Synthesis and characteristics of silica-supported N-heterocyclic carbene catalyst for ring-opening polymerization of D-lactide to produce polylactide

Homogeneous N-heterocyclic carbene (H-NHC) catalyst was synthesized by the reaction of 3-chloropropyl trimethoxysilane with 1-methyl imidazole and followed by carbene generation with potassium tert-butoxide. A silica-supported N-heterocyclic carbene (S-NHC) catalyst was obtained by immobilizing 1-methyl-3-(3-trimethoxysilylpropyl) imidazolium chloride on the calcined silica, followed by carbene generation. A series of solution polymerizations of D-lactide were conducted with the H-NHC and S-NHC catalyst to compare the characteristics of the catalysts and resulting polylactide (PLA). The PLA produced with S-NHC catalyst showed higher polymer bulk density and better morphology than that produced with H-NHC catalyst. However, the catalytic activity and the molecular weight of the PLA produced with S-NHC catalyst were a little lower than those of PLA prepared with H-NHC catalyst. The molecular weight distribution of PLAs produced with S-NHC catalyst was narrower than that of PLAs produced with H-NHC catalyst. This can be attributed to the formation of a similar environment for the active sites as a result of immobilization. The molecular weights of PLAs produced by both H-NHC and S-NHC catalyst increased with an increase in the D-lactide/catalyst molar ratio. However, the changes in the molecular weight distribution of PLA with increasing D-lactide/catalyst molar ratios were different with the different catalyst systems.