This Perspective article summarizes efforts paid in our group to develop efficient metal-based catalysts for the immortal ring-opening polymerization (iROP) of cyclic esters in the presence of large amounts of alcohols (ROH) as chain transfer agents. The catalyst systems reviewed include discrete organometallic complexes based on rare earths, magnesium, calcium and more specifically zinc, as well as simple systems employing metal triflate salts, notably Al(OTf)₃ , for the (stereo)controlled iROP of lactide (LA), β-butyrolactone (BBL) and trimethylenecarbonate (TMC). Special emphasis is given to systems that allow the use of minute amounts of metal catalysts and large loadings of both monomer and alcohol for the rapid and productive formation of functional polyesters (H–Pol–OR) with controlled molecular features.

A new heteroleptic ethyl-zinc complex stabilized by a chelating bis(morpholinomethyl)phenoxy ligand has been synthesised and shown to be a highly potent initiator for the immortal ring-opening polymerisation (ROP) of lactide and β-butyrolactone, being able to convert up to 50 000 equiv. of monomer in the presence of up to 1000 equiv. of an alcohol as transfer agent. Two related heteroleptic complexes of magnesium and calcium have also been prepared. These complexes are stable in solution, even in the presence of large amounts of alcohol, and constitute effective catalysts for the immortal ROP of lactide under mild conditions.

The reactions of the bulky amino-bis(phenol) ligand Me₂NCH₂CH₂N{CH₂-3,5-Bu^t ₂-C₆H₂OH-2}₂ (1-H₂) with Zn[N(SiMe₃)₂]₂ (4), [Mg{N(SiMe₃)₂}₂]₂ (5) and Ca[N(SiMe₃)₂]₂(THF)₂ (6) yield the complexes 1-Zn, 1-Mg and 1-Ca in good yields. The X-ray structure of 1-Ca showed the complex to be dimeric, with calcium in a distorted octahedral coordination geometry. Five of the positions are occupied by an N₂O₃ donor set, while the sixth is taken up by an intramolecular close contact to an o-Bu^t substituent, a rare case of a Ca⋯H–C agostic interaction (Ca⋯H distances of 2.37 and 2.41 Å). Another sterically hindered calcium complex, Ca{2-Bu^t-6-(C₆F₅NCH)C₆H₃O}₂(THF)₂·(C₇H₈)_2/3 (7), was prepared by reaction of 6 with the iminophenol 2-Bu^t-6-(C₆F₅NCH)C₆H₃OH (3-H). According to the crystal structure, 7 is monomeric and octahedral, with trans THF ligands. The complex Ti[N{CH₂-3-Bu^t-5-Me-C₆H₂O-2}₂{CH₂CH₂NMe₂}](OPrⁱ)₂ (2-Ti) was prepared by treatment of Ti(OPrⁱ ₄) with the new amino-bis(phenol) Me₂NCH₂CH₂N{CH₂-3-Bu^t-5-Me-C₆H₂OH-2}₂ (2-H₂). The reduction of 2-Ti with sodium amalgam gave the titanium(III) salt Ti[N{CH₂-3-Bu^t-5-Me-C₆H₂O-2}₂{CH₂CH₂NMe₂}](OPrⁱ)₂·Na(THF)₂ (8). A comparison of the X-ray structures of 2-Ti and 8 showed that the additional electron in 8 significantly reduced the intensity of the π-bonding from the oxygen atoms of the isopropoxide groups to titanium. 1-Ca and 8 were active initiators for the ring-opening polymerisation of ε-caprolactone (up to 97% conversion of 200 equivalents in 2 hours) and yielded polymers with narrow molecular weight distributions.