Positional Isomerism: A Molecular Switch for Substantial Modulation of Polymer Mechanics

Abstract

Positional isomerism, a universal structural regulation strategy in nature, achieves precise control over biological functions through minute variations in substituent positions on a molecular backbone. Despite its extensive application in small-molecule chemistry, the systematic introduction of positional isomerism as the sole structural variable into polymer main chains remains unexplored. Herein, inspired by biological systems, we employ three isomeric aromatic diols, 1,2-, 1,3-, and 1,4-benzenedimethanol (o-, m-, and p-BDM) as chain extenders to synthesize three thermoplastic polyurethanes with identical chemical composition, differing only in the substitution pattern on the aromatic ring. The para-substituted polyurethane (p-PU) exhibits a tensile strength of 13.1 MPa, an elongation at break of 2016%, and a toughness of 151.8 MJ/m³, representing a 7.7-fold increase in tensile strength and a 19.2-fold increase in toughness over its ortho-substituted counterpart (o-PU, with a tensile strength of 1.7 MPa, an elongation at break of 604%, and a toughness of 7.9 MJ/m³), while the meta-substituted polyurethane (m-PU) shows intermediate values (tensile strength: 7.4 MPa; elongation at break: 1271%; toughness: 47.0 MJ/m³). Combined characterization using small-angle X-ray scattering (SAXS), density functional theory (DFT) calculations, and molecular dynamics simulations reveals that the para isomer, with minimal steric hindrance, promotes ordered hydrogen bonding and π–π stacking within hard segments, inducing the most well-defined microphase-separated morphology among the three isomers. This work demonstrates the feasibility of positional isomerism as an independent dimension for tuning polymer properties and offers a molecular design strategy for precisely modulating mechanical performance without altering chemical composition.