Biodegradability of poly(ester-thioether)s containing chiral biomass <i>via</i> a Michael-type thiol-ene click reaction

We prepared dianhydrosugar-based diacrylates: isosorbide bis(acrylate) (ISDA) and isomannide bis(acrylate) (IMDA) from the corresponding dianhydrosugars. Using a scandium triflate as the catalyst, the chemoselective dehydration of d-, l-, and <i>meso</i>-tartaric acids with 2-mercaptoethanol selectively synthesized three dithiols: l-bis(2-mercaptoethyl)tartrate (META), d-META, and <i>meso</i>-META. The thiol-Michael click polyaddition of ISDA or IMDA with the three dithiols proceeded in <i>N</i>,<i>N</i>-dimethylformamide (40 °C) using triethylamine as the catalyst, yielding the expected six poly(ester-thioether) diastereomers (<i>M</i> _n, 8.2 × 10³ to 9.2 × 10³; molecular weight distribution (<i>M</i> _w/<i>M</i> _n), 1.29-1.51). The synthesized poly(ester-thioether)s showed a single glass transition temperature (<i>T</i> _g) between -8 and 14 °C. In biodegradation measurements, monitored by biochemical oxygen demand (BOD) values in active sludge, poly(IMDA-<i>alt-meso</i>-META), poly(ISDA-<i>alt</i>-l-META), and poly(ISDA-<i>alt</i>-d-META) showed lower biodegradability. In contrast, poly(IMDA-<i>alt</i>-l-META), poly(IMDA-<i>alt</i>-d-META), and poly(ISDA-<i>alt-meso</i>-META) showed 16% to 28% biodegradation after 30 days, indicating diastereomeric effects on biodegradability. Since enzymatic hydrolysis using lipase showed a similar trend to the BOD tests, we concluded that biodegradability depends on the stereochemistry along the polymer backbone.