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

Abstract

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)-[Me 2 Ga(m-OCH(Me)CO 2 Me)] 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 Me 2 Ga(ON) (where ON is monoanionic bidentate ligand possessing organosuperbase functionality) shows that the presence of an organosuperbase may substantially weaken GaÀO alkoxide ÀGa 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.

NMR diffusion: an update

“…Mononuclear dialkylgallium alkoxides that have been activated by organosuperbases can be used as catalysts in the ring-opening polymerization of racemic lactide (rac-LA). However, DOSY data support the presence of dimeric gallium species (Horeglad et al, 2013).…”

Probing the Effect of Six-Membered N-Heterocyclic Carbene—6-Mes—on the Synthesis, Structure and Reactivity of Me2MOR(NHC) (M = Ga, In) Complexes

“…However, irrespective of the origin, the presence of transesterification in the case of the ROP of rac-LA with 1 at -20 • C, which is in contrast with the essentially no transesterification observed for the ROP of rac-LA with Me 2 GaOR(NHC) (NHC = SIMes, IMes) [1,2], led to a considerable decrease of isoselectivity (P m (probability of meso linkages in PLA) = 0.66-0.69 at −20 • C, P m = 0.63-0.66 at r.t.) in comparison with the ROP of rac-LA with Me 2 GaOR(NHC) (P m = 0.76-0.78 at −20 • C, P m = 0.65-0.68 at r.t.). Importantly, the ROP of rac-LA with 1 showed the considerable effect of the structure of NHC on the catalytic properties of Me 2 GaOR(NHC), which should be of importance and is of our current interest as Me 2 GaOR(NHC) are still rare examples of highly active metal alkoxides for the isoselective ROP of rac-LA [5][6][7][8][9][10][11][12][13].…”

“…As we were unable to isolate any stable alkoxide Me2InOR(6-Mes) complex, we focused on the activity of 1 in the ROP of rac-LA. Due to the reactivity of 1 towards CH2Cl2, which led to the almost quantitative and instant formation of [Me2Ga(μ-OCH2CH2OMe)]2 [9,35], no activity of 1 in the ROP of rac-LA in CH2Cl2, at −20 °C was observed. On the other hand, the polymerization of rac-LA with 1 (25:1) in toluene led to essentially full conversion after 12 hours (−20 °C) or 10 minutes (room temperature).…”

“…Moreover, they represent isoselective catalysts for the ring-opening polymerization (ROP) of rac-lactide (rac-LA), which are additionally highly active at low temperatures. Therefore, they still constitute rare examples of isoselective catalysts operating at mild conditions, both among group 13 metal complexes [5][6][7][8][9], and beyond [10][11][12][13]. Importantly, both the structure and reactivity of Me 2 MOR(NHC) (M = Ga, In; NHC = SIMes, IMes) were found to be strongly dependent on the M-C NHC bond.…”

Multifunctional Biodegradable Polymers with Room Temperature Self-Healing and Rewritable Shape Memory Properties via Diels-Alder Reaction

“…5 In general, PLA is mechanically rigid, but its weakness of brittleness, low flexibility and non-recoverable fracture toughness after failure limits its durability and lifetime usages for aforementioned applications. [6][7] In order to address these challenges for developing mechanically robust (tough) PLA, many strategies have been demonstrated including improving stereochemistry and crystallinity, [8][9] using plasticizers [10][11] or nanoparticle fillers 12 and blending with other polymers with desired material properties such as thermoplastics polyurethane (TPU). [13][14] Alternatively, using crosslinking also enhances the toughness of PLAs.…”