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Influence of tert-amine groups on the solubility of polymers in CO2
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There is a need to develop new, non-fluorous polymers that are highly soluble in CO2. Experimental evidence indicates that tertiary amine and pyridine groups may exhibit favorable Lewis acid-Lewis base type interactions with CO2. It is therefore reasonable to assume that incorporation of tertiary amines into the side chain or backbone of non-fluorous polymers may impart a degree of CO2-solubility to the polymer. We present experimental results for eight different tert-amine-containing polymers. Of these polymers, only propyl dimethylamine-functionalized poly(dimethylsiloxane) is soluble in CO2 at temperatures and pressures accessible in our experiments, but even this polymer is less soluble than non-functionalized poly(dimethylsiloxane) at the same chain length. We have performed ab initio calculations on tertiary amine-containing moieties representative of some of the polymers examined experimentally. Our calculations confirm that amine-CO2 interactions are indeed energetically favorable. However, we also find that the moiety self-interactions are typically more favorable than the CO2-moiety interactions. This indicates that the lack of solubility of amine-containing polymers in CO2 is a direct result of strong polymer-polymer interactions.