Effect of substituent volume on the adsorption and separation performance of thermo-responsive ion-imprinted polymers

Moving beyond empirical approaches, this work seeks to formulate a rational design principle for ion-imprinted polymers by deciphering the influence of monomer substituent volume. A systematic series-varying from H to CH₃ to C₂H₅-was employed to probe how substituent volume modulates the binding energy toward Ru(III) and ultimately governs the adsorption-desorption efficacy of the synthesized thermo-responsive imprinted polymers. The DFT-calculated binding energies ranked as follows: DMAM (-451.7 kcal/mol) > DEAM (-378.5 kcal/mol) > AM (-341.5 kcal/mol), delineating a distinct trend in monomer-template affinity. Employing a common PDEAM-b-P(DEAM-co-AM) block copolymer as a smart scaffold, we synthesized three Ru(III)-imprinted polymers (TIIPs) differentiated solely by the functional monomer (AM, DMAM, or DEAM). By correlating a suite of characterization data (chemical, porous, morphological, surface) with adsorption performance, definitive structure-function principles were elucidated. Specifically, the structural simplicity and moderate binding affinity of the AM monomer promoted the assembly of a well-defined pre-polymerization complex. This, in turn, yielded a TIIP material with an optimal performance balance: Accessible pore channels, optimal surface charge, and robust stability. Although AM-TIIP did not exhibit the highest absolute adsorption capacity, its integrated performance profile was exceptional, featuring efficient mass transfer, excellent selectivity (evidenced by separation factors), and superior reusability (67.98% capacity retention after multiple cycles). In contrast, DMAM's exceptionally high binding energy, coupled with methyl-induced steric hindrance, and DEAM's strong hydrophobicity from ethyl groups, led to less favorable polymer morphologies-such as constricted pores or overly hydrophobic environments-which ultimately compromised practical performance parameters like desorption efficiency and adsorption kinetics. A critical finding was that an exceedingly high binding energy, while theoretically favorable for affinity, can create kinetic barriers for desorption and hinder mass transfer. Therefore, this work established that a monomer with structural simplicity and a balanced binding energy-exemplified by AM-is pivotal for designing high-performance, intelligent adsorbents that harmonize high selectivity with efficient regenerability.