An unexpected discovery points the way to breaking down polycarbonate under mild conditions and promises a potentially cost-neutral process for upcycling the common waste, believe researchers at the University of Bath, Bath, U.K.
Based in Bath’s Centre for Sustainable and Circular Technologies (CSCT), the researchers were investigating a number of ZnII complexes bearing half-salan ligands in the mild and selective chemical upcycling of various commercial polyesters and polycarbonates.
When they tested Zn(2)2 and Zn(2)Et complexes in methanol at room temperature, the researchers were surprised to see commercial poly(bisphenol A carbonate) (BPA-PC) beads broken down into their chemical constituents in 12–18 minutes.
The resulting bisphenol A (BPA) and dimethyl carbonate (DMC) molecules are high quality, with the latter reportedly ideal as a green solvent and building block for synthesis of industrial chemicals. The ability to recover BPA prevents leakage of a potentially damaging environmental pollutant, too.
CSCT researcher Jack Payne describes the results as remarkable. It is the first example of a discrete metal-mediated poly(BPA-PC) methanolysis reaction being appreciably active at room temperature, he stresses.
“This was extremely surprising since state-of-the-art systems prior to this relied on elevated temperatures (≥70°C), high catalyst loadings (≥5 mol%) and/or prolonged reaction times — especially for those that had reported activity at room temperature. Given the lack of discrete metal-based catalytic examples in the field, we expected our systems to be competitive with organocatalytic examples, but to complete polycarbonate consumption within 20 minutes at room temperature exceeded our expectations ten-fold,” he declares.
The CSCT team now is focusing on optimizing the catalyst, mainly by trying to fine-tune the ligand that binds the zinc.
“Once we have a clearer understanding of the catalyst design features that promote such high activity, we will then explore applicability to other reactions and polymer classes,” Payne notes.
At the same time, the CSCT is collaborating with Bath’s chemical engineering department in a project to demonstrate proof-of-concept scale-up work.
“The major challenges here have been catalyst scale-up and stability since we need the catalyst to be stable in air. Looking on to pilot and commercial scale, catalyst recovery will be a primary challenge to address since our system is homogenous. One possible solution to this is to immobilize the catalyst on a support to allow easy separation from the product(s)/solvent,” he explains.
Since being described in a recent issue of ChemSusChem, the work has attracted significant interest from industry, says Payne; the CSCT is in the preliminary stages of exploring possible collaborations with several companies.
