Anionic Polymerization of Lactams in the Presence of Metal Dialkoxyaluminum Hydrides: Presentation of a New Mechanism

Metal dialkoxyaluminum hydrides (1) form a new class of catalysts for anionic polymerization of lactams. Propagation proceeds in their presence—as it does in the presence of conventional catalysts such as alkali metal hydrides—by successive additions of the anionically active species to the end-standing N-acyllactam function carried by the polymer chain. However, this is the only similarity between the two polymerization processes. The active species generated when ε-caprolactam (2) is treated with sodium dialkoxyaluminum hydride is the sodium salt of 2-(dialkoxyaluminoxy)-1-azacycloheptane (6) and is therefore quite different in terms of nucleophilicity from the mere activated monomer, sodium caprolactamate. The new reactive salt (6) is obtained upon deprotonation of a lactam monomer at its amide position followed by the reduction of its carbonyl function. During lactam polymerization, after each monomer addition, the active species is re-formed in two steps: first, proton exchange followed by instantaneous reduction of the lactamate formed by transfer of both hydrogen and the Al(OR)₂ group from the previous monomer unit added. This displacement regenerates the end-standing N-acyllactam and the salt of the (dialkoxyaluminoxy)-azacycloheptane (6), allowing propagation to proceed by addition of 6 to this latter function. This new mechanism still propagates by an “activated monomer” mechanism but it also involves the transfer of the dialkoxyaluminum hydride group throughout polymerization.