Microbial synthesis and characterization of poly(3-mercaptoproprionate-co-3-mercapto-2-methylproprionate) [P(3MP-co-3M2MP)]: Co-polythioesters consisting of achiral 3MP unit and (RS)-3M2MP unit.

Polythioesters (PTEs) can be microbially polymerized from thiol carboxylate precursors by polyhydroxyalkanoate (PHA)-producing bacteria. Poly[(RS)-3-mercapto-2-methylpropionate] [P(3M2MP)], characterized in our previous study, is an amorphous and highly elastic polymer. In this study, achiral 3-mercaptopropionate (3MP) was polymerized in recombinant Escherichia coli DH5α, and the copolymerization of 3MP with (RS)-3M2MP was subsequently investigated to characterize a PTE copolymer. P(3MP) was hard and brittle, and insoluble in most solvents, but its amorphous form immediately after extraction from cells was soluble in hexafluoroisopropanol. P(3MP) exhibited very fast crystallization behavior, with a small temperature gap (39 °C) between the melting and crystallization temperatures and a high spherulite growth rate of 25.5 μm/s at 120 °C. P(3MP-co-3M2MP) exhibited a blocky microstructure attributable to the properties of the PHA synthase used and could be fractionated into chloroform-soluble and -insoluble fractions. As the 3M2MP fraction in P(3MP-co-3M2MP) increased, the polymers' melting temperature and crystallinity decreased, yielding soft, flexible polymers. The temperatures at 5% weight loss of P(3MP-co-3M2MP) increased from 235 °C to 282 °C as the 3M2MP fraction increased from 17 to 100 mol%. P(3MP-co-3M2MP) is the first microbial PTE copolymer containing thiol carboxylate monomers with different properties, enabling the tailored design of a wide range of material properties.