Deprotonation usually occurs as an unwanted side reaction in the Lewis pair polymerization of Michael acceptors, for which the conjugated addition of the Lewis base to the acidactivated monomer is the commonly accepted initiation mechanism. This has also been reported for B À P-based bridged Lewis pairs (BLPs) that form macrocyclic addition products.W en ow show that the formerly unwanted deprotonation is the likely initiation pathway in the case of AlÀP-based BLPs.I nadetailed study of as eries of AlÀP-based BLPs, using ac ombination of single-crystal diffraction experiments (X-ray and neutron) and mechanistic investigations (experimental and computational), an active role of the methylene bridge was revealed, acting as ab ase towardst he a-acidic monomers.A dditionally,t he polymerization studies proved al iving behavior combined with significantly high activities, narrowm olecular mass distributions,a nd the possibility of copolymerization.