Chapter 21 - Supercritical Carbon Dioxide as a Green Solvent for Polymer Synthesis

Abstract

This chapter focuses on the application of supercritical carbon dioxide (scCO2) as a solvent for polymer synthesis. ScCO2 is an inexpensive, nontoxic, and nonflammable solvent for material synthesis and processing. It has emerged as the most extensively studied environmentally benign medium for organic transformations and polymerization reactions. One of the fundamental issues that should be considered while implementing CO2 for polymer synthesis or processing is polymer solubility. A number of research groups have synthesized “CO2-philic” fluoropolymers or silicone-based materials for use as steric stabilizers in dispersion polymerization, phase transfer agents for liquid–liquid extraction, supports for homogeneous catalysis, and surfactants for the formation of water/CO2 emulsions and microemulsions. It has been demonstrated that inexpensive poly(ether carbonate) (PEC) copolymers are soluble in CO2 under moderate conditions and could function as building blocks for inexpensive surfactants, although there are numerous practical difficulties. These hydrocarbon systems involve PECs synthesized by aluminum-catalyzed copolymerization of cyclic ethers with CO2. These copolymers were found to be soluble in liquid CO2 at concentrations of 0.2–1.5% (w/v) at ambient temperatures and pressures in the range 120–160 bar. These statistical copolymers were generated from inexpensive feedstocks and are thus appealing as “building blocks” for cheap surfactants. Similarly, sugar acetates are highly soluble and have been proposed as renewable “CO2-philes.” Such materials could, in principle, function as “CO2-philic” building blocks for inexpensive ligands and surfactants, but this potential has not been realized and numerous practical difficulties remain. Poly(vinylacetate) (PVAc) has also been shown to exhibit high solubility in CO2 with respect to other vinyl hydrocarbon polymers, although the polymer is soluble only at relatively low molecular weights under conditions of practical relevance.