Herein we report on the catalytic polymerization of diverse Michael-type monomers with high precision by using simple but highly active combinations of phosphorus-containing Lewis bases and organoaluminum compounds. The interacting Lewis pair catalysts enable the control of molecular weight and microstructure of the produced polymers. The reactions show a linear Mn vs consumption plot thus proving a living type polymerization. The initiation has been investigated by end-group analysis with ESI mass spectrometric analysis. With these main-group element Lewis acid base pairs, it is not only possible to polymerize sterically demanding, functionalized as well as heteroatom containing monomers but also, for the first time, to catalytically polymerize extended Michael systems, like 4-vinylpyridine.

Unlike different types of Lewis pairs as polymerization catalysts for acrylic monomers, organometallic aluminum(III) compounds are reported that show a surprisingly high polymerization activity even without an additional Lewis base. DFT calculations, end group analysis and kinetic investigations clearly suggest a main group element (MGE) group transfer polymerization (GTP) mechanism analogous to the known metal-mediated GTP mechanism. The novel catalysts perform a precision polymerization of a broad variety of monomers, ranging from 2-isopropenyl-2-oxazoline to tert-butylmethacrylate and N,N-dimethylacrylamide. Additionally, extended Michael-type structures like 4-vinyl pyridine are accessible. Especially the Al(III) half-metallocenes show an almost quantitative initiator efficiency, and, combined with the living character of the polymerization reactions, they enable the synthesis of block copolymers, even with unconventional monomers like vinyl phosphonates.

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.

Die Deprotonierung tritt in der Regel als unerwünschte Nebenreaktion bei der Lewis-Paar-Polymerisation von Michael-Akzeptoren auf,bei der die konjugierte Addition der Lewis-Base an das säureaktivierte Monomer der gängige Initiationsmechanismus ist. Dies wurde auchf ürB -P-basierte verbrückte Lewis-Paare (Bridged Lewis Pairs,B LPs) berichtet, die makrozyklische Additionsprodukte bilden. Wirk onnten zeigen, dass bei Al-P-basierten BLPs die bisher unerwünschte Deprotonierung der wahrscheinliche Initiationsweg ist. In einer detaillierten Studie mit einer Reihe von Al-P-basierten BLPs unter Verwendung von Einkristalldiffraktometrie (Rçntgen und Neutronen) und mechanistischen Untersuchungen (experimentell und rechnerisch)w urde eine aktive Rolle der Methylenbrückea ufgedeckt, die als Base fürd ie aaziden Monomere agiert. Darüber hinaus bewiesen Polymerisationsstudien ein lebendes Verhalten in Kombination mit signifikant hohen Aktivitäten,e ngen Molmassenverteilungen und der Mçglichkeit zur Copolymerisation.

Invited for the cover of this issue is the group of Bernhard Rieger and colleagues at the Technical University of Munich. The image depicts a polymerization process with an aluminum‐based catalyst and its cyclopentadienyl‐ligands. Read the full text of the article at https://doi.org/10.1002/chem.201802075.