Thermally Stable Biodegradable Polyesters with Camphor and Tartaric Acid Coupled Cyclic Diester Monomers for Controlled Hydrolytic Degradations

Abstract

The development of biodegradable engineering plastics is challenging, because increases in biodegradability mostly degrade other material properties, such as mechanical and thermal properties of polymers. Here, we use bioderivatives of diester monomers, dimethyl (1S,4R)-1,7,7-trimethylspiro[bicyclo[2.2.1]heptane-2,2′-[1,3]dioxolane]-4′,5′-dicarboxylate (camphor dimethyl dl-tartrate, Ct diester), for the synthesis of phthalate-based copolyesters to enhance degradability without a notable loss of thermal stability. Ct diester synthesized by acetalization of tartaric acid and camphor contains a bridged type of rigid bicycling ring with high thermal stability and controlled degradation via hydrolysis of acetal groups within the spiro-ring structure. Various alkyl-length diols of ethylene, butylene, and hexylene are copolymerized with mixtures of dimethyl phthalate (DMT) and Ct diester in a 9:1 molar ratio. The glass transition temperature and degradation temperature of the copolyesters are comparable to those of DMT-based homopolyesters. Due to decreases in crystallinity, copolyesters exhibit slight decreases in melting temperature of 10 to 20 °C, which can be advantageous for reducing processing temperatures. Notably, the copolyesters demonstrate enhanced hydrolytic degradation of 9.3 to 26.2% in pH 2 environment for 18 days, as measured by the degree of molecular weight reductions. These values are between 1.7 and 3.5 times higher than the degradation rates of their control samples of DMT-based homopolyesters under the same degradation conditions.