The influence of organosuperbases on the structure and activity of dialkylgallium alkoxides in the polymerization of rac-lactide: the road to stereo diblock PLA copolymers

• Published in: Applied organometallic chemistry Vol. 27; no. 6; pp. 328 - 336
• Main Authors: Horeglad, Paweł, Litwińska, Anna, Żukowska, Grażyna Z., Kubicki, Dominik, Szczepaniak, Grzegorz, Dranka, Maciej, Zachara, Janusz
• Format: Journal Article
• Language: English
• Published: Blackwell Publishing Ltd 01.05.2013
• Subjects: Alkoxides; Catalysis; Catalysts; gallium; lactide; metal alkoxides; Polylactides; Polymerization; ring-opening polymerization; Stereoselectivity; Switches; Synthesis
• ISSN: 0268-2605; 1099-0739
• DOI: 10.1002/aoc.2978

Stereoselective polymerization of rac‐lactide is one of the most important issues as the properties of polylactide (PLA) depend strongly on its tacticity. There is, however, a paucity of catalysts that allow for easy switching between heteroselectivity and isoselectivity, which limits the synthesis of stereo copolymers of PLA and modification of polylactide properties. Dialkylgallium alkoxides activated by organosuperbases have been used as catalysts in the ring‐opening polymerization of racemic lactide (rac‐LA). The reaction of (S,S)‐[Me2Ga(μ‐OCH(Me)CO2Me)]2 (1) with 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) or 7‐methyl‐1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (MTBD) resulted in the formation of isoselective gallium species, highly active in the polymerization of rac‐LA. DOSY (diffusion‐ordered spectroscopy) NMR was indicative for the presence of dimeric gallium species. However, the structure of model monomeric gallium alkoxide Me2Ga(ON) (where ON is monoanionic bidentate ligand possessing organosuperbase functionality) shows that the presence of an organosuperbase may substantially weaken Ga−Oalkoxide−Ga bridges. The facile switch of stereoselectivity upon addition of organosuperbase to nonselective/heteroselective 1 allowed for the first time the synthesis of diblock polylactide comprised of isotactically and heterotactically enriched blocks. Copyright © 2013 John Wiley & Sons, Ltd. Interaction of DBU or MTBD with dialkylgallium alkoxides leads to stereoselectivity switch in the polymerization of rac‐lactide and allows to synthesize heterotactic‐b‐isotactic stereo diblock polylactide.