Phase Equilibrium Thermodynamics for a Polycarbonate Production Process using Supercritical Carbon Dioxide

Thermodynamic phase equilibrium data, which are relevant to the development of a novel production process for polycarbonates from CO2 and epoxides, are presented. In this solventfree process, CO2 is coupled to epoxides using various transition-metal catalyst systems. Data on the performance of these systems are discussed. Additionally, the polycarbonate solubility in CO2 and CHO is considered, both inside and beyond the P-T ranges that have been used for synthesis of the polycarbonate. Moreover, CHO-CO2 vapour pressure isotherms have been measured and modelled. Experimental results indicate that following the pressure decay during the polymerisation reaction offers a feasible and reliable method for on-line monitoring of monomer conversion. Results are finally discussed in terms of their implication on the development of novel production processes for polycarbonates. Introduction Using various transition-metal catalyst systems, CO2 is alternatingly coupled to epoxides in order to produce polycarbonates without the further need for additional solvents. Supercritical CO2 is used both as a renewable reactant and simultaneously as a means to effectively control the physical and chemical properties of the reaction medium. Although coupling reactions between epoxides and CO2 have been under investigation since 1969 [1], little is known about the phase equilibrium thermodynamics of the systems involved. The majority of studies have focussed on catalyst performances based on overall batch yield and average turnover rates. Little attention is paid, however, to the effects of phase behaviour due to e.g. discrepancies between local and overall monomer and catalyst concentrations. As has been underlined by Beckman [2], phase behaviour plays a significant role for the accurate comparison of catalyst performance and analysis of reaction products in relation to initial pressure, temperature and composition. Another problem when using experimental data from batch-wise experiments is that catalyst activity and selectivity are likely not to be constant during the reaction period, which cannot be taken into account if no on-line data is available. Very recently, initial formation rates of polycarbonate and cyclic carbonate from in situ infrared spectroscopic measurements have been reported [3]. Because of high viscosities and pressures, few alternative methods are available. However, one reaction parameter, namely pressure, can be monitored relatively easy. In this paper it is shown that following the pressure decay during reaction provides an experimentally feasible and reliable method to monitor monomer conversion on-line. Furthermore, in order to exploit fully the potential of this novel process, it is imperative to study the phase behaviour both inside and outside the pressure and temperature range used for polycarbonate synthesis thus far. Knowledge of the phase behaviour will provide useful information for basic process development guidelines. Data on the performance of different catalyst systems for the copolymerisation of cyclohexene-oxide (CHO) and CO2 into poly(cyclohexene-carbonate) (PCHC) are briefly discussed. Additionally, experimental data regarding the thermodynamic phase equilibrium behaviour of the systems PCHC-CO2, PCHC-CHO-CO2 up to 200 oC and 4000 bar are presented. CHO-CO2 vapour pressure isotherms have been measured and modelled up to151 oC. The data are subsequently interpreted in terms of PCHC and CO2 solubility in CHO and translated into basic process development guidelines. Catalyst performance For this work various transition-metal based catalyst systems are synthesised [4] and tested in batch-wise polymerisation runs. Yield, selectivity towards coversus homopolymerisation, Mw and Mw/Mn of some of these systems are presented in Table 1. TOF’s are calculated averages, based on overall reaction time. Table 1: Performance of some catalyst systems