Organocatalyzed chemoselective ring-opening polymerizations

Ning Zhu,Junhua Liu,Weijun Huang,Kai Guo,Jun Ling,Yihuan Liu,Xin Hu,Weiyang Feng

doi:10.1038/s41598-018-22171-6

Abstract

A novel metal-free and protecting-group-free synthesis method to prepare telechelic thiol-functionalized polyesters is developed by employing organocatalysis. A scope of Brønsted acids, including trifluoromethanesulfonic acid (1), HCl.Et2O (2), diphenyl phosphate (3), γ-resorcylic acid (4) and methanesulfonic acid (5), are evaluated to promote ring-opening polymerization of ε-caprolactone with unprotected 6-mercapto-1-hexanol as the multifunctional initiator. Among them, diphenyl phosphate (3) exhibits great chemoselectivity and efficiency, which allows for simply synthesis of thiol-terminated poly(ε-caprolactone) with near-quantitative thiol fidelity, full monomer conversion, controlled molecular weight and narrow polydispersity. Kinetic study confirms living/controlled nature of the organocatalyzed chemoselective polymerizations. Density functional theory calculation illustrates that the chemoselectivity of diphenyl phosphate (3) is attributed to the stronger bifunctional activation of monomer and initiator/chain-end as well as the lower energy in hydroxyl pathway than thiol one. Moreover, series of tailor-made telechelic thiol-terminated poly(δ-valerolactone) and block copolymers are efficiently generated under mild conditions.

Highlights

A novel metal-free and protecting-group-free synthesis method to prepare telechelic thiolfunctionalized polyesters is developed by employing organocatalysis
Et2O (2), diphenyl phosphate (3), γ-resorcylic acid (4) and methanesulfonic acid (5), were investigated respectively in ε-caprolactone (CL) polymerizations initiated by 6-mercapto-1-hexanol (MH) as the multifunctional initiator
It was noteworthy that no large distinction was observed between the strong acid and weak acid (1 vs 5, 2 vs 4) with respect to the reaction temperature, time, thiol fidelity, molecular weight and polydispersity

Summary

Introduction

A novel metal-free and protecting-group-free synthesis method to prepare telechelic thiolfunctionalized polyesters is developed by employing organocatalysis. Diphenyl phosphate (3) exhibits great chemoselectivity and efficiency, which allows for synthesis of thiol-terminated poly(ε-caprolactone) with near-quantitative thiol fidelity, full monomer conversion, controlled molecular weight and narrow polydispersity. Density functional theory calculation illustrates that the chemoselectivity of diphenyl phosphate (3) is attributed to the stronger bifunctional activation of monomer and initiator/chain-end as well as the lower energy in hydroxyl pathway than thiol one. Chemoselective polymerization in the presence of multifunctional initiator/monomer is the ideal yet challenging green synthetic strategy to prepare functional polymers[20,21]. Thiol-functionalized polymers have significant applications in polymer chemistry and nanoscience, which requires quantitative thiol fidelity, controlled molecular weight and narrow polydispersity[22,23,24,25]. Quantitatively chemoselective and highly efficient synthetic strategies are extremely desirable to meet the requirement of green and sustainable chemistry

Methods

Results

Conclusion