Tetrabutylammonium Halides as Selectively Bifunctional Catalysts Enabling the Syntheses of Recyclable High Molecular Weight Salicylic Acid-Based Copolyesters.

To synthesize high molecular weight poly(phenolic ester) via a living ring-opening polymerization (ROP) of cyclic phenolic ester monomers remains a critical challenge due to serious transesterification and back-biting reactions. Both phenolic ester bonds in monomer and polymer chains are highly active, and it is difficult so far to distinguish them. In this work, an unprecedented selectively bifunctional catalytic system of tetra-n-butylammonium chloride (TBACl) was discovered to mediate the syntheses of high molecular weight salicylic acid-based copolyesters via a living ROP of salicylate cyclic esters (for poly(salicylic methyl glycolide) (PSMG), Mn =361.8 kg/mol, Ð<1.30). Compared to previous catalysis systems, the side reactions were suppressed remarkably in this catalysis system because phenolic ester bond in monomer can be selectively cleaved over that in polymer chains during ROP progress. Mechanistic studies reveal that the halide anion and alkyl-quaternaryammonium cation work synergistically, where the alkyl-quaternaryammonium cation moiety interacts with the carbonyl group of substrates via non-classical hydrogen bonding. Moreover, these salicylic acid-based copolyesters can be recycled to dimeric monomer under solution condition, and can be recycled to original monomeric monomers without catalyst under sublimation condition.

Efficient Access to Sequence-Controlled Poly(α-hydroxy acids) with Ax(BC)yDz-Type and Ax(BC)yAz-Type Triblock Structures via Self-Switchable Ring-Opening Polymerization of Monomer Mixtures.

Synthesizing block-sequence-controlled poly(α-hydroxy acids) of three or four α-hydroxy acids remains challenging in one step. In this study, a strategy was employed using three monomers of O-carboxyanhydrides (OCAs) consisting of one α-hydroxy acid (A), asymmetric cyclic diester (B and C, two different α-hydroxy acids of B and C), and symmetric cyclic diester (one α-hydroxy acid of D) with remarkably different activities toward a stereoselective, regioselective, and chemoselective initiator of a zirconium complex. Then, via a self-switchable approach, these monomers can be copolymerized in a well-controlled block sequence of Ax(BC)yDz and Ax(BC)yAz without an external stimulus. Moreover, upon addition of more monomer mixtures during the copolymerization process, more complicated sequence-controlled poly(α-hydroxy acids) can be achieved with up to 15 blocks.

Isotactic-Alternating, Heterotactic-Alternating, and ABAA-Type Sequence-Controlled Copolyester Syntheses via Highly Stereoselective and Regioselective Ring-Opening Polymerization of Cyclic Diesters.

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

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 ring-opening polymerization (ROP) of LacOCA was achieved using a bulky amine tris(phenolate) hafnium alkoxide initiator with a high Pr value of 0.91. However, the stereoselectivities of amine tris(phenolate) hafnium alkoxide initiators for the ROP of malic acid O-carboxyanhydride (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.

Isoselective Ring-Opening Polymerization of rac-Lactide Catalyzed by Sodium/potassium Tetradentate Aminobisphenolate Ion-paired Complexes.

Two sodium/potassium tetradentate aminobisphenolate ion-paired complexes were synthesized and structurally characterized. These ion-paired complexes are efficient catalysts for the ring-opening polymerization of rac-lactide (rac-LA) in the presence of 5 equivalents BnOH as an initiator and the side reaction of epimerization can be suppressed well at low temperatures. The polymerizations are controllable, affording polylactides with desirable molecular weights and narrow molecular weight distributions; the highest molecular weight can reach 50.1 kg mol-1 in this system, and a best isoselectivity of Pm =0.82 was achieved. Such polymerizations have rarely been reported for isoselective sodium/potassium complexes without crown ether as an auxiliary ligand. The solid structures suggest that BnOH can be activated by an interaction with the anion of sodium/potassium complex via a hydrogen bond and that the monomer is activated by coordination to sodium/potassium ion.

Alternating Sequence Controlled Copolymer Synthesis of α-Hydroxy Acids via Syndioselective Ring-Opening Polymerization of O-Carboxyanhydrides Using Zirconium/Hafnium Alkoxide Initiators.

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 Pr 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.