastomers, are produced by sequential anionic polymerisation (see Chapter 2). 

The polymerisation casting of nylon 6 in situ in the mould has been developed in recent years. Anionic polymerisation is normally employed a typical system uses as a catalyst 0.1-1 mol.% of acetic caprolactam and 0.15-0.50 mol.% of the... 

The described experimental rig for the anionic polymerisation of dienes has been shown to behave as an ideal CSTR. The mathematical model developed allows the prediction of the MWD at future points in the reactor history, once suitable kinetic parameters have been estimated. 

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. 

As is the case for cationic polymerisation, anionic polymerisation can terminate by only one mechanism, that is by proton transfer to give a terminally unsaturated polymer. However, proton transfer to initiator is rare - in the example just quoted, it would involve the formation of the unstable species NaH containing hydride ions. Instead proton transfer has to occur to some kind of impurity which is capable for forming a more stable product. This leads to the interesting situation that where that monomer has been rigorously purified, termination cannot occur. Instead reaction continues until all of the monomer has been consumed but leaves the anionic centre intact. Addition of extra monomer causes further polymerisation to take place. The potentially reactive materials that result from anionic initiation are known as living polymers. 

Monodisperse polymers for this purpose may be obtained from commercial suppliers, but they are expensive and for many polymers not available in sufficiently narrow molar mass fractions. For this reason, the not very satisfactory procedure is sometimes adopted of calibration with polystyrene, since this polymer can be relatively easily prepared in discrete, monodisperse fractions using anionic polymerisation. The unknown polymer is then... 

Thermal explosion which occurred during fast anionic polymerisation of styrene, catalysed by butyllithium, was prevented by addition of low MW polystyrene before the catalyst. 

Anionic polymerisation mechanism is illustrated in Figure 26 with the example of styrene polymerisation. 

The active species in anionic chain polymerisations are anionic growing chain ends. The main characteristic of such a process is the almost total absence of termination and transfer reactions. For this reason, anionic polymerisation is often called "living polymerisation". 

Synthesis of vinyl block copolymers is accomplished by living polymerisation, mostly by anionic polymerisation. Several strategies can be used, illustrated here by the example of the Styrene-Butadiene-Styrene (or SBS) triblock copolymer. 

Making use of the higher reactivity of butadiene in anionic polymerisation y <1, 2 > 1) to get the triblock SBS copolymer in two steps. The first step is the synthesis of a PS sequence with a living end, then, upon addition of a mixture of styrene and butadiene, butadiene will add first, building a "pure" PB sequence, and styrene will finally build the third sequence (two steps). 

Figure 20 Reaction scheme for the anionic polymerisation of propylene oxide to form PPG (R is the part of alcohol initiator, M the metal from the catalyst and P the propylene oxide).

It is possible to produce a block copolymer by the anionic polymerisation of styrene and butadiene as depicted below. The polystyrene and polybutadiene are mutually incompatible and hence phase separate to give the morphology also depicted below ... 

For the PDMS-grafted systems a somewhat different method was used since PDMS is not soluble in DMF. An anionic polymerisation method was again used to produce "living" PDMS chains, but in this case these were reacted with acetic acid to give hydroxyl-terminated chains. The silica particle dispersions in ethanol were stable, and remained stable on adding n-heptane to give a 1 1 (by volume) solvent mixture in which PDMS is still soluble. 

Polystyrene was prepared by the anionic polymerisation of styrene in toluene plus THF mixtures (4 1 volume ratio) using n-butyl lithium as initiator. After removing a sample for analysis at this stage, the remainder of the living polystyrene was reacted with a five molar excess of trichloromethylsilane for 15 min and then excess methanol introduced. The methoxy-terminated polystyrene was freeze-dried from dioxan. The method described here essentially follows the route proposed by Laible and Hamann (6). 

Polydimethylsiloxane was prepared by the anionic polymerisation of hexamethylcyclotrisiloxane, in a solvent mixture of cyclohexane and THF (4 1 volume ratio), using n-butyllithium as the initiator. After a reaction period of 48 hr termination was effected by adding a small quantity of acetic acid. 

As the propagation end of the Polymer chain is negatively charged, the reaction may be considered as an anionic polymerisation reaction. 

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