Researchers at the University of Edinburgh and RPTU University Kaiserslautern-Landau in Germany have developed a one-step chemical process that converts polyesters into faster-degrading materials by swapping oxygen atoms for sulphur in the polymer chain. The method, published this month in the Cell Press journal Chem Circularity, produces what are known as polythionoesters - polymers whose carbon-sulphur bonds are weaker than the carbon-oxygen bonds they replace, and which therefore break down more readily.

The process (known as thionation) works by applying a molecule that installs sulphur atoms in place of oxygen to an existing polyester under standard lab conditions. Because carbon-sulphur bonds require less energy to break than their carbon-oxygen equivalents, the resulting material degrades faster while keeping its structural properties intact. By controlling the degree of substitution, the researchers could tune crystallinity, thermal behaviour and rate of degradation to suit different applications.

So far the method has been demonstrated on polycaprolactone (PCL), a polyester used in food packaging, 3D printing and biomedical implants. PCL is already classed as biodegradable, but degrades slowly under ambient conditions. Thionation accelerated its breakdown substantially.

The researchers now plan to apply the same chemistry to polyesters that are not currently biodegradable. Some 99 per cent of plastics in circulation resist biological breakdown, and existing biodegradable alternatives often require high temperatures or harsh chemicals to decompose. Dr Jennifer Garden of the University of Edinburgh's School of Chemistry, who co-led the study, told resourcemedia that the process has so far been demonstrated for polycaprolactone, "with potential to extend to other polyesters," adding that the team planned to test the approach on post-consumer plastic waste in future studies.

The modified material has a second useful property. Polythionoesters produced by this method can be chemically recycled back to their original monomer with high yield and purity - what the paper calls a circular lifecycle. A polyester waste stream could, in principle, be manipulated to produce a faster-degrading material that, at end of life, yields clean monomer feedstock for remanufacture rather than residual waste.

"The thionation of polyesters is a challenging task, as these materials are less reactive towards thionation than many other polymers, and accessing polythionoesters via traditional routes can be difficult," said Dr Garden. "What makes this discovery so exciting is that we've successfully developed a strategy that opens the door to a whole new range of sulfur-containing materials."

The process also showed selectivity between different polyester types. Polylactic acid (PLA) - another common biodegradable polymer - resisted thionation under the same conditions. In copolymers containing both PCL and PLA segments, only the PCL portions were modified. For mixed plastic waste streams, this selectivity could eventually allow targeted modification of specific polymer fractions without affecting others, though this application remains theoretical at the current stage of research.

Further work is needed to assess the environmental safety of the breakdown products from polythionoesters, and to determine whether the bench-scale process can be applied economically at industrial volumes and to the heterogeneous, contaminated feedstocks typical of post-consumer waste.