The impact of various Lewis bases on the miscibility of siloxane polymers in CO 2 was investigated using both ab initio calculations and experimental phase behavior studies. A series of sidechain functional silicones were synthesized containing various Lewis bases in the side chain, and their phase behavior was compared in CO 2 at 295 K. Calculations showed that interactions between CO 2 and ethers (either a dialkyl ether or the ether oxygen in an ester group) should be as favorable as interactions between CO 2 and a carbonyl oxygen. Indeed, phase behavior results seemed to support this, as ether-functional silicones exhibited miscibility pressures as low or lower than acetate-functional analogues. Further, a keto-functional material was not nearly as CO 2 -philic as the acetate functional analogue. In general, the location of the phase boundary in CO 2 is governed by a balance between forces working to increase miscibility pressures, such as increased cohesive energy density of the polymer or factors suppressing the entropy of mixing, and those working to lower miscibility pressures, such as enhanced specific interactions with CO 2 and increased free volume or chain flexibility.

The cloud point curves of a series of oxygen-containing polymers in CO 2 were measured to attempt to deduce the effect of oxygen functional groups within a polymer on the polymer/CO 2 phase behavior. The addition of an ether oxygen to a hydrocarbon polymer, either in the backbone or the side chain, enhances "CO 2philicity" by providing sites for specific interactions with CO 2 as well as by enhancing the entropy of mixing by creating more flexible chains with higher free volume. Ab initio calculations show that both ether and ester oxygens provide very attractive interaction sites for CO 2 molecules. The binding energy for an isolated ether oxygen with CO 2 is larger in magnitude than that for a carbonyl oxygen/CO 2 complex. However, acetate functionalized polymers are more CO 2 -soluble than polymers with only ether functionalitiesspossibly because acetate functional groups contain a total of three binding modes for CO 2 interactions, compared with only one for the ether functional group. Experiments clearly indicate that adding a single methylene group as a spacer between a polymer backbone and either an ether or acetate group exhibits a strong deleterious effect on phase behavior. This effect cannot be explained from our ab initio calculations.

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