Screening of different organocatalysts for the sustainable synthesis of PET

Industrially, poly(ethylene terephthalate) (PET) is produced using organometallic catalysts which have shown excellent ability to promote the polymerization at elevated temperatures. However, the catalysts remain permanently in the fabricated item because their removal is very difficult both from a technical and economical point of view. In this work, several organocatalysts have been evaluated as alternative to the industrially applied organometallic catalysts for the bulk polymerization of PET from dimethyl terephthalate (DMT) and ethylene glycol (EG) at high temperatures. The effectiveness of the organic catalysts in PET synthesis is evaluated by SEC, DSC, TGA, H-1 NMR and FTIR spectroscopy. After screening various organocatalyst families, our results demonstrate that the catalytic activity of 1,8-Diazabicyclo[5.4.0]undec-7-ene: benzoic acid salt (DBU:BA) is competitive in comparison with the conventional organometallic-based catalyst employed in PET synthesis. This ionic salt combines the excellent catalytic ability of organic compounds with the thermal stability of metal-based catalysts, resisting degradation up to > 250 degrees C which is close to the industrial polymerization temperature of PET. Moreover, the dimerization degree of ethyleneglycol in the presence of DBU:BA is significantly reduced in comparison to organometallic catalysts which enhances the thermal properties of the synthesized PET using DBU:BA. Thus, organic salts have shown to be promising catalyst for the bulk polymerization of PET and in addition may be a viable option to obtain metal-free PET.