The Lewis Acid-Catalyzed Synthesis of Hyperbranched Oligo(glycerol-diacid)s in Aprotic Polar Media

The Lewis acid, titanium (IV) butoxide [15% (w/w; catalyst/reactants)], was used to catalyze the condensation of 0.05 mol glycerol with 0.10 mol of succinic acid, glutaric acid, and azelaic acid to produce oligomers. The reactions were refluxed in dilute solutions of dimethylsulfoxide (DMSO) or dimethylformamide (DMF) for 24 h. The oligomers were obtained, on average, in 84% yield and were soluble in polar organic solvents. Analysis by gel permeation chromatography determined that the oligomers had a number of average molecular weights (M (n) ) ranging from 2,118 to 3,245 g/mol, with degrees of polymerization (DOP) ranging from 12.2 to 13.4 repeat units. The oligomers had low polydispersities (M (w) /M (n) ) that averaged a parts per thousand 1.33. Degrees of branching were determined by one-dimensional and two-dimensional H-1 NMR and C-13 NMR and varied from 25 to 80%. Like M (n) and the DOP values, the degrees of branching were dependent on the aliphatic chain length of the diacid. MALDI-TOF mass spectrometry was used to detect ionated species that were unique to branched molecules. It was also used to validate NMR studies that suggested that some diacids were terminated with dimethylamine, generated from the hydrolysis of DMF, by as much as 36%.