Marine-biodegradable alternating copoly(ester-amide)s from (R)-3-hydroxybutyric acid and amino acid with high glass-transition temperature

• Alternating copoly(ester-amide)s composed of 3HB and amino acids prepared. • P[3HB- alt -(amino acid)]s exhibited high T g values of 55–85°C. • Alternating copolymers comprising l -Ala, Gly, and β-Ala degraded in seawater. • PHB depolymerases hydrolyzed ester bonds in all synthesized copolymers. • Proteinase K selectively degraded the copolymers containing l -Ala and Gly units. Plastic pollution is a serious environmental issue worldwide. Although biodegradable polymers offer a promising solution, improving their physical properties while maintaining biodegradability remains a major challenge. Poly(ester-amide)s combine thermal properties with biodegradability; however, increasing the amide moiety content typically reduces enzymatic degradability, creating a trade-off. In this study, we synthesized the alternating poly(ester-amide)s with alternating sequence structure of ( R )-3-hydroxybutyric acid and amino acids (P[3HB- alt -(amino acid)]s). Four types of amino acids, l -alanine ( l -Ala), d -alanine ( d -Ala), glycine (Gly), and β-alanine (β-Ala), were used an amino acid moiety, and the thermal properties and biodegradability of the synthesized P[3HB- alt -(amino acid)]s were evaluated. Obtained P[3HB- alt -(amino acid)]s exhibited high glass-transition temperatures (55‒85°C). The alternating sequence enabled incorporation of amide units at a 1:1 ratio with ester units without suppressing degradability, which contrasts with conventional poly(ester-amide) designs. Biodegradation tests with seawater showed that P(3HB- alt - l -Ala), P(3HB- alt -Gly), and P(3HB- alt -β-Ala) reached 37‒58% biodegradation within 60 days based on BOD measurements and were metabolized by marine microorganisms, whereas P(3HB- alt - d -Ala) degraded slowly but generated water-soluble oligomers that microorganisms could utilize. Enzymatic degradation tests with hydrolases demonstrated that PHB depolymerase hydrolyzes ester linkages in all copolymers, while proteinase K selectively degrades l -Ala– and Gly-containing sequences due to stereochemical recognition. These results suggest that controlling the monomer sequence and stereo-configuration is the key to balancing biodegradability and physical properties, providing a strategy for designing sustainable polymers.