Alcohol-initiated ring-opening alternating copolymerization (ROAP) of phthalic anhydride (PA) and a variety of mono-, di-, and trisubstituted epoxides has been performed with a weak phosphazene base (t-BuP1) as the catalyst. Each product exhibits a perfectly alternating sequence distribution, controlled molar mass (M n up to 124 kg mol–1), and low dispersity (Đ M < 1.15, mostly). Full conversion of PA can be reached in 0.5–24 h depending on the substituent of the epoxide, the targeted degree of polymerization, and the amount of t-BuP1 used (0.2–5 mol % of PA) when the reactions are conducted under solvent-free conditions at 100 °C with a small excess of the epoxide (0.5 equiv of PA). The glass transition temperature of the polyester ranges from −14 to 135 °C. The living nature of the ROAP allows one-pot construction of well-defined block-alternating copolymers through sequential addition of two epoxides. Statistical-alternating copolymers have also been synthesized by copolymerization of PA and two mixed epoxides. Thus, the structural diversity of aromatic alternating polyesters synthesized by this simple organocatalysis has been largely enriched.

One-step synthesis of block copolymer from mixed monomers is of great interest and challenge. Using a simple non-nucleophilic organobase as the catalyst, we have achieved sequence-selective terpolymerization from a mixture of phthalic anhydride (PA), an epoxide, and rac-lactide (LA). Alcohol-initiated alternating copolymerization of PA and epoxide occurs first and exclusively because PA is substantially more active than LA for reacting with base-activated hydroxyl. When PA is fully consumed, LA polymerizes from the termini of the first block while excess epoxide stays intact because of the mild basicity of the catalyst. The two polymerizations thus occur tandemly, both in chemoselective manners, so that an aromatic–aliphatic block copolyester is generated in this one-step synthesis. The effectiveness and versatility of this approach is demonstrated by the use of ethylene oxide and several monosubstituted epoxides as well as mono-, di-, or tetrahydroxy initiators.

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Recently, Lewis pairs composed of organobases and organoboranes have shown high catalytic activity for the synthesis of polyethers by ring-opening polymerization of epoxides. Surprisingly, copolymerization of cyclic anhydride and a large excess of epoxide we conducted at elevated temperatures were free of polyether formation when catalyzed by a phosphazene base and triethylborane (Et3B), even after complete anhydride consumption. As a result, polyesters with (near) perfect alternating sequence distribution, controlled and narrowly distributed molar mass were readily obtained. Experimental and calculational results attributed the unexpected chemoselectivity to the protonolysis of Et3B and the newly formed oxygenated boron species. Most importantly, the reversible interchange of acyloxyborane and borinic ester at the propagating chain ends suppressed the reaction between hydroxy species and epoxide (formation of ethers) while maintaining the catalytic activity for the reaction between carboxy species and epoxide (formation of esters). The polyesters containing catalyst residues exhibited good cytocompatibility despite these changes. The in situ structure and activity evolvement of boron species revealed here will pave new pathways for rational design of metal-free catalysts.

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Overactive inflammatory cascade accompanied with oxidative stress in nucleus pulposus exacerbates intervertebral disc degeneration (IVDD). Hydrogels have been demonstrated promising in treating IVDD, yet remains less efficacious in anti-inflammation supplemented...

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