Nowadays, it is important to think about degradability and recyclability when designing new polymers. A good example is poly(thio)lactides, which consist of ester compounds or thioesters (with sulphur). Both have their advantages and disadvantages. An Sofie Narmon, Michiel Dusselier and colleagues at KU Leuven wanted to combine these ‘pros’ by making a monomer that combines the ester and the thio-ester.

Poly(thio)lactides are composed of lactides and thiolactides, ring-shaped molecules with ester and thioester moieties, respectively. The main difference between the two is the rate at which they polymerise and the way they degrade. Both polymerise by opening the ring, but the sulphur bonds in thiolactides make this easier, so you usually have to cool the polymerisation reaction if you want it to progress completely. The disadvantage of thiolactides is that they are unlikely to be biodegradable, whereas esters are.

The Leuven team has now combined the two characteristic components of lactides and thiolactides in a previously unpublished monothiolactide, a ring-shaped molecule with both an ester and a thio-ester group. They wanted to do this with maximum atom economy by direct condensation reactions, but this resulted in very low yields. They therefore used less ‘green’ reaction conditions and encourage others to find a better, greener synthesis.

Despite the disappointing yields, the ring-opening polymerisation (ROP) of monothiolactide was very efficient and was better than that of thiolactide. The resulting polymers had high molar masses of up to 258 kg/mol. In comparison, attempts to copolymerise lactide with thiolactide resulted in polymers with a molar mass of only 3 - 15 kg/mol.

The researchers observed that the ROP was more similar to that of lactide. So they tried a copolymerisation of monothiolactide with lactide. This went surprisingly well, paving the way for polyesters with adjustable sulphur content. Monothiolactide also depolymerises more efficiently than polythiolactide, making it a promising monomer.

Narmon, A.S. et al. (2024) ChemSusChem e202400134, DOI: 10.1002/cssc.202400134