Synthesis and drug controlled release of block copolymers of poly(l-lactide) with poly(d,l-lactide) and related monomers

Four types of new biodegradable block copolymers AB, ABA, AC and AD, where A is poly(L-lactide) (PLLA), B is poly(D, L-lactide) (PDLLA), C is poly(p-dioxanone) (PDON) and D is poly(ϵ-caprolactone) (PCL) with different block lengths were synthesized and characterized by GPC, IR, ¹H-NMR and DSC. There are phase-separated and biodegradable block copolymers. Their in vitro biodegradation rates with the change of composition ratio were studied as well as the biodegradation rates of homopolymers with the series as PDON > PDLLA > PLLA in parallel with their crystallinities, i.e. from amorphous to semicrystalline. All these block copolymers were used as matrix to test their controlled release behavior of levo-norgestrel (LNG) in the form of microspheres through solvent evaporation preparation with thoroughly long-time washing to minimize the generally occurring bursting effect. As a result all of them showed almost constant rate of release even from the initial stage,