Facile green ring-opening polymerization of l-lactide catalyzed by natural kaoline
This work demonstrates a facile and green route for the solvent-free ring-opening polymerization (ROP) of L-lactide, which is catalyzed by natural kaoline for the first time. poly(L-lactide) (PLA) in 68.5% yield was obtained under the conditions of catalysis with a mass ratio of m(kaoline)/m(LA) = 1/1500, at 140 °C for 24 h, resulting in a high molecular weight (Mn = 4.55 × 104 g mol−1) and a low polydispersity index (PDI = 1.34). The results from DSC and XRD revealed that the interactions between PLA chains weakened by the plasticizing effect of kaoline. The rigid structure of kaoline can prevent PLA chains from crystallizing, thus causing a decrease in the glass transition temperature of the obtained PLA product. O, Si and Al were shown to be the main components of kaoline, with the existence of less than 2% of Ti, Fe, K, Ca, Mg by XPS and XRF. The layered structure of kaoline was examined by SEM and TEM, which determined the polymer chain growth model and catalytic mechanism. XRF showed that the content of heavy metals in the product was lower than 5 ppm without removing the catalyst, which meets the requirement for medical devices. The catalytic behavior of kaoline lays the foundation for natural mineral catalysts in the field of medical aliphatic ester polymerizations.