Micellization of Sequence-Controlled Polyurethane Ionomers in Mixed Aqueous Solvents

While the impact of compositional parameters such as block length and ionic content on the micellization of (polymeric) amphiphiles is widely investigated, the influence of monomer sequence has received far less attention until recently. Here, we report the synthesis of two sequence-controlled polyurethane ionomers (PUIs) prepared via a stepwise coupling-deprotection strategy, and compare their solution association in aqueous-organic mixtures. The two PUIs are highly similar in mass and overall composition, yet differ markedly in the sequence of building blocks. PUI-A2 comprises a polytetrahydrofuran (pTHF) block connected to an alternation of isophorone diamine (IPDA) and dimethylolpropionic acid (DMPA) units that together are also arranged in a blockwise manner. The result is a macromolecular structure with a comparatively hydrophobic tail (pTHF) and a hydrophilic headgroup, which structure is reminiscent of those of traditional surfactants, albeit much larger in size. PUI-S2 instead resembles a bolaamphiphilic architecture with a pTHF midblock connected on either end to a singly charged segment comprising DMPA and IPDA. We detect micellization below a threshold cosolvent volume fraction (φ_solv) of 0.4 in aqueous-organic mixtures with tetrahydrofuran (THF), ethanol, and isopropyl alcohol. We use scattering tools to compare the aggregation number (N _agg) and hydrodynamic radius (R _h) of PUI-S2 and PUI-A2 micelles. Irrespective of the solvent composition, we observe in the micellar window of φ_solv < 0.4, lower N _agg for PUI-S2 micelles compared to PUI-A2, which we attribute to packing restraints associated with its bolaamphiphilic architecture. The increase in micellar size with increasing φ_solv is much more pronounced for PUI-S2 than for PUI-A2. The micellar mass decreases with increasing φ_solv for both PUIs; the effect is modest for PUI-S2 compared to PUI-A2 and is not observed in the most apolar cosolvent studied (THF). Upon the approach of the micellization boundary φ_solv ≈ 0.4, both types of PUI micelles become less compact in structure, as (in most cases) PUIs are released and as micellar dimensions increase.