Group 3 and Group 13 initiators for rac-lactide ring-opening polymerization

摘要

This thesis describes the synthesis of a series of bis(quinolinolato)aluminium ethyl, (quinolinolato) gallium and (quinolinolato)zinc ethyl compounds which have been synthesised and fully characterised using spectroscopic techniques, elemental analyses and in some cases single crystal X-ray diffraction. The compounds are shown to be active initiators for the ring-opening polymerization of rac-lactide, with the activity being dependent on the metal, zinc > gallium > aluminium. All initiators show characteristics consistent with a controlled polymerization: predictable molecular weights and narrow polydispersities. The bis(quinolinolato)aluminium ethyl initiators all promote iso-selective ring-opening, with iso-selectivity values between 0.57 and 0.76. The degree of iso-selectivity is attributed to the steric hindrance imposed by the substituents on the ligands. The related gallium compounds show increased activity, whilst maintaining the good degree of stereocontrol. udA series of yttrium phosphasalen compounds are also described and their activity for the ring-opening polymerization of rac-lactide is reported. The series of compounds vary at the diimine linker, the phenoxide substituents and at the alkoxide group. The compounds all show the rapid and controlled initiation of the ring-opening polymerization of lactide. The activity of the initiators was found to be dependent on the diimine linker: ethylene > cyclohexene > phenylene > propylene, and on the electron donating ability of the phenoxide substituent. All the initiators exerted a good degree of stereocontrol on the polymerization. Interestingly, the tacticity of the polymer was seen to switch from heterotactic polylactide (Ps 0.87) to isotactic polylactide (Pi 0.86), with the inclusion of an additional bonding nitrogen moiety in the diimine linker. The formation of isotactic polylactide using Group 3 initiators is extremely rare.