Heteroleptic phenoxyimino tin(ii) bis(trimethylsilyl)amides for the synthesis of poly(diester-alt-ethers) from cyclohexene oxide and succinic anhydride†

The search for biodegradable polyesters with tunable physical properties has gone beyond the ring-opening polymerisation of cyclic esters to the copolymerisation of epoxides and cyclic anhydrides. Tin(II) complexes are active for the reaction between cyclohexene oxide and succinic anhydride monomers to produce a psuedo-periodic polymer: multiple epoxide insertions occur for every cyclic anhydride. In this paper, six heteroleptic phenoxyimino tin(II) bis(trimethylsilyl)amide complexes were synthesised and characterised by single crystal X-ray diffraction. When used as catalysts for the polymerisation of cyclohexene oxide and succinic anhydride under neat conditions at 110 °C, an average of four cyclohexene oxide molecules were incorporated for every succinic anhydride, while the proportion of pure polyester (AB)_n units was low (less than 8.5%). To obtain evidence for the average numbers of cyclohexene oxides inserted between each succinic anhydride, polymer samples were hydrolysed at the ester linkages and the resulting fragments were analysed by electro-spray ionisation mass spectrometry. When solvents such as toluene or tetrahydrofuran were added to the polymerisation system, less epoxide was incorporated. Tetrahydrofuran could also be incorporated into the polymer at a ratio of two-fifths with respect to succinic anhydride.