Weak Lewis Pairs as Catalysts for Highly Isoselective Ring-Opening Polymerization of Epimerically Labile rac-O-Carboxyanhydride of Mandelic Acid

Abstract

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.

Synthesis and Stereocomplexation of New Enantiomeric Stereo Periodical Copolymers Poly(l-lactic acid–l-lactic acid–d-lactic acid) and Poly(d-lactic acid–d-lactic acid–l-lactic acid)

“…DSC measurements were performed to evaluate thermal properties of the specimens (Figure 6). The glass transition, cold crystallization, and melting temperatures (T g , T cc , and T m , respectively) as well as cold-crystallization and melting enthalpies (ΔH cc and ΔH m , respectively) obtained from the DSC thermograms in Figure 6 remained unchanged and decreased for enantiomeric P-(2H3MB) homopolymers 71,76 and PMA homopolymers 19,20 with large side groups. Therefore, this paper reports the first case wherein the enantiomeric polymers with small side groups such as P(LLA-LLA-DLA) and P(DLA-DLA-LLA) did not induce a large T m increase upon SC formation.…”

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

“…83 Wu and coworkers also reported that an aminobisphenolate Zn complex (Zn-5), paired with 3-fluoropyridine, a weak Lewis base, along with rac-(methyl mandelate) as an initiator, could mediate isoselective ROP of rac-5 with a high P m of 0.92 at À50°C (Scheme 4c). 104 Severe epimerization occurred in the absence of 3-fluoropyridine,…”

“…83 Wu and coworkers also reported that an aminobisphenolate Zn complex (Zn-5), paired with 3-fluoropyridine, a weak Lewis base, along with rac-(methyl mandelate) as an initiator, could mediate isoselective ROP of rac-5 with a high P m of 0.92 at À50°C (Scheme 4c). 104 Severe epimerization occurred in the absence of 3-fluoropyridine, Review and polymerization almost ceased to happen without the Zn complex, suggesting the necessity of a Lewis pair to eliminate epimerization. Notably, poly(sb-5) (P m ¼ 0.92) showed a higher T g value of 93°C when compared to the atactic poly(rac-5) (P m ¼ 0.60, T g ¼ 83°C).…”

Constructing ABA- and ABCBA-Type Multiblock Copolyesters with Structural Diversity by Organocatalytic Self-Switchable Copolymerization

“…O -Carboxyanhydrides (OCAs), which can be prepared from amino acids or hydroxyl acids with a rich variety of side-chain functionalities, have emerged as an alternative class of highly active monomers for polyester synthesis. − ROP of OCAs bearing different pendent groups (methyl-, phenyl-, and benzyl-) will afford poly­(α-hydroxy acids) with demanded physicochemical properties such as heat resistance, ductility, and tensile strength. , For example, poly­(mandelic acid) (PMA), an aryl analogue of polylactide (PLA), could be readily prepared by ROP of the OCA derived from mandelic acid. It was demonstrated that PMA exhibits polystyrene-like physical and mechanical properties but is available from renewable resources and degradable as PLA. , Accordingly, OCAs are promising heterocyclic candidates for synthesis of sequence-controlled multiblock copolyesters by self-switchable copolymerization.…”