Physical and Chemical Responses of Amidine-Containing Polymers in the Capture and Release of CO2

Abstract

Polymeric materials containing amidine motifs are of high interest due to their ability to reversibly capture and release CO₂ at ambient temperature. Here we probe physical and chemical responses of styrene-based copolymers containing linear amidine motifs as functions of CO₂ and inert gas exposures and temperature. A copper-catalyzed azide–alkyne cycloaddition “click” reaction involving N′-propargyl-N,N-dimethylacetamidine is used to modify random copolymers, resulting in an array of linear amidine motifs along the chain backbone with the amount of CO₂-active amidine controlled by the copolymer composition. Through thermogravimetric measurements, we demonstrate that the amidine-functionalized copolymers efficiently capture CO₂ upon exposure to a stream of CO₂ (at 27 °C) and release it at a slightly elevated temperature (50 °C) when exposed to an inert gas stream (N₂). In addition to displaying a maximum adsorption capacity of 22 wt % in the presence of pure CO₂, the copolymers show composition-dependent direct air capture (DAC) behaviors. Small molecule analogs are used to definitively understand degradation via chemical hydrolysis of the amidine moiety, which leads to insolubility and a large reduction in CO₂ adsorption capacity (1.7 wt %). Neutron vibrational spectroscopy and DFT calculations confirm that CO₂ binds strongly to the amidine motif, inducing a strong bending of the CO₂ molecule from its linear geometry. The coupled insights into mechanisms and behaviors of CO₂ adsorption in amidine-functionalized polymers provides a foundation for future investigations of CO₂-responsive polymers and soft materials to improve carbon capture and sequestration technologies.