nanoparticles formed from a random polymer of at least one compound of formula in which a represents a group or a group ; r 1 and r 2 , identical or different , represent a linear or branched c 1 - c 6 alkyl group ; n = 1 , 2 , 3 , 4 or 5 , having a diameter of less than 500 nm and an average molecular mass between about 8000 and 100000 , the nanoparticles being formed from homogeneous molecular species , and optionally containing a biologically active material .