Multi-arm star poly(L-lactide) with hyperbranched polyglycerol core

Biocompatible multi-arm star block copolymers based on poly(L-lactide) (PLLA) have been prepared by a core-first approach, using hyperbranched polyglycerol (PG), a polyether polyol, as a polyfunctional initiator. The molecular weight of the hyperbranched initiator-core was varied from 2 200 to 5 200 g.mol(-1), molecular weights of the resulting multi-arm stars were in the range of 6 700-107 000 g.mol(-1) (NMR), depending on the amount of dilactide (LA) added. Various monomer/initiator ratios have been employed in the Sn-catalyzed LA polymerization in order to vary the length of the lactide arms from (DP) over bar (n)(arm) = 2 to 20 units. Detailed NMR analysis using conventional and 2D-NMR techniques (e.g., HSOC NMR) revealed that the monomer/initiator-core ratio indeed permits control of the arm length. SEC measurements showed that the narrow polydispersities of the core molecules ((M) over bar (w)/(M) over bar (n) = 1.5 and 1.6) became even lower after grafting of PLLA for the multi-arm star polymers. Size exclusion chromatography (SEC) also demonstrated that the competing homopolymerization of LA could be avoided using suitable reaction conditions. The resulting PG-PLLA star polymers exhibited low polydispersities ((M) over bar (w)/(M) over bar (n)) between 1.15 and 1.7, depending on the length of the PLLA arms. Attachment of the hydrophobic PLLA chains to the hydrophilic polyether structure leads to amphiphilic, core-shell type structures suitable for guest encapsulation.