Preparation and characterization of polylactide-poly(ethylene glycol)-polylactide triblock polymers and a preliminary in vivo examination of the blood circulation time for the nanoparticles made therefrom

Fractionated samples of polylactide-poly(ethylene glycol)-polylactide triblock copolymer were prepared and characterized. It was found that their molecular weights were related to the feed-ratios of monomer and initiator by the equation based on a simple chain reaction model. Progestrone-containing nanoparti-cles prepared from the block copolymers were injected in rats to measure the retention of the medicine during their circulation in the blood. The results confirmed the general belief that the presence of hydrophilic groups in the particle is essential for extending the retention time and also showed that the initial concentration and retention half-life of progesterone are increased by using a fractionated base polymer for the particle.