Influence of CO₂ on Acyl Compound Initiated Polymerization of 2-Pyrrolidone

The influence of catalyst and CO₂ on the synthesis of bio-based poly(γ-aminobutyric acid) (PGABA) was investigated in the presence of an acyl compound initiator. The molecular structure and crystal form of the products were examined using magnetic resonance spectroscopy (¹H-NMR), Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD). Thermogravimetry (TG) and differential scanning calorimetry (DSC) were used to evaluate the thermal properties of PGABA prepared via different ways. Results indicated that the addition of CO₂ had an adverse effect on the yield of PGABA. When the mole fraction of acyl catalyst was fixed at 6% or 7%, the molecular weight increased at first and then decreased with increasing the dosage of CO₂ , while the yield decreased successively. When the mole fraction of catalyst was increased to 9%, increasing the dosage of CO₂ less affected the molecular weight of PGABA, but the yield kept decreasing. On the other hand, the absence of the initiator dramatically decreased the yield of PGABA in the CO₂ containing system. At the different dosages of CO₂, the molecular weight rose firstly and then went down with increasing catalyst mole fraction. Such a trend became not evidence at a high dosage of CO₂. The impact of catalyst mole fraction on the yield was a little complex. The yield could reach a maximum with increasing the catalyst mole fraction when CO₂ mole fraction was less than 13.2%, above which a positive effect of the catalyst on the yield was presented. The crystals of PGABA samples prepared through various methods were all of the α form and independent of the mole fraction of catalyst, initiator, and CO₂. Moreover, introducing CO₂ less affected the melting point of PGABA, but led to the increase of the thermal decomposition temperature and the improvement of the thermal stability of PGABA.