  Macromolecular solubility in solvent mixtures often exhibit striking and
paradoxical nature. For example, when two well miscible poor solvents for a
given polymer are mixed together, the same polymer may swell within
intermediate mixing ratios. We combine computer simulations and theoretical
arguments to unveil the first microscopic, generic origin of this
collapse-swelling-collapse scenario. We show that this phenomenon naturally
emerges at constant pressure in mixtures of purely repulsive components,
especially when a delicate balance of the entropically driven depletion
interactions is achieved.
