Polymerization of ɛ-caprolactone using ruthenium(II) mixed metallocene catalysts and isopropyl alcohol: Living character and mechanistic study

A series of ruthenium(II) complexes with the general formula [Ru(η5-C5H5) (η6-substituted arene)]+[PF6]− (substituted arene = 2-phenylpyridine (1), dibenzosuberone (2) and toluene (3)), in combination with isopropyl alcohol were used for the polymerization of ɛ-caprolactone. The polymerization was found to be quantitative and controlled, with PDI in the range 1.1–1.3. By means of MALDI-ToF analyses, functionalization studies with d,l-lactide and NMR monitoring techniques, it has been found that the polymerization proceeds via a living Activated Monomer mechanism (AM) involving an η6–η4 change of the coordination mode of the arene. These experimental results were corroborated by DFT studies. The growth of several polymer chains per ruthenium atom highlights interesting potentialities for molecular weight control and catalyst economy. The stability of the ruthenium complexes allows their recovery at the end of the polymerization, which can be viewed as a further advance in a green chemistry frame.

Change in the coordination mode of the arene ligand upon ɛ-caprolactone interaction during an Activated Monomer mechanism promoted by ruthenium(II) mixed metallocenes.Figure optionsDownload high-quality image (125 K)Download as PowerPoint slideHighlights
► First Ru(II) mixed metallocene catalysts for living Activated Monomer mechanism.
► Innovative polymerization mechanism based on arene η6–η4 slippage.
► DFT studies performed for the first time for a ROP via Activated Monomer mechanism.
► Recovery of the ruthenium catalytic complexes at the end of reaction.
► Growing of several polymer chains per metal: catalyst economy for green chemistry.