Poly(lactic Acid) and Chain-Extended Poly(lactic acid)−Polyurethane Functionalized with Pendent Carboxylic Acid Groups

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 ∼3 h at 130 °C, using a catalyst concentration of 3 × 10-2 wt % Sn. The polymers were characterized by gel permeation chromatography, equipped with a multiangle laser light scattering detector (GPC−MALLS), 1H NMR, 13C 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 (Mn,GPC) and 1H NMR (Mn,NMR) were in good agreement with theoretical values (Mn,theo) calculated assuming a strictly linear structure. The average number of branches per molecule, N̄br, 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.