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.

本文报道了首例单体型酯烯醇铝锂配合物的合成与分子结构——该类配合物可作为单位点阴离子增长中心的结构模型，同时报道了其在共轭有机铝路易斯酸催化下引发甲基丙烯酸酯聚合的反应机理。α-锂代异丁酸异丙酯（2）与合适的解聚稳定型有机铝化合物（如MeAl(BHT)2，其中BHT为2,6-二叔丁基-4-甲基苯酚盐（2,6-di-tert-butyl-4-methylphenolate））在烃类溶剂中可高选择性地生成酯烯醇铝锂配合物Li+[Me₂C=C(OiPr)OAlMe(BHT)₂]⁻（3）。值得注意的是，配合物3可分离得到，且在固态与溶液态下均以单体形式存在。与聚集型烯醇盐2引发甲基丙烯酸酯的不可控聚合不同，单体型配合物3引发的甲基丙烯酸酯聚合虽可控，但活性较低。不过，通过使用3/MeAl(BHT)2增长剂/催化剂体系，可实现兼具高可控性与高活性的聚合——该体系可通过将2与2当量的MeAl(BHT)2原位混合方便地制备。外加有机铝化合物的结构对单体活化程度、烯醇铝盐的生成与反应活性以及聚合可控性均具有显著影响。对3/MeAl(BHT)2体系引发聚合的动力学研究表明，其遵循以单位点酯烯醇铝锂增长中心为核心的双分子活化单体阴离子聚合机理。活化单体1、聚集型引发剂2以及单体型增长剂3的分子结构均通过X射线衍射（X-ray diffraction）表征得以确定。