We have synthesized linear poly(lactic acid) (BHMBA-PLA) possessing a pendent carboxylic acid group via ring-opening polymerization (ROP) of L- or D,L-lactide using bis(hydroxymethyl) butyric acid (BHMBA) as initiator and stannous octoate as catalyst. Polymerizations were typically conducted in the bulk for similar to 3 h at 130 degrees C, using a catalyst concentration of 3 x 10(-2) wt % Sri. The polymers were characterized by gel permeation chromatography, equipped with a multiangle laser light scattering detector (GPC-MALLS), H-1 NMR, C-13 NMR, homonuclear correlation spectroscopy.(COSY), and heteronuclear single quantum coherence spectroscopy (HSQC). Branching, via possible condensation reaction between the pendent carboxylic acid group of the initiator fragment and the hydroxyl chain ends, was investigated and determined to be negligible under our synthesis conditions. Number-average molecular weights by GPC-MALLS (M-n,M-Gpc) and H-1 NMR (M-n,M-NMR) were in good agreement with theoretical values (M-n,M-theo calculated assuming a strictly linear structure. The average number of branches per molecule, (N) over bar (br), was calculated from NMR data and found to be nearly identical to that of a strictly nonbranched poly(lactide) synthesized using 1,4-butandiol as the initiator. We have chain extended BHMBA-PLA using a diisocyanate, proving its utility in the synthesis of hydrolytically degradable poly(ester-urethane)s containing regularly spaced pendent carboxylic acid groups.