Catalyst separation in atom transfer radical polymerization

Atom transfer radical polymerization (ATRP) is a living radical polymerization process utilizing transition-metal complexes as catalysts to mediate the propagation of the polymerization. It is a very versatile process and can synthesize a wide spectrum of polymers with controlled structures. However, a high concentration of soluble catalyst is required in the ATRP process. These catalysts generally co-precipitate in the products as contaminants. Thus, a remaining challenge in ATRP is how to efficiently and economically remove/reduce the catalyst residue from its products, especially for large-scale industrial productions. Post-purification such as reprecipitation, washing, adsorbing with ion-exchange resins, and passing columns of alumina or silica gel has been used on small lab scales. Biphasic catalysis of liquid-liquid biphase (fluorocarbon-organic solvents, ionic liquid-organic solvents) and liquid-solid biphase (solid phase ATRP, solid-supported catalysts by physical adsorption and by covalent bonding, soluble/recoverable supported catalysts, immobilized/soluble hybrid system) has been explored for ATRP. In spite of the advantages of easy catalyst separation/recovery and possibility of scaling up, its control over the polymerization generally deteriorates compared with homogeneous catalysis. Finally, a reversible catalyst supporting concept that is homogeneous for catalysis but heterogeneous for separation/recovery is presented. The development and characters of each system are critically reviewed. (C) 2004 Elsevier Ltd. All rights reserved.