Alkali Metal Borate Conjugated Block Polyelectrolytes as Tuneable Mixed Ionic-Electronic Conductors

Abstract

Conjugated polyelectrolytes (CPEs) combine ionic conductivity from tethered ionic groups with electronic conductivity from intrinsically-doped π-conjugated backbones, enabling applications in energy storage, bioelectronics, and neuromorphic computing. Conjugated block polyelectrolytes (CBEs), in which the conjugated and ionic functions are spatially segregated into distinct blocks, represent an emerging and underexplored variant of this materials class. Despite multiple studies of various pendant ionic groups, neither borate ionic functionalisation nor block copolymer architectures bearing non-sulfonate ionic groups have been explored. Here, we report CBEs, accessible through controlled Suzuki–Miyaura catalyst-transfer polymerisation (SCTP) and cyclic carbonate ring-opening polymerisation (ROP), incorporating alkali metal borate polycarbonate segments: poly(3-hexylthiophene)-block-poly(ethylene oxide-graft-poly(ethylene glycol))-block-polycarbonate bearing lithium, sodium, or potassium borate moieties. Systematic variation of P3HT chain length (DP = 35 and 110) and content (10–50 wt%) reveals distinct cation-dependent transport behaviour. Electrochemical characterisation via impedance spectroscopy, chronoamperometry, and linear sweep voltammetry demonstrates purely ionic conduction at low applied potentials, with p-type electronic transport activated above 1–1.5 V. Notably, lithium-ion conductivity remains independent of P3HT incorporation, whereas sodium transport improves with longer conjugated blocks and potassium conductivity is enhanced with shorter segments. Thermal analysis, DFT calculations, rheological and tensile measurements establish structure–property relationships linking polymer ionic networking to mechanical and ionic-electronic transport properties. These findings position anionic borate CBEs as a promising addition to the mixed ionic-electronic conductor platform, with tuneable properties for emerging electrochemical technologies and a modular synthetic approach expected to extend to alternative conjugated backbones and other polyelectrolyte groups easily installed by ligand coordination chemistry.