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.
Control of stereoregularity is an integral part of a precision polymerization method and for the development of functional materials. Yttriumand aluminum-based catalysts are known for converting diethyl vinylphosphonate (DEVP) into its stimuli-responsive polymer in a precise but stereoirregular way. Herein, we present Y-and Al-based constrained geometry complexes (CGCs) to induce isotacticity without losing control over other macromolecular parameters. After having established convenient synthesis routes and detailed structural analyses, these CGCs showed exceedingly high turn-over frequencies (up to 45 000 h −1 ) in the group-transfer polymerization of DEVP. The initiator efficiencies (≤99%) and dispersities (≤1.02) strongly depended on the substitution pattern of the applied ligands. An analysis of the microstructure using multidimensional NMR ( 1 H− 1 H and 1 H− 13 C(− 31 P)) correlation experiments demonstrated significant disparities for the stereospecificity of the CGCs and enabled a reliable signal assignment. The yttrium catalysts produced highly isotactic poly(diethyl vinylphosphonate), likely following a chain-end control mechanism, whereas the aluminum complexes produced less defined polymers.
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.
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