Air and Thermally Stable Cyclic (Alkyl)(amino)carbene Ruthenium Complexes for Efficient Ring Expansion Metathesis Polymerization.

Ring expansion metathesis polymerization (REMP) has emerged as a potent strategy for obtaining cyclic polymers over the past two decades. The scope of monomers, however, remains limited due to the poor functional group tolerance and stability of the catalyst, necessitating a rational catalyst design to address this constraint. Here, we present ruthenium complexes featuring tethered cyclic (alkyl)(amino)carbene ligands for REMP, aiming to deepen our understanding of the structure-property relationship in newly designed catalysts. Notably, these ruthenium catalysts exhibit remarkable thermal stability even in air, as confirmed through monitoring the air-exposed solution at 80 °C. In addition, the initiation rate of the catalysts was effectively modulated by tuning the steric hindrance of the -aryl substituent, adjusting tethered chain lengths, or introducing a Blechert-type ligand. Based on systematic studies of catalysts, we successfully demonstrate challenging REMP of a cyclic allene (2,8-dimethylnona-4,5-diene) for the first time, as well as methyl-5-norbornene-2-carboxylate, resulting in cyclic polymers. We also note that the exceptional stability of the catalyst enables REMP under air. This study provides valuable insights into the rational design of catalysts and introduces a novel, user-friendly platform for the synthesis of cyclic polymers.