Abstract: The present application relates to new peptide polymers comprising monomeric units derived from the residues of glutamic acid, aspartic acid and lysine, or their protected derivatives, and which are functionalized through the introduction of side chains containing thiol groups or protected thiol groups. The new peptide polymers can be crosslinked in aqueous medium, and the resulting polymer matrices have the capacity to entrap drugs and, subsequently, release them in a controlled way when introduced into a physiological medium. This enables new pharmaceutical compositions to be developed for the controlled release of drugs, especially peptide- and protein-based ones.