Anionic 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... 

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. 

Ionic polymerisation is a well-known technique for the preparation of graft copolymers but the fate of these reactions is determined by the reaction conditions. Since the discovery of living polymerisation , (anionic polymerisation) [67] it has become an excellent method for the synthesis of block and graft copolymers. In anionic polymerisation the graft copolymerisation is initiated by the anion generated by the reaction of bases with acidic protons in the polymer chain as shown in Scheme 2. 

Li-NMR has been widely applied to the structural characterisation of lithium initiators for anionic polymerisation. Anionic polymerisation of diene monomer with lithium... 

A whole series of very similar salts of composition MP5O14 (M = La, Ce, Pr, Nd, Sm, Eu, Gd, Ho, Tb, Dy, Er, Tm, Yb, Lu, Y, Bi) have been prepared and shown to contain highly polymerised anions in which some of the PO4 tetrahedra share three of their comers with neighbouring tetrahedra. Among these room-temperature-stable salts, three types of structure have been found. One of these is characterised by anionic ribbons as shown below [75]. 

Ito and co-workers [39] applied C-NMR to the determination of end groups in PS polymerised anionically with n-butyl lithium as the initiator. Polymers with molecular weights between 1000 and several million were included in this study. 

The electron-withdrawing (electrophilic) cyano group makes these monomers very susceptible to polymerisation by nucleophilic species, i.e., they polymerise anionically. Even very weak nucleophiles such as the OH" ions from atmospheric moisture on surfaces are sufficient to polymerise the monomers in seconds if they are confined between two surfaces ... 

Polymerisations may be categorised by both the polymerisation mechanism (e.g. radical polymerisation, anionic polymerisation etc.) and by the polymerisation technique (e.g. solution polymerisation, emulsion polymerisation etc.). A third factor is how the reactor is operated in batch mode, or by adding monomers during the process (semi-continuous) or by continuous operation. Mechanism, technique and process strategies (mode of operation) all have an influence on rates of polymerisation and characteristics of the formed polymer. In this chapter, we will focus on the special characteristics that can be distinguished in an emulsion polymerisation related to rate, development of molar mass and chemical composition. In Chapter 4 the effects of the process strategy will be discussed. 

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