Synthesis of polylactide/poly(ethylene glycol) diblock copolymers with functional endgroups

Amphiphilic polylactide/poly(ethylene glycol) (PLA–PEG) diblock copolymers with functional groups at the PEG chain ends were synthesized by coupling PLA and PEG homopolymers using different coupling agents. PLA precursors with different endgroups were synthesized by ring‐opening polymerization of l ‐lactide in the presence of different initiators such as octanol, acetic acid or benzoic acid, or water, using non‐toxic zinc lactate as catalyst. The mechanism of the ring‐opening polymerization of lactide initiated by carboxyl groups was investigated and discussed in comparison with the literature. N,N'‐carbonyldiimidazole was used to couple the two hydroxyl groups of PLA and PEG, using 4‐dimethylaminopyridine (DMAP) as catalyst. Dicyclohexylcarbodiimide (DCC) and DMAP were adopted to couple the carboxyl group and the hydroxyl group of PLA and PEG, respectively, while DCC and N‐hydroxysuccinimide were used to connect PLA and PEG by coupling their carboxyl and amine groups. Comparison of different coupling routes shows that the DCC/DMAP one exhibits the highest efficiency. A common tumor targeting ligand, folic acid, was attached to PLA–PEG with hydroxyl endgroups using the DCC/DMAP route. The resulting PLA–PEG copolymers bearing folic acid present great interest for targeted delivery of anti‐cancer drugs. © 2012 Society of Chemical Industry