Equilibrium and non-equilibrium polycondensation process of segmented poly-perfluoro(oxymethylene-ran-oxyethylene)-carbonates

The melt transesterification polyreaction between diphenyl carbonate and poly-perfluoro(oxymethylene-ran-oxyethylene)dimethylol terminated oligomers (briefly perfluoropolyethers or PFPE diols) to give linear, highly fluorinated polycarbonates was investigated. The PFPE-polycarbonates were characterised by GPC and 19F NMR spectroscopy. The polycondensation was catalysed by Zn(Ac)2 at temperatures comprised between +120 and +200 °C. The polymerisations carried out in closed systems (no by-product distillation) gave information on some aspects of the equilibrium thermodynamics of the process. The effects of temperature and stoichiometric ratio were particularly studied. It resulted that the equilibrium degree of polymerisation (Xn)eq is inversely proportional on a semi-log scale to the temperature of polymerisation as typical of exothermal processes. The standard enthalpy of polymerisation was calculated (ΔH0=−31±3 kJ/mol). It was found, in apparent disagreement with theory, that maximum molecular weights were achieved when an excess of diphenyl carbonate monomer is used. In equilibrium conditions low molecular weights (Mn≤104) were generally obtained. The non-equilibrium conditions focused to the obtainment of high molecular weight polycarbonates were also investigated, using temperature ramps and pressure steps to get the phenol off the reaction medium. A comparison between two different functionality PFPE oligomers was finally done in non-equilibrium conditions, showing that Mn as high as 105 can be obtained starting from a PFPE macromonomer with a OH functionality as high as 2.