Lipase Catalysis. A Direct Route to Linear Aliphatic Copolyesters of Bis(hydroxymethyl)butyric Acid with Pendant Carboxylic Acid Groups

Immobilized Lipase B from Candida antartica (CAL-B, Novozyme 435) catalyzed terpolymerizations of bis(hydroxymethyl)butyric acid, BHB (AB<INF>2</INF>) and 1,8-octanediol (B<INF>2</INF>) with adipic acid (A<INF>2</INF>). The copolymerizations of these AB<INF>2</INF>, B<INF>2</INF>, and A<INF>2</INF> monomers were conducted in bulk, at 80 °C, without activation of the acid groups. Carbon (¹³C) NMR studies using a series of model BHB derivatives showed that CAL-B was strictly selective for esterification of BHB hydroxyl groups while leaving the carboxylic acid unchanged. Thus, all polymerizations were conducted as if BHB were a B<INF>2</INF> monomer and then formulating a 1 to 1 ratio of carboxylic acid to hydroxyl groups in the monomer feed. By varying the monomer feed ratio, copolyesters with 9−45 mol % BHB−adipate units were formed with M<INF>w</INF> values between 21 900 and 2300 g/mol. By this direct polymerization without protection−deprotection chemistry, a series of linear aliphatic copolyesters with controlled quantities of pendant free acid groups was prepared. Thermal and crystalline properties of the copolyesters were studied by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and wide-angle X-ray scattering (WAXS). Increasing the BHB content in the copolyesters resulted in melting temperature depressions that were well described by Baur's equation for random copolymers where BHB−adipate units are excluded from the crystal phase of the crystallizable 1,8-octanediol−adipate units.