Disclosed are nanoparticles comprising an end-modified poly(β-amino ester) and an additive that is a sugar or sugar derivative, such as a sugar, a sugar alcohol or chitosan. The nanoparticles may be used in any field where polymers have been found useful, including in medical fields, particularly in drug delivery. The polymers are useful in delivering a polynucleotide such as DNA, RNA or siRNA, a small molecule or a protein. Also disclosed are compositions comprising said nanoparticles and an active agent, methods for preparing said nanoparticles, said nanoparticles and compositions for use in medicine, and in vitro methods using said nanoparticles and compositions.
