Novel pentablock polymers having a PCL-PLA-PEG-PLA-PCL or PCL-PGA-PEG-PGA-PCL configuration, wherein PEG is polyethylene glycol, PCL is poly(8-caprolactone), PGA is poly(glycolic acid), and PLA is poly(lactic acid), and methods of making nanoparticles from the pentablock polymers, are disclosed. The invention is also directed to a method for preparing nanoparticle compositions comprised of polymers with high levels of bioactive or diagnostic agents.