The present invention relates to a selectively soluble polymer capable of binding to a desired biomolecules in a mixture containing various biological materials and the methods of using such a polymer to purify a biomolecule from such a mixture. The polymer is soluble in the mixture under a certain set of process conditions such as pH or temperature and/or salt concentration and is rendered insoluble and precipitates out of solution upon a change in the process conditions. The polymer is capable of binding to the desired biomolecule (protein, polypeptide, etc) and remains capable of binding to that biomolecule even after the polymer is precipitated out of solution. The precipitate can then be filtered out from the remainder of the stream and the desired biomolecule is recovered such as by elution and further processed.