Anionic epoxide polymerization reaction scheme

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). 

Aluminum tetraphenylporphyrin with axial bound chloride, (TPP)AICI, Ic is an effective initiator for the anionic ringopening polymerization of epoxides (11). - - This complex can be easily synthesized by reacting free-base tetraphenylporphyrin, (TPP)H2, with chlorodiethylaluminum in CH2CI2 under nitrogen, where the reaction proceeds rapidly and quantitatively at room temperature with evolution of two equivalents of ethane with respect to (TPP)H2-According to Scheme 1, a variety of aluminum porphyrins can be synthesized using some organoaluminum compounds and they are easily identified by H NMR (Table 2). 

Scheme 3 Polymerization of epoxide in an anionic mechanism and a possible side reaction.

The conventional anionic ring-opening polymerization of epoxides can be described by the set of reactions shown in Scheme 1 for a monosubstituted epoxide. 

In contrast to the living character of the polymerization of EO, the anionic polymerization of substituted epoxides like POx initiated by alkali metal derivatives is subject to transfer reactions to the monomer. Indeed, the highly basic alkoxide propagating species can pull out a proton of the monomer substituent, leading to chain termination and the formation of a new growing chain bearing a terminal double bond, as indicated in Scheme 12 in the case of POx. 

In contrast to conventional anionic polymerizations, in which epoxide insertion involves an inversion of the configuration at the carbon atom of the epoxide ring where cleaved, some porphyrin systems were shown to operate by a nondisso-ciarive reaction mechanism, with retention of configuration at the carbon atom of the epoxide ring where cleaved (Scheme 28). 

Imidazole is a well-known initiator for the anionic ring-opening polymerization of epoxides. The photogeneration of imidazole and its subsequent use as an initiator for the polymerization of multifunctional epoxy-novolak resins have been reported by Nishikubo et Photolysis of the nitro-benzyl derivative 119 as shown in Scheme 36 generates imidazole, 120, which forms zwitterionic intermediate, 121, by reaction with the epoxide. The latter species is a rather weak nucleophile and the application of heat (120 °C) was required to drive the polymerization to completion. 

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