Anionic chain polymerisation

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. 

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". 

Figure 26 Overall anionic chain polymerisation mechanism of styrene initiated by n-butyllithium.

Table 5 Typical end-functionalisation reactions in anionic chain polymerisation...

Reactivity in anionic chain polymerisation of monomers possessing a double carbon=carbon bond is increased by the presence of electro-attractive groups on one of these carbon atoms. Depending on the reactivity of the monomer and on the solvent used, different types of initiator can be used. 

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. 

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

Before vitrification, a heat capacity change as a result of chemical reaction, ACp,react, is noticcd. For the anhydride-cured epoxy and the polyester-styrene resin a minor, but reproducible, and almost linear decrease of Cp with conversion is observed. The former system is supposed to be an anionic chain-growth living polymerisation (without termination), the latter is a chain-growth copolymerisation with termination. 

Figure 1.15 (a) The normal head-to-tail addition propagation mode in the anionically catalysed polymerisation of propylene oxide (b) the chain transfer reaction generating an allyl alkoxide. 

The anionically initiated polymerisation gives highly monodisperse polymers of controlled molecular weight and the silicon in the soluble chain pemits easy and accurate estimation of the amount of soluble polymer associated with the particleso... 

It was pointed out in Section 2.16.9 that anionic living polymerisation can be used to prepare ABA tri>block copolymers suitable for use as thermoplastic elastomers. In such copolymers the A blocks are normally of a homopolymer which is glassy and the B block is of a rubbery homopolymer (e.g. a polydiene such as polybutadiene or polyisoprene). The characteristic properties of these materials stems from the fact that two polymers which contain repeat units of a different chemical type tend to be incompatible on the molecular level. Thus the block copolymers phase separate into domains which are rich in one or the other type of repeat unit. In the case of the polystyrene-polydiene-polystyrene types of tri-block copolymers used for thermoplastic elastomers (with about 25% by weight polystyrene blocks), the structure is phase-separated at ambient temperature into approximately spherical polystyrene-rich domains which are dispersed in a matrix of the polydiene chains. This type of structure is shown schematically in Fig. 4.36 where it can be seen that the polystyrene blocks are anchored in the spherical domains. At ambient temperature the polystyrene is below its Tg whereas the polydiene is above its Tg. Hence the material consists of a rubbery matrix containing a rigid dispersed phase. 

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... 

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. 

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... 

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. 

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

Polymers having one or two carboxyl endgroups are prepared by anionic polymerisation and subsequent chain termination with C02+ and further reaction with mineral acids. 

In case of the anionic polymerisation, since all the chains grow at the same rate, a monodisperse system is obtained in this case. 

The anionic mechanism is similar to that postulated to explain the thermal polymerisation. The apparent general similarity of the polymers produced by the two methods of initiation justifies this. Cross-linking involves additions to the free CN groups, and regular networks with two or more interconnected polymer chains are possible. Thus the structure of the azulmine is highly complex. Termination must occur by reactions of the growing anions with H2CN+ ions formed in the reaction... 

This reaction has been shown to be very rapid77. Sulphuric and acetic acids sup press the polymerisation. Evidently their anions are ineffective as initiators, and the enhanced proton concentration provided by them must reduce the chain lifetime. The slight retarding effect of oxygen could be due to electron scavenging. However, the authors suggest that there may be a small free radical component of the chain reaction, which is inhibited in the presence of oxygen. 

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. 

In this first attempt at a systematic definition of the problem it is recognized explicitly that there may be a multiplicity of chemically distinct chain-carriers growing simultaneously in the same reaction mixture (enieidic polymerisation). The fact that these may include paired and unpaired ions is considered from the point of view of conventional ionic equilibria, and a warning is given that there may be tight and solvent-separated ion-pairs to be considered. This idea, taken over from the theory of anionic polymerisations, was shown much later to be inappropriate for cationic polymerisations 154. ... 

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