Thermoresponsive Core−Shell Brush Copolymers with Poly(propylene oxide)-block-poly(ethylene oxide) Side Chains via a “Grafting from” Technique

Thermoresponsive brush copolymers with poly(propylene oxide)-block-poly(ethylene oxide) side chains were synthesized via a “grafting from” technique. Near-monodisperse poly(p-hydroxystyrene) was used as the backbone, and the brush copolymers were prepared by sequential metal-free anionic ring-opening polymerization of the oxyalkylene monomers, using the phosphazene base (t-BuP4) and the phenolic hydroxyl groups in the backbone to generate the complex multifunctional initiating system. The length and composition of the side chains were varied by changing the feed ratios of the backbone and the side-chain monomers. By inverting the sequence of the monomer addition, two different molecular structures were achieved, with either poly(propylene oxide) or poly(ethylene oxide) linked to the backbone. In all cases, brush copolymers with high molecular weights and low molecular weight distributions were synthesized. The thermoresponsive behavior of the brush copolymers in dilute aqueous solutions was investigated by dynamic/static light scattering and fluorescence measurements. Temperature-induced intramolecular chain contraction/association and intermolecular aggregation could both be observed at different stages of the heating process. Intermolecular aggregation was more pronounced for the sample with the poly(propylene oxide) blocks located at the periphery. The results from fluorescence spectroscopy indicate the incompletely solvated state of the brush copolymer in aqueous solution at low temperature and the absence of compact hydrophobic domains in some of the aggregates due to the core−shell brushlike molecular structure of the copolymers.