Migratory insertion polymerization (MIP) of P FpR [ P Fp = (PPh 2 (CH 2 ) 3 Cp)Fe(CO) 2 , R = (CH 2 ) 4 CHCH 2 or (CH 2 ) 5 CH 3 ] (1) involved competitive MIP ring-closing polymerization and monomer cyclization (MC), producing P( P FpR) macrocycles (2) and P FpR 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 ringclosing 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 M n 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.