In many polymerization processes, diffusion of monomers, solvents, and other species is critical in the overall polymerization process. Coupled to these diffusion processes are often equilibrium distribution considerations, which can significantly impact the properties of the final product. The Center has studied such phenomena in various polymerization processes ranging from concentration gradients in bulk reactors to the distribution of co-monomers in high pressure gas phase reactors and aqueous emulsion polymerization reactors. As in other Center activities, the keystone in these studies has been expertise in molecular diffusion, mass transfer, and the thermodynamics of polymer-solvent systems.