2. Phase Behavior of PEG/CO₂ System

High-pressure processes are widely applied in the polymer industry. Near-critical and supercritical fluids (SCFs) (e.g. scCO₂) are used owing to their easily tunable density, which enhances control of polymer solubility and good separability from polymer. On the other hand, for homogeneously catalytic reaction using polyethylene glycol (PEG) as a solvent, CO₂ can act as a miscibility switch to shift the system from homogeneous at atmospheric conditions to heterogeneous under CO₂ pressure. This allows for extraction of the products into the organic solvent phase and immobilization of the homogeneous catalyst in the PEG phase. Understanding of phase behavior in a biphasic solvent system such as PEG/CO₂, where a chemical reaction takes place in one phase and the products can be extracted to another phase, would be critical for the design of efficient and environmentally friendly reaction and separation process. In this chapter, phase behavior of different PEG/CO₂ systems from 1.13–29.00 MPa CO₂ pressure at 313.15–348.15 K with PEG molecular weights (MWs) in the range of 200–35000 is discussed. Ternary systems such as CO₂/PEG/ethanol, CO₂/PEG/1-pentanol, CO₂/PEG/1-octanol, CO₂/PEG/1, 4-dioxane, CO₂/PEG/acetonitrile and CO₂/PEG/1-octene are also investigated. Phase equilibrium data, solid–liquid–vapor (S–L–V) curve, influence of CO₂ addition on viscosity of PEG, solubility data of CO₂ in PEG or PEG in combination with an organic solvent and so on are explored.