Methods to prepare quaternary ammonium groups-containing alternating poly(chlorotrifluoroethylene-alt-vinyl ether) copolymers†
The radical copolymerisation of electron-acceptor chlorotrifluoroethylene (CTFE) with electron-donor vinyl ethers (VEs) produced poly(CTFE-alt-VE) alternating fluorinated copolymers with a high content of functional groups. This article describes methods to synthesise poly(CTFE-alt-VE) copolymers that bear ammonium-groups insensitive to Hofmann degradation. Direct preparation and post-polymerisation functionalisation methods are presented. First, the radical copolymerisation of CTFE with a tertiary amine-carrying vinyl ether led to low molecular weight-copolymers with an imperfect structure. That copolymerisation and the quaternisation of the amino groups using methyl iodide were however accompanied by side reactions, including transfer reactions and significant loss of functionality. In contrast, the second strategy dealt with the post functionalisation of an alternated poly(CTFE-alt-2-chloroethyl vinyl ether) copolymer. Halogen exchange of poly(CTFE-alt-CEVE) copolymers was carried out using the efficient Finkelstein reaction. Then, the nucleophilic substitution of the iodine atoms on the obtained poly(CTFE-alt-IEVE) copolymer led to a poor yield of functionalisation and was accompanied by a substantial amount of side reactions. Then, the iodine atoms in poly(CTFE-alt-IEVE) copolymers were chemically changed quantitatively into azides (especially from microwave activation) and the resulting azido-containing copolymer was reacted with 1-dimethylamino-2-propyne. An optimisation of this Huisgen's or thermal cycloaddition showed that (i) a copper catalyst led to cross-linked copolymers, (ii) whereas in the absence of the copper catalyst, the cycloaddition was slow and only converted ca. 60% of the azides; (iii) then, microwave activation led to high yield in a few minutes. Dimethylamino groups were inserted onto the copolymeric backbone and subsequently quaternised using iodomethane efficiently. Such copolymers, potentially resistant to alkaline media and able to promote hydroxide anion conduction, may exhibit interesting properties as a component of solid alkaline fuel cell membranes.