Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 2 / Section A / 19 / e / Solution

Replacing lactide by ethylene oxide givesThe first chain grows by alkoxide attack on ethylene oxide, followed by protonation at work-up; the alkoxyamine subsequently controls styrene polymerization. The final material is an amphiphilic block copolymer. In water, polystyrene avoids solvent exposure while the poly(ethylene oxide) block is hydrated. Above a concentration-dependent assembly threshold, micelles with a polystyrene core and hydrated poly(ethylene oxide) corona are a reasonable morphology. Unimers remain in equilibrium with aggregates. Core–corona assembly lowers unfavourable water contact but costs chain-stretching and translational entropy. Block volume fractions, solvent history and concentration can instead favour cylinders or vesicles, so the chemical structure alone does not uniquely establish spherical micelles.