Ring-Opening Alternating Copolymerization of O-Carboxyanhydrides of Lactic Acid and Malic Acid Using Hafnium Alkoxide Initiators with Different Stereo Selectivities and Activities

Abstract

Monomer sequence controllable syntheses of copolymers, including copolyesters, remain a challenge in polymer science. Although alternating sequence-controlled copolymerization of O-carboxyanhydrides (OCAs) can be achieved via using syndioselective initiators, the alternating copolymerization of lactic acid-derived O-carboxyanhydride (LacOCA) with other monomers still suffers from a lack of highly syndioselective initiators. In this work, a highly syndioselective system for the ringopening polymerization (ROP) of LacOCA was achieved using a bulky amine tris(phenolate) hafnium alkoxide initiator with a high P r value of 0.91. However, the stereoselectivities of amine tris(phenolate) hafnium alkoxide initiators for the ROP of malic acid Ocarboxyanhydride (MalOCA) change to be modestly or lowly isoselective. Interestingly, despite the different stereoselectivities of this system for the two different monomers, the high syndioselectivity of the initiator for the ROP of LacOCA and the low activity of MalOCA in the ROP allow comparatively high rates of cross-propagation; consequently, the ring-opening alternating copolymerization (ROAP) of LacOCA and MalOCA was achieved successfully.

Sequence‐Controlled Alternating Copolyesters Synthesis via Selective Ring‐Opening Polymerization

“…In 2020, Wu et al reported a ROAP of MalOCA and LacOCA with different enantiomerically pure using zirconium/hafnium alkoxide initiators supported by sterically bulky ligand (Zr-2 and Hf-2, as shown in Figure 10). [48] Because of the bulky adamantyl groups in Zr-2 and Hf-2, the syndioselective ROP rac-LacOCA was achieved with a high P r value up to 0.91, which is the highest value until now. In addition, Zr-2 and Hf-2 have almost no stereoselectivity in the polymerization of rac-MalOCA (P r ≈ 0.5).…”

“…However, due to its smaller side groups, LacOCA has significantly higher polymerization activity than MalOCA. Based on the [48] Copyright 2020, American Chemical Society.…”

“…The ROP of rac-OCAs and ROAP of LacOCA and MalOCA initiated by Zr-2 and Hf-2 complexes. Reproduced with permission [48]. Copyright 2020, American Chemical Society.…”

Controlled Ring-Opening Polymerization of O-Carboxyanhydrides to Synthesize Functionalized Poly(α-Hydroxy Acids)

“…Wu and coworkers further reported another Hf alkoxide complex with bulky adamantyl groups (Hf-2; Scheme 5a) that exhibited excellent syndioselectivity for rac-4 with a P r of 0.91, but a poor syndioselectivity for rac-11 with a P r of 0.46. 107 The polymerization rate for rac-11 was slow, as the bulky adamantyl group likely impeded the monomer incorporation. Intriguingly, when the copolymerization of D-4 and L-11 was carried out at room temperature, alternating poly(D-4-alt-L-11) was successfully synthesized with an alternation level of 86% (Scheme 5b).…”

Synthesis of Sustainable Polyesters via Organocatalytic Ring‐Opening Polymerization of O‐carboxyanhydrides: Advances and Perspectives

“…In 2006, ROP of O ‐carboxyanhydrides (OCAs), a class of five‐numbered cyclic anhydrides derived from sustainable amino acids, emerged as a promising route towards functionalized polyesters. [ 27–37 ] OCAs have a rich variety of functional groups, and can be regarded as an alternative class of highly active monomers for polyester synthesis ( Figure A). Many review articles have documented the development of polymerization of OCAs.…”