Biomass (e.g., plant biomass, animal biomass, and municipal waste biomass) is processed to produce useful intermediates and products, such as aliphatic hydroxy-carboxylic acid and hydroxyl-carboxylic acid derivatives. These aliphatic hydroxy-carboxylic acids are, in turn, polymerized. The polymerization is carried out using a thin film evaporator or a thin film polymerization/devolatilization device. Conversion of lactic acid to poly lactic acid is an especially useful product to this process.