Homogeneous free radical polymerizations in supercritical carbon dioxide

The goal of the research described herein is to explore the utilization of carbon dioxide as a solvent for the synthesis and processing of polymers. We have discovered that a series of amorphous fluoropolymers are highly soluble in supercritical carbon dioxide. These fluoropolymers are used in many technologically demanding applications because of their balance of high-performance properties. A significant impediment to the synthesis of variants of commercially available amorphous fluoropolymers is their general insolubility in most solvents except chlorofluorocarbons (CFCs). The environmental concerns about CFCs can be circumvented by preparing these technologically important materials in supercritical fluids. We have successfully synthesized a series of high molar mass amorphous fluoropolymers, both acrylic and styrenic fluoropolymers, in supercritical carbon dioxide. This represents the first successful homogeneous polymerization in an inert supercritical fluid.; In order to understand the polymerization mechanisms and to optimize the polymerization conditions, the initiation kinetics in supercritical carbon dioxide was investigated. The thermal decomposition of 2,2{dollar}spprime{dollar}-azobis(isobutyronitrile) (AIBN) in supercritical CO{dollar}sb2{dollar} was studied by using high pressure UV/vis spectroscopic methods. The decomposition rate and initiation efficiency of AIBN in supercritical CO{dollar}sb2{dollar} as a function of pressure and temperature were determined and modeled using Kirkwood solution theory. The decomposition rate of AIBN in supercritical CO{dollar}sb2{dollar} was found to be considerably lower than that observed in benzene at the same temperatures and pressures. But the initiation efficiency of AIBN in supercritical CO{dollar}sb2{dollar} was shown to be higher than that measured in benzene.; Based on the solubility in CO{dollar}sb2{dollar}, we categorize polymers into "CO{dollar}sb2{dollar}-philic" or "CO{dollar}sb2{dollar}-phobic" materials. The CO{dollar}sb2{dollar}-philic amorphous fluoropolymers were used to molecularly engineer macromolecules for carbon dioxide solution applications. We successfully synthesized two types of triblock copolymers via a living free radical polymerization method: (a) fluorocarbon-hydrocarbon-fluorocarbon triblock copolymers for use as surfactants for CO{dollar}sb2{dollar}; (b) hydrocarbon-fluorocarbon-hydrocarbon triblock copolymers for use as CO{dollar}sb2{dollar} viscosity modifiers. The bulk properties of these heterophase materials are currently used to prepare asymmetric membranes for reverse osmosis and pervaporation applications.