Strong Variation of Micelle–Unimer Coexistence as a Function of Core Chain Mobility

Polymeric micelles coexist in solution with unassembled chains (unimers). We have investigated the influence of glass transition temperature (T_g) (i.e., chain mobility) of the micelle core-forming blocks on micelle–unimer coexistence. We synthesized a series of seven PEG-b-P­(nBA-ran-tBA) amphiphilic block copolymers [PEG = poly­(ethylene glycol), nBA = n-butyl acrylate, and tBA = tert-butyl acrylate] with similar molecular weights (12 kg/mol). Varying the nBA/tBA molar ratio enabled the broad modulation of core block T_g with no significant change in core hydrophobicity or micelle size. NMR diffusometry revealed increasing unimer populations from 0 to 54% of total polymer concentration upon decreasing core block T_g from 25 to −46 °C. Additionally, unimer population at fixed polymer composition (and thus core T_g) increased with temperature. This study demonstrates the strong influence of core-forming block mobility on polymer self-assembly, providing information toward designing drug delivery systems and suggesting the need for a new dynamical theory.