DBU-catalyzed CO₂ fixation in polypropargylamines under solvent-free conditions

CO₂ is a useful and attractive C1 resource in polymer chemistry, given its direct applicability to polymer synthesis and modification. The use of CO₂ as a sustainable precursor of valuable polymers has aroused much interest, from both environmental and industrial perspectives. Thus far, research efforts have focused on the direct (co)polymerization of CO₂, the polymerization of monomers synthesized from CO₂, and the incorporation of gaseous CO₂ by postmodification approaches [1,2,3,4]. Among the most environmentally benign attempts is metal- and solvent-free CO₂ fixation in polymeric substrates under moderate conditions. However, this approach is hindered by the relatively low reactivity of CO₂ and the difficulty in performing polymer reactions in solvent-free conditions. Endo and coworkers reported an efficient base-catalyzed gas–solid phase reaction that incorporates CO₂ into oxirane-derived polymers [5,6,7,8], but current reports on this topic are limited. We have reported highly efficient CO₂ fixation in polypropargylamines with a 2-pyridyl group using 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU) as a base catalyst [9]. Within a few minutes, the reaction yielded the corresponding polyoxazolidinone under atmospheric CO₂ and room temperature conditions. Owing to the highly reactive substrate, the reaction occurred even under conditions with very low CO₂ concentrations in air. Herein, we report the efficient CO₂ fixation in polypropargylamines under solvent-free conditions. Two types of polymers with a propargylamine-repeating unit are synthesized: one with a main chain of repeating units and the other with side chains of repeating units. The side chain-type polypropargylamines are newly designed for the sake of simplification of copolymerization with other monomers in order to incorporate functional groups. Both types of polymers could be used for solvent-free CO₂ fixation. For side chain-type polymers, the effect of the incorporation of a tertiary amine moiety into the polymer chain was evaluated.

Commercially available solvents and reagents were used as received. (6-Bromo-pyridin-2-yl)methanol and N-benzyl-1,1-dimethylpropargylamine were synthesized as described in the literature [9]. ¹H and ¹³C NMR spectra were recorded by a Bruker Biospin AVANCE DPX-300 spectrometer and a Bruker AVANCEIIIHD500 using deuterated solvents. Spectra were calibrated using residual undeuterated solvent, tetramethylsilane as the internal standard. The IR spectra were recorded on a JASCO FT/IR-230 spectrometer. High-resolution mass spectrometry–electrospray ionization (HRMS–ESI) data were collected using a JEOL JMS-700 mass spectrometer at the Center for Advanced Materials Analysis, Tokyo Institute of Technology and are available on request. Size exclusion chromatography (SEC) was carried out at 30 °C in dimethylformamide (DMF) (5 mM LiBr, 0.85 mL/min) using a JASCO PU-2080 system equipped with a set of TOSOH TSKgel G2500H and G4000H columns. The number average molecular weight (M_n), weight average molecular weight (M_w), and polydispersity index (PDI) of the obtained polymers were calculated based on a polystyrene calibration. The 5% and 10% weight decomposition temperatures T_d5 andT_d10, at which the polymers lose 5% and 10% of their weight, respectively, were determined by thermogravimetric analysis (TGA). The TGA was carried out on a Shimadzu TGA-50 instrument under N₂ atmosphere (flow rate 50 mL/min) at a heating rate of 10 °C/min. The glass transition temperatures (T_gs) of the polymers were determined by differential scanning calorimetry (DSC) using a Shimadzu DSC-60 instrument in N₂ atmosphere (flow rate 50 mL/min) with liquid N₂ as a refrigerant. The DSC heating rate was 10 °C/min.

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