The ring-opening polymerization (ROP) of O-carboxyanhydrides (OCAs) can give diverse poly(α-hydroxy acid)s (PAHAs) with different functional groups because of easy modification of the side group of OCAs, which can extend applications of PAHAs widely. The stereoselective polymerization of O-carboxyanhydrides and further sequence controlled alternating copolymerization of OCAs were still big challenges until now for lack of suitable catalysts/initiators. In this work, a highly syndioselective ROP of OCAs system as the first stereoselective example in this area is reported using zirconium/hafnium alkoxides as initiators with the highest P value up to 0.95. Furthermore, these initiators were successfully applied in the precisely alternating sequence controlled copolymerization of PheOCA and Tyr(Bn)OCA, and alternating copolymerization of LacOCA and PheOCA was also achieved.

Synthesizing different types of sequence-controlled copolyesters can enrich the diversity of copolyesters and modify their properties more precisely, but it is still a challenge to synthesize a complicated sequence-controlled copolyester using different hydroxy acids in a living polymerization manner. In this work, a highly regioselective and stereoselective catalytic system was developed to synthesize biorenewable and biodegradable copolyesters of mandelic acid and lactic acid with isotactic-alternating, heterotactic-alternating, and ABAA-type precise and complicated sequences. Because of the regular incorporation of mandelic acid into polylactide, these sequence-controlled copolymers of mandelic acid and lactic acid show higher glass-transition temperatures than polylactide and a random copolymer. A stereocomplexation interaction between two opposite enantiomeric isotactic polymer chains was also discovered in the isotactic-alternating copolymer.

To control the monomer sequence in copolymers of different hydroxyl acids remains a big challenge so far, which is valuable for tuning the properties of copolyesters. In this work, the perfectly alternating sequence-controlled copolymer of mandelic acid and glycolic acid, as the first example, was synthesized via the highly regioselective ring-opening polymerization (ROP) of a cyclic diester of mandelic acid and glycolic acid (3-phenyl-1, 4-dioxane 2, 5-diketone, PDD). The high molecular weight of poly­(mandelate-alt-glycolate) (66.8 kg/mol) was achieved too, and the glass transition temperature of the alternating copolymer can reach 91.3 °C (M n = 45.7 kg/mol). The high alternating level of this polymerization system may be interpreted by the high regioselectivity of a methyl glycolate zirconium complex as an intermediate mode complex in the propagation progress. The regioselectivity in the ROP of PDD can be changed from the initiation step to the propagation progress when the alkoxy group in zirconium complexes is methyl lactate or methyl mandelate.

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.

Twos odium/potassium tetradentate aminobisphenolate ion-paired complexes were synthesized and structurally characterized. These ion-paired complexesa re efficient catalysts for the ring-opening polymerization of rac-lactide (rac-LA) in the presence of 5equivalents BnOH as an initiator and the side reactiono fe pimerization can be suppressed well at low temperatures. The polymerizations are controllable, affording polylactidesw ith desirable molecular weights and narrow molecular weightd istributions;t he highest mo-lecular weight can reach5 0.1 kg mol À1 in this system, anda best isoselectivityo fP m = 0.82 was achieved. Such polymerizations have rarely been reported for isoselective sodium/ potassium complexes without crown ether as an auxiliary ligand.T he solid structures suggest that BnOH can be activated by an interaction with the anion of sodium/potassium complex via ah ydrogen bond and that the monomer is activated by coordinationt os odium/potassium ion.[a] X.