In the Sn(Oct)(2) catalyzed bulk copolymerization of L-lactide (LLA) and a large amount of branching comonomer 2,2-bis(hydroxymethyl)butyric acid (BHB) (LLA/BHB/Sn(Oct)(2) ratio is 5:1:0.05, 135 degrees C), low molecular weight of copolymers with inhomogeneous structure were generated. MALDI-TOF mass spectrum demonstrated that the lower molar mass fraction was dominated with the BHB homo-polycondensation products: whereas the higher molar mass fraction was mainly composed of linear chains of poly(L-lactide) (PLLA) bearing one BHB unit. The addition of aliphatic tertiary amine to this polymerization system could effectively enhance the molecular weight of the obtained copolymers, whereas, aromatic tertiary amines and aliphatic primary and secondary ones were not so effective. The obtained PLLA copolymers were characterized by GPC, H-1 NMR, C-13 NMR and MALDI-TOF MS, which verified that they were composed of LLA and BHB units and had the highly branched structure. The degree of branching was about 0.23-0.30. Thermal analyses by TGA and DSC demonstrated that the resulting highly branched PLLA copolymers were amorphous. Compared with linear PLLA, the highly branched PLLA copolymers were thermally more stable and had lower T-g. The mechanism of aliphatic tertiary amine mediated synthesis of medium molecular weight of highly branched PLLA copolymers was proposed. (C) 2011 Elsevier Ltd. All rights reserved.