Reactivities of anionic species for the ring opening of ethylene oxide

Epoxide derivatives constitute a broad family of monomers which are able to be polymerized by anionic or related nucleophilic ring-opening mechanism. The main synthetic strategies developed are reviewed in terms of polymerization rates, side reactions, and possibilities for controlling such polymerizations. Ring-opening polymerization of ethylene oxide and substituted epoxides utilizing alkali metal derivatives or other initiating systems in conjunction or not with activating systems are discussed. Emphasis is also given on the use of organic initiators or catalysts to trigger the metal-free ring-opening polymerization of epoxides. Functionalization of polyethers is also described, which includes the use of functional initiators, post-chemical modification of reactive functions carried by the polyether backbone, or functional-monomer insertion. A focus is particularly given on the preparation of polyethers bearing hydroxy, amine, allyl, azide and other reactive groups, at the chain ends or in the chains.

The polymerizations of epoxides by anionic and related nucleophilic ring-opening reactions are reviewed and discussed in this chapter. The elementary mechanisms involved and the main synthetic strategies developed for the preparation of homopolymers and copolymers are described. The chapter is organized as follows. A brief introduction to the subject is provided in the first section. The second section deals with the anionic polymerization of ethylene oxide and substituted epoxides involving alkali metal derivatives as initiators. The use of organic derivatives as either counterion or catalyst is presented in the third section. The fourth section is devoted to the coordinated polymerizations of epoxides by mono- and bimetallic catalytic systems. Finally, the last section examines polymerizations involving monomer activation and the recent progress achieved by this approach in the control of the polymerization of an increasing number of epoxides, including functional ones.

Reactivities of cryptated ion pairs and of free ions were determined by spectrophotometry for the ring-opening of ethylene oxide by 9-methylfluorenylsodium complexed by [222], in THF at +20, −10 and −30 C. These ionic species have the same reactivity which is much lower than that of tight ion pairs. This behaviour may be explained by the increase of delocalization of the negative charge when the interionic distance becomes larger, but coordination of ethylene oxide with the cation may also occur with tight ion pairs leading to electrophilic activation of the monomer.

This chapter discusses the alkali and alkaline earth metal cryptates. The classical approach to cryptands with bridgehead nitrogen atoms involves the high dilution reaction of a secondary bis-amine coronand with the required bis-acid chloride, followed by reduction of the resultant bisamide with diborane. Such an approach has been used to synthesize, for example, and also the related pyridinophane cryptand. The high dilution technique requires gradual mixing of reagents in order to avoid polymerization reactions, although Dye has shown that the reaction may be effected in under a minute if the reagents are efficiently mixed in a suitable flow cell. The simple approach to diazamacrocyclic ligands is in stark contrast to the rather lengthy and/or low-yielding procedures required to synthesize the cryptands with carbon atoms at the bridgehead.

A kinetic study of the anionic polymerization of ethylene oxide has been made in tetrahydrofuran at 20°C, with the cryptate Cs⁺ + [TC] as counterion, [TC] being a spheroidal macrotricyclic ligand. Conductance measurements have been made on THF solutions of ∅₄BCs + [TC]. Ionic associations higher than cryptated ion pairs are negligible for living end concentrations lower than 3 × 10⁻⁴ moll⁻¹. k_± and the alkoxide ion pair dissociation constant K_D were determined from both sets of kinetic data obtained with and without added salt knowing the value of k₋ from kinetic data performed with K⁺ + [222] as counterion. Free alkoxide ions are about twenty times more reactive than cryptated caesium ion pairs.

the reaction of benzaldehyde mth LiCH₂CN in THF is controlled by carbonyl complexation while uith KCH₂CN the reaction is controlled by ionic association. In the reaction of m.Cl.C₆H₄CHCN⁻Li⁺ with substituted benzaldehydes carbonyl complexation prevails with p.CH₃OC₆H₄CHO while ionic association dominates uith PhCHO and p.CNC₆H₄CHO.