Experimental and DFT investigation of reactivity in the near-azeotropic R1233zd(E)/R1336mzz(E) mixture

Halogenated olefin refrigerants and their mixtures are increasingly considered as low-GWP working fluids for high-temperature heat pumps and organic Rankine cycles. However, their unsaturated nature raises chemical stability concerns that conventional assessment methods may fail to capture. This work demonstrates that the standard static-isochoric thermal stability test - while effective for pure components, yielding safe-use temperatures of 200 °C for R1233zd(E) and 350 °C for R1336mzz(E) - fails to detect polymerisation pathways through pressure monitoring alone, as no non-condensable gases are produced. Using the near-azeotropic R1233zd(E)/R1336mzz(E) mixture as a case study, polymerisation is shown to occur at 110 °C - conditions relevant to ORC and HTHP applications - with liquid and solid reaction products observed in dedicated aging experiments at saturation pressure (below 15 bar), and independently during bubble point measurements at higher pressures. Density Functional Theory simulations reveal energetically favourable chain-growth reaction pathways that explain this behaviour, and combined Raman spectroscopy and ATR-FTIR characterisation of the solid products identifies functional groups consistent with a fluorinated oligomeric structure, in agreement with the DFT-predicted mechanism. These findings establish a multi-method framework - combining standard stability testing, DFT simulations, and spectroscopic analysis - as essential for the reliable chemical assessment of olefin-containing refrigerant mixtures, and raise concerns regarding the suitability of the R1233zd(E)/R1336mzz(E) mixture as a working fluid under the investigated conditions.