Soluble Hyperbranched Poly(glycolide) Copolymers

Abstract

A series of (hyper)branched poly(glycolide) copolymers has been prepared by copolymerization of glycolide (GA) with 2,2-bis(hydroxymethyl)butyric acid (BHB) via combined ROP/AB2-polycondensation. Polymerization was conducted in bulk and catalyzed by stanneous-2-ethyl hexanoate (Sn(Oct)2). The branched topology of the resulting polyesters was studied in detail by 1D- and 2D-NMR spectroscopy and confirmed by the synthesis and characterization of model compounds. The AB2 monomer BHB was incorporated either as a dendritic or focal unit, but hardly in linear or terminal mode. As expected for multifunctional polycondensation, SEC measurements showed polydisperse products with polydispersity index in the range of 1.88 to 3.40. Mn of the copolymers varied from 1100 to 4000 g/mol. MALDI−TOF MS analysis allowed to verify the main polymeric species. Furthermore, MALDI−TOF evidenced incorporation of several BHB units per macromolecule, confirming a successful condensation reaction and the formation of branched copolymers. Detailed 1H NMR characterization (1D and 2D methods) permitted calculation of the molar composition, the conversion and the degree of branching (DB), which ranged between 0.12 and 0.44. Differential scanning calorimetry (DSC) measurements showed that in contrast to linear PGA (Tm > 220 °C) the melting behavior and the glass-transition temperature of the branched poly(glycolide) copolymers changed drastically. The presence of dendritic units in the polymer backbone resulted in a depression of the melting point and amorphous materials at amounts of BHB exceeding 15%. The amorphous hyperbranched poly(glycolide) copolymers show enhanced solubility in common solvents (e.g., acetone, ethyl acetate, THF) and improved processability in contrast to linear PGA and possess potential for use in slow or controlled drug release systems.