  We introduce a polymer model where the transition from swollen to compact
configurations is due to interactions between the monomers and the solvent.
These interactions are the origin of the effective attractive interactions
between hydrophobic amminoacids in proteins. We find that in the low and high
temperature phases polymers are swollen, and there is an intermediate phase
where the most favorable configurations are compact. We argue that such a model
captures in a single framework both the cold and the warm denaturation
experimentally detected for proteins. Some consequences for protein folding are
discussed.
