Disclosed are polymers that are poly(beta-amino ester)s (PBAEs) modified with at least one oligopeptide. The polymers may be used in any field where polymers have been found useful including in medical fields, particularly in drug delivery. The polymers are particularly useful in delivering a polynucleotide such as DNA, RNA and siRNA, a small molecule or a protein. Also disclosed are compositions comprising said polymers and an active agent, methods of encapsulating an agent in a matrix of said polymers, and said polymers and compositions for use in medicine.