Disclosed are compositions, systems and methods for treating keratinous fibers such as hair. The compositions and systems comprise an oxazoline functionalized polymer and a compound having at least one nucleophilic site capable of reacting with the at least one oxazoline functionalized compound and chosen from hydroxyl group-containing compounds, thiol group-containing compounds, and a mixture thereof. The compositions and methods are employed to improve the quality of the keratinous fibers.