Synthesis and Complexation Behavior of Well-Defined Polyester-Based Polyelectrolytes with Varying Charge Densities and Hydrophobicities

In this work, we describe the synthesis of polyester-based polyelectrolytes with varying charge densities and examine the complexation behavior of the corresponding polyelectrolyte complexes (PECs). The polyelectrolytes were prepared via ring-opening polymerization (ROP) of α-bromo-ε-caprolactone and subsequent postmodification with functionalized thiols, yielding oppositely charged polyelectrolytes with different charge densities ranging from 59 to 100%. From this, nine combinations of complexes were produced, and their complexation behavior and critical salt concentration (CSC) were investigated. All polyelectrolyte mixtures show a strong tendency to undergo solid-liquid phase separation even at salt concentrations close to the CSC. Liquid-liquid complex coacervation is only observed, close to the CSC, for the complex containing polyelectrolytes with the highest charge densities. A nonmonotonic dependence of the CSC on the charge density is observed, where the combination of the polyelectrolytes with the lowest charge densities and highest hydrophobic contents yields the highest CSC. Furthermore, complexes containing polyelectrolytes with very different charge densities tend to have a lower CSC. These findings imply that interactions other than electrostatics play a role in complexation. We interpret our results using a mean-field theory for polyelectrolyte complexation, accounting for electrostatic and nonelectrostatic interactions. Our findings pave the way for developing novel polyester-based materials with controllable material properties.