Recent Progress in Ring-opening Polymerizations Catalyzed by Rare Earth Catalysts

Rare earth (RE) catalysts exhibit extremely high catalytic activity in ring-opening polymerization. This work reviews the application of rare earth catalysts in ring-opening polymerizations of lactones, lactides, cyclic ethers, cyclic carbonates and cyclic carboxyanhydrides in the past decade. Compared to other transition metals and main group elements, RE elements have larger cationic radii which allow for higher coordination numbers and more coordination modes, endowing RE catalysts with high catalytic activity. These characteristics not only fuel the widespread use of new RE catalysts in pursuing high molecular weight and polymerization rate of intensively studied monomers, such as epsilon-caprolactone (CL), lactide (LA) and beta-butyllactone (BBL), but also unleash the polymerization of traditionally non-polymerizable gamma-butyrolactone (gamma-BL) and its derivatives. The inherent low strain energy of the five-membered ring imparts monomer recyclability into the corresponding polymer. Furthermore, stereoselective polymerization of racemic monomers can be realized by RE catalysts containing chiral ligands with large steric hindrance. Slight modification of ligands gives rise to tailored or even inverted stereoregularity of polymers. The unique advantages of RE catalysts in the controllability of polymerization process and polymer structure are illustrated by discussing the relationship between catalyst structures and polymerization results. In addition, a novel ring-opening polymerization catalyzed by RE compounds, named Janus polymerization is introduced. This new mechanism combines cationic and anionic polymerizations at two ends of a single propagating chain and thus provides a facile methodology to synthesize polymers with sophisticated chemical structures and topologies in one step. New catalysts and mechanisms of ring-opening polymerizations have attractive potentiality in begetting new materials.

Automated summary

Rare earth (RE) catalysts exhibit high catalytic activity in ring-opening polymerization, allowing for high molecular weight and polymerization rate and expanding the polymerization scope to traditionally non-polymerizable monomers. They also enable stereoselective polymerization and tailored polymer structures through slight modifications of ligands. The unique advantages of RE catalysts in controlling polymerization and structure are illustrated, along with a novel Janus polymerization mechanism providing a facile methodology to synthesize polymers with sophisticated chemical structures and topologies in one step. New catalysts and mechanisms of ring-opening polymerizations show potential in creating new materials.