Solvent-dependent chain conformation for ring closure of metal carbonyl oligomers via migration insertion polymerization (MIP) of CpFe(CO)₂(CH₂)₆PPh₂

The solution migration insertion polymerization (MIP) of CpFe(CO)₂(CH₂)₆PPh₂ (FpC6P) in THF generates linear PFpC6P (l-PFpC6P) oligomers. When THF/hexane mixed solvents are used to replace THF, cyclic PFpC6P (c-PFpC6P) without end groups is produced. In contrast, the MIP of FpC6P in THF/pyrrole, THF/DMSO, and THF/pyridine and the MIP of CpFe(CO)₂(CH₂)₃PPh₂ (FpC3P) in THF/hexane generate a mixture of c- and l-oligomers with NMR detectable end groups. Molecular dynamics simulations indicate that both PFpC6P and PFpC3P are extended chains in THF. PFpC6P, regardless of the DP, forms a cyclic structure in THF/hexane with end-to-end distances (Re) less than 10 Å, which facilitates ring closure during the MIP. In other systems, both cyclic and non-cyclic chain conformations are observed. This simulation rationalizes the experimental results and illustrates that PFpP chain conformation is solvent-dependent, and potentially useful as protein-like materials.