Single-Site Anionic Polymerization. Monomeric Ester Enolaluminate Propagator Synthesis, Molecular Structure, and Polymerization Mechanism

The synthesis and molecular structure of the first examples of monomeric lithium ester
enolaluminates that serve as structural models for single-site anionic propagating centers, as well as the
mechanism of their polymerization of methacrylates catalyzed by conjugate organoaluminum Lewis acids,
are reported. Reactions of isopropyl α-lithioisobutyrate (2) with suitable deaggregating and stabilizing
organoaluminum compounds such as MeAl(BHT)2 (BHT = 2,6-di-tert-butyl-4-methylphenolate) in hydrocarbons cleanly generate lithium ester enolaluminate complexes such as Li+[Me2CC(OiPr)OAlMe(BHT)2]-
(3). Remarkably, complex 3 is isolable and exists as a monomer in both solid and solution states. Unlike
the uncontrolled polymerization of methacrylates by the aggregating enolate 2, the methacrylate
polymerization by the monomeric 3 is controlled but exhibits low activity. However, the well controlled and
highly active polymerization can be achieved by using the 3/MeAl(BHT)2 propagator/catalyst pair, which is
conveniently generated by in situ mixing of 2 with 2 equiv of MeAl(BHT)2. The structure of the added
organoaluminum compounds has marked effects on the degree of monomer activation, enolaluminate
formation and reactivity, and polymerization control. Kinetics of the polymerization by the 3/MeAl(BHT)2
pair suggest a bimolecular, activated-monomer anionic polymerization mechanism via single-site ester
enolaluminate propagating centers. The molecular structures of activated monomer 1, aggregated initiator
2, and monomeric propagator 3 have been determined by X-ray diffraction studies.