pH-Dependent Structure of Block Copolymer Micelles Featuring a Polyampholyte Corona: A Combined Experimental and Theoretical Approach

Herein, we describe the preparation as well as experimental and theoretical investigations of block copolymer micelles based on amphiphilic, pH-responsive block copolymers featuring bipyridine ligands in the side chain. Several well-defined polystyrene-block-poly­(acrylic acid) (PS-b-PAA) and polystyrene-block-poly­((acrylic acid)-co-(2-(4-(4′-methyl-2,2′-bipyridyl))­ethylacrylate)) (PS-b-P­(AA-co-bpyEA)) block copolymers were synthesized via nitroxide-mediated polymerization. Morphological investigations of different PS-b-PAA and PS-b-P­(AA-co-bpyEA) block copolymer micelles in water using dynamic light scattering (DLS) and cryogenic transmission electron microscopy (cryo-TEM) revealed spherical micelles with pH-dependent corona size. The coronae of the micelles can be regarded as polyelectrolytes in the case of PS-b-PAA and polyampholytes in the case of PS-b-P­(AA-co-bpyEA). To describe the conformational behavior of the micellar coronae, we performed combined molecular dynamics (MD) and Monte Carlo (MC) simulations of pH-sensitive brushes with comparable grafting density and matching composition. We have studied the brush thickness, degree of dissociation of monomer units, and distributions of functional groups at different solution pH values. Particular emphasis has been put on distribution and average distance between bipyridyl units as model functional groups and the theoretically obtained macrocharacteristics are in good agreement with the experimental results. Afterward, a platinum­(II) complex was immobilized covalently at the bipyridine anchoring sites of a selected polystyrene-block-poly­((tert-butyl acrylate)-co-(2-(4-(4′-methyl-2,2′-bipyridyl))­ethyl acrylate)) (PS-b-P­(^tBuA-co-bpyEA)) block terpolymer. After deprotecting the ^tBuA groups to AA, the obtained amphiphilic diblock terpolymer formed core–corona micelles with pH-dependent corona thickness in water according to DLS and TEM investigations.