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 $\overline {DP} _{\rm n}$ (arm) = 2 to 20 units. Detailed NMR analysis using conventional and 2D‐NMR techniques (e.g., HSQC 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 ( $\overline M _{\rm w} /\overline M _{\rm n}$  = 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 ( $\overline M _{\rm w} /\overline M _{\rm n}$ ) 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.