Ring‐opening Polymerization of Enantiopure Bicyclic Ether‐ester Monomers toward Closed‐loop Recyclable and Crystalline Stereoregular Polyesters via Chemical Upcycling of Bioplastic

Abstract

Although great successes have been achieved, the preparation of closed-loop recyclable polyesters with high working temperatures still remains as a big challenge. Herein, we present the syntheses of a series of enantiopure bicyclic ether-ester monomers by upcycling of poly(3-hydroxybutyrate) bioplastic. The "living"/controlled ring-opening polymerizations of these enantiopure monomers to produce stereoregular polyesters with controlled molecular weights and welldefined chain ends were achieved. The effects of stereoconfiguration and substituent on polymerization kinetics and thermodynamics as well as the thermal properties of resultant polyesters were investigated. Of note, the stereoregular polyesters are semi-crystalline materials with melting temperatures up to 176 °C, even higher than the commodity polyolefin plastics. These polyesters can be depolymerized back to recover pristine monomers, thus successfully establishing a closed-loop life cycle.

Crystalline Stereoregular Poly(ether-ester) via MeAl[Salen]-Catalyzed Well-Controlled Ring-Opening Polymerization of Enantiopure Cyclic Ether-Ester Monomer

“…Scheme shows the synthetic route to the O -heterocyclic lactones by reacting ( R )-HBMe with racemic styrene oxide (SO) according to the previously reported method . It has been demonstrated that enantiopure ( R )-HBMe without racemization can be easily obtained by methanolysis of P3HB using H 2 SO 4 as the catalyst. − It is supposed that good thermal properties of the resultant polyesters can be achieved by introducing the rigid phenyl moiety into the polyester side chain using SO as the starting reagent. The regiospecific ring opening reaction of SO was achieved in the presence of the Lewis acid BF 3 as the catalyst due to the electron-donating and conjugated stabilizing effect of the phenyl substituent on the carbenium ion intermediate.…”

“…The absolute configurations of both monomers were verified using X-ray crystallographic characterizations (Scheme and Table S1). It is worth pointing out that no detectable epimerization was observed for the C7 carbon, which remained as R -stereoconfiguration during the course of monomer preparation . The high enantiopurity of SR -M1 and RR -M2 was characterized by supercritical fluid chromatography (SFC).…”

“…The high stereoregularity of SR -M1 was further examined with circular dichroism (CD) spectroscopy (Figure S24). The recycled SR -M1 can be further repolymerized to produce P­( SR -M1) with almost the same molar mass and dispersity as those of the original monomer ( M n = 22.6 kDa, D̵ = 1.04) (Figure S25). P­( RR -M2) can also be recycled in the same way (Figure S26) despite having a much lower depolymerization conversion of 43% at 160 °C (monomer recovered yield = 38%).…”

“…Recently, we developed a chemical recycling strategy for P3HB toward polymerizable heterocyclic lactones and corresponding poly(ether- alt -ester) (PEE) and poly(amine- alt -ester) (PAE) with closed-loop recyclability. , Nevertheless, both polymers are amorphous due to their low stereoregularity, thus hampering their practical applications. The low crystallinity and slow crystallization rate were also observed for most other reported poly(ether-ester)s. , Most recently, we addressed this problem by preparing enantiopure bicyclic ether-ester monomers from degraded products of P3HB and obtained closed-loop recyclable crystalline polyesters with a high melting temperature . The polymerization kinetics and thermodynamics as well as the thermal properties were strongly dependent on the stereoconfiguration and substituent of the bicyclic ether-ester monomers.…”

“…The low crystallinity and slow crystallization rate were also observed for most other reported poly(ether-ester)s. 23,30 Most recently, we addressed this problem by preparing enantiopure bicyclic ether-ester monomers from degraded products of P3HB and obtained closed-loop recyclable crystalline polyesters with a high melting temperature. 31 The polymerization kinetics and thermodynamics as well as the thermal properties were strongly dependent on the stereoconfiguration and substituent of the bicyclic ether-ester monomers. Despite the advancements, the crystalline P(RSS-M1) and P(RSS-M3) exhibited poor mechanical properties, probably due to their rigid polymer backbones and associated high-chain entanglement molar masses.…”