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.

Ring-opening polymerization (ROP) of rac-Ocarboxyanhydride of mandelic acid (rac-manOCA) is a promising method to synthesize isotactic biodegradable polyester of poly-(mandelic acid), which has a high glass-transition temperature (T g ) and a high melting temperature (T m ) due to the formation of a stereocomplex. However, it is still a big challenge to synthesize isotactic poly(rac-manOCA) so far because of the easy epimerization of manOCA in the polymerization progress. In this work, a highly isoselective ROP of the rac-manOCA system as the first stereoselective example is reported using weak Lewis pairs of zinc complexes and 3-fluoropyridine (3-F-Py) as catalysts with the highest P m value of 0.92, and no obvious epimerization happens in the polymerization progress. T g of poly(rac-manOCA) (∼5.3 kg/ mol) increases from 84 to 92 °C when the isotacticity increases from P m = 0.60 to 0.92. Isothermally crystallized isotactic poly(rac-manOCA) (5.3 kg/mol, P m = 0.92) demonstrates a melting point at 116 °C, and no obvious melting point can be found for the almost atactic poly(rac-manOCA) (5.4 kg/mol, P m = 0.60). The T m value of a 1:1 mixture of enantiopure poly(L-manOCA) and poly(D-manOCA) can reach 173 °C after isothermal crystallization.

Three potassium crown ether complexes supported with bulky amidinate ligands were synthesized for the ring-opening polymerization (ROP) of rac-lactide. The side polymerization reaction initiated directly by ligand anion was suppressed well in the presence of alcohol as our design, and the synthesis of linear polylactide with a molecular weight as high as 117.7 kg/mol was successful together with an isoselectivity value of P = 0.88 at -70 °C. In this system, lactide can be deprotonated by amidinate anion to give lactide enolate, which can initiate the ROP of lactide as a side reaction in the absence of alcohol; however, this side reaction can also be suppressed well in the presence of alcohol by a decrease in temperature. An interesting anti-Arrhenius-like behavior in the polymerization was discovered, which can be attributed to the fact that the active catalyst can be converted to a less active lactide enolate potassium complex at a high temperature.

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.

A series of mononuclear salen–sodium anions, as the first examples, were synthesized with tetra-alkyl ammonium as a counterpart cation. These complexes are efficient catalysts for the isoselective ring-opening polymerization of rac-lactide; the molecular weights of polymers are under control and molecular weight distributions are narrow when five equivalents of BnOH is used as an initiator. The best isoselectivity value of P m = 0.82 was achieved at −70 °C. The experimental results together with a density functional theory calculation show that a ligand-assisted activated monomer mechanism is more reasonable than an activated monomer mechanism for this system.

A highly isoselective ROP system using just a zinc alkoxide as an initiator for the isoselective ROP of OCAs with the best Pm value of 0.97 at −70 °C.

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.

The synthesis of highly stereoregular poly­(mandelic acid) (PMA) via controllable polymerization is a considerable challenge because of the easy racemization of monomers during polymerization; consequently, precise stereoblock copolymers of PMA have not been reported so far. In this work, highly stereoregular PMA was synthesized through a living ring-opening polymerization of O-carboxyanhydrides (OCAs) of mandelic acid using highly active OOO-tridentate bis­(phenolate)/zinc catalysts. In this polymerization system, the side reaction of racemization was suppressed very well via decreasing the basicity of ligands in zinc complexes. The molecular weight increases linearly as the ratio of monomer to initiator increases and can be high up to 32.5 kg/mol with narrow molecular weight distributions. Meanwhile, for the living polymerization feature, stereodiblock and stereotriblock copolymers with controllable molecular weights were obtained upon sequential one-pot monomer addition within 45 min. Thermal and X-ray diffraction characterizations indicated that these stereoblock copolymers crystallized in a stereocomplex phase with a distinct melting temperature (T m) up to 178 °C, and the solubility behaviors of stereoblock PMAs and enantiomeric homopolymers of PMAs are remarkably different.