Block copolymers for drug solubilisation: Relative hydrophobicities of polyether and polyester micelle-core-forming blocks

Published values of the critical micelle concentration are tabulated for diblock copolymers EmPn, EmBn, EmSn, EmLn, EmVLn and EmCLn, where E denotes a chain unit derived from ethylene oxide, P from propylene oxide, B from 1,2-butylene oxide, S from styrene oxide, L from dl-lactide, VL from γ-valerolactone and CL from ɛ-caprolactone, and the subscripts denote average chain lengths. Noting that log(cmc/mol dm−3 is proportional to the standard Gibbs energy of micellisation, the dependence of this quantity on hydrophobic block length (n) is explored for a given E-block length. Superposition of data allows ranking of the hydrophobicities of the chain units. The ratios relative to the least hydrophobic unit are:P:L:B:VL:S:CL=1:4:6:10:12:12P:L:B:VL:S:CL=1:4:6:10:12:12Transitions in the slope of log(cmc) versus n are assigned to changes in the unimer-micelle equilibrium and related to the formation of unimolecular micelles and, at high values of n, to the completion of that process. The formation transition is seen in the plots for all the copolymers except the least hydrophobic, EmPn. The completion transition is seen in the plots for EmCLn and EmLn copolymers, as these alone include results for copolymers with very lengthy hydrophobic blocks.