Hyperbranched Polylactide Copolymers

A series of hyperbranched poly( l -lactide) (PLLA) copolymers were prepared by copolymerization of dilactide with 2,2-bis(hydroxymethyl)butyric acid (BHB) as an AB 2 comonomer via a combined ring-opening polymerization/AB 2 polycondensation. Stannous 2-ethylhexanoate was used as a catalyst. Apparent molecular weights determined by size exclusion chromatography (SEC) (polystyrene standards) were in the range of 700−60 000 g/mol, decreasing with the degree of branching (DB). All branched copolyesters showed monomodal molecular weight distributions with low apparent polydispersities (SEC) in the range of 1.3−2.2. The branched comonomer was randomly incorporated in the copolyesters ( 13 C and 1 H NMR), thereby varying the DB. The degree of crystallization of the materials decreases with increasing DB. With more than 7% incorporation of the BHB monomer, amorphous materials were obtained (differential scanning calorimetry (DSC)). Solubility and solution viscosity depended strongly on the DB.