Polymerization, anionic controlled

Once the initiating cation has been formed the polymerization is controlled by temperature (technical difficulties may arise if the control is not adequate). A reaction scheme for cationic polymerization of an epoxide using triarylsulphonium hexafluoroantimonate as photo-initiator is shown in Figure 105. Termination of the polymerization often is adventitious, particularly with anions and other bases (theoretically the reaction can continue until the supply of monomer is exhausted). 

Under these reaction conditions, the use of ammonium and related onium salts with nucleophilic anions has been found effective at converting the HCl/SnCU-initiated, uncontrolled polymerizations into controlled/ living processes [105], Similar results are reported for TiCl4-based polymerizations [174,175], Effective salts include tetraalkylammonium and phosphonium salts R4N+Y and R4P+Y (Y = I, Br, Cl, CH3COO R = CH3, C2Hs, 71-C4H9, etc.). As added nucleophiles do in nonpolar solvents, the added salts retard the polymerization, narrow the MWD of the polymers, and render their M values directly proportional to conversion and close to the calculated values (one living chain per initiator molecule). 

Phenyl and alkenyl (—CH=CH2) substituents are electron releasing but they stabilize the product anions by resonance, and so styrene and butadiene can undergo both cationic and anionic polymerizations. Anionic mechanisms are more important, however, since they provide better control over the polymer structure (Chapter 9). 

A number of general reviews on polyphosphazenes have appeared. Specific reviews on polyphosphazenes deal with radiation graft polymerization, anionic polymerization, hydrogel microspheres,controlled biodegradability,coatings, and membrane separation. 

The particular features of anionic polymerization that made the polymer chains living were discussed above. The main requirement for a living polymerization is the absence of any process for spontaneous termination so that the degree of polymerization is controlled by the ratio of monomer to initiator concentrations. The molar-mass of the polymer therefore increases linearly with monomer conversion. On exhaustion of the monomer, the initiation centres remain, so chains may be re-initiated by addition of further monomer. Termination or chain transfer is controlled by the delibemte addition of a reagent to remove the living end. The resulting polymers will also have very narrow molar-mass distributions since rapid initiation ensures that all chains are initiated at the same time. 

Since the discovery of living polymerizations by Swarc in 1956 [1], the area of synthesis and application of well-defined polymer structures has been developed. The livingness of a polymerization is defined as the absence of termination and transfer reactions during the course of the polymerization. If there is also fast initiation and chain-end fidelity, which are prerequisites for the so-called controlled polymerization, well-defined polymers are obtained that have a narrow molar mass distribution as well as defined end groups. Such well-defined polymers can be prepared by various types of living and controlled polymerization techniques, including anionic polymerization [2], controlled radical polymerization [3-5], and cationic polymerization [6, 7]. 

This chapter aims to provide an update on the role of anions as templates. The review is divided in two main sections (a) anion-templated synthesis of assemblies linked together by irreversible bonds (or bonds that are inert under mild experimental conditions) (b) anion templates in systems where the bonds linking the components are reversible and lead to anion-controlled dynamic combinatorial libraries. Since some comprehensive reviews in the area of anion temptation have appeared over the past few years [5-7], this chapter will mainly focus on papers published recently and will aim to show the principles of anion temptation rather than being a comprehensive account of the literature. In addition, the scope of the chapter will be restricted to finite assemblies (molecular or supramolecular) and not polymeric (for a review on molecularly imprinted polymers using anions see Steinke s chapter in this volume). 

The formation of living polymer obeys a completely different mechanism than faced in radial polymerization. With living polymers, highly uniform materials can be synthesized. Living polymerization includes not only anionic polymerization but also atom transfer radical polymerization, living controlled free radical polymerization. 

Acar, M.H., Matyjaszewski, K., 1999. Block copolymers by transformation of living anionic polymerization into controlled/ living atom transfer radical polymerization. Macromol. Chem. Phys. 200 (5), 1094-1100. 

There is already a large number of different conductive polymers. A typical monomer is 3-methylthiophene, which can be electrically polymerized to a polymer coupled by the 2-and 5-positions of the monomer. In the oxidized form, usually called doped , the chains contain positive charges at about every fourth monomer unit. In order to keep the polymer layer electrically neutral, also counter anions should be present in the polymer matrix. It is analytically interesting that the diffusion rate of these counter anions controls the rate of oxidation and reduction of the polymer, and the diffusion rate depends on the size, degree of solvation etc. of the anion. Hence, by a suitable choice of the polymer, it should be possible, at least in principle, to tailor-make sensors for different anions. In addition, it has been shownthat electrically neutral polymers can be incorporated from the solution into the polymer matrix during the polymerization process. This of course extends enormously the possibilities for developing selective sensors without undue efforts to synthesize new electrically polymerizable monomers. 

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