Competition between Ring-Closing Migratory Insertion Polymerization and Monomer Cyclization

Migratory insertion polymerization (MIP) of PFpR [PFp = (PPh2(CH2)3Cp)­Fe­(CO)2, R = (CH2)4CH═CH2 or (CH2)5CH3] (1) involved competitive MIP ring-closing polymerization and monomer cyclization (MC), producing P­(PFpR) macrocycles (2) and PFpR rings (3), respectively. MC, generating 3, occurred at the early stage of MIP, while the growing polymer chains exclusively underwent ring-closing cyclization without producing any linear analogues. The effect of solvent, temperature, and the concentration of 1 on the competition between the ring-closing MIP and MC was investigated. 3 was synthesized as the only product in THF with a low concentration of 1 (1 wt %), while the ring-closing MIP predominated under the condition at 60°C with a high concentration of 1 in THF (>70 wt %), resulting in 2 with Mn up to 17 500 g/mol. This effective synthesis of ring molecules is attributed to the piano-stool coordination geometry and the low rotation barrier of Cp–Fe bond and will facilitate further exploration of ring molecules as functional materials and supramolecular building blocks.