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

## TL;DR

This paper studies how polyester-based polyelectrolytes with different charges and hydrophobicities form complexes and how their properties change.

## Contribution

The study introduces a new method to synthesize polyelectrolytes with tunable charge densities and examines their complexation behavior.

## Key findings

- Polyelectrolyte mixtures show solid–liquid phase separation even near the critical salt concentration.
- Complexes with highest charge densities exhibit liquid–liquid coacervation close to the critical salt concentration.
- Hydrophobic content and charge density differences influence the critical salt concentration nonmonotonically.

## Abstract

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.