Heterogeneous catalysts based on built-in N-heterocyclic carbenes with high removability, recoverability and reusability for ring-opening polymerization of cyclic esters

Both activity and reusability are critical issues for developing new generation metal-free catalytic systems. In this work, we designed a heterogeneous catalyst with mesoporous microspheres as the carrier and N-heterocyclic carbenes (NHCs) as built-in catalytic sites, and it was used to catalyze the ring-opening polymerization of cyclic esters. Owing to the mesoporous structure of the microsphere carrier, the built-in NHCs can turn back to their precatalyst form (i.e. PrIm+Cl−), and therefore achieve reuse of the catalyst at least 5 times without obvious inactivation. After the reaction was completed, the microspheres could be efficiently separated and collected by simple filtration with a very high recovery ratio (>95%) because of its insolubility. Therefore, the residual catalyst in the polymerization product could be effectively decreased to a very low level.