Heteroleptic Aluminum Allyls as Initiators for Lactide Polymerization

Organoaluminum complexes with the formula [(NHC)AlCl3‑nA′n] (NHC = IMes (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) or IDipp (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene); A′ = [1,3-(SiMe3)2C3H3]−; n = 0–2) were synthesized either through solution or mechanochemical methods. Although an NHC adduct with [AlA′3] did not form, [(IDipp)AlCl2A′] and [(IMes)AlClA′2] were obtained and X-ray crystallography confirmed their construction around Al in a distorted tetrahedral environment. The [(NHC)AlCl3‑nA′n] complexes, [AlA′3], and [Al(OiPr)3] (as a reference) were examined for their ability to polymerize l-lactide via ring-opening polymerization to produce polylactide. All of them did so with varying degrees of effectiveness. Of the organoaluminum species, the [(NHC)AlCl3] complexes were very weak initiators, and [AlA′3] the most effective, producing polymer molecular weights up to 49 kDa (Mw), with the mixed chloro/allyl complexes in between. All initiators were slower than [Al(OiPr)3].