ABSTRACT
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 acid-activated monomer is the commonly accepted initiation mechanism. This has also been reported for B-P-based bridged Lewis pairs (BLPs) that form macrocyclic addition products. We now show that the formerly unwanted deprotonation is the likely initiation pathway in the case of Al-P-based BLPs. In a detailed study of a series of Al-P-based BLPs, using a combination 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 a base towards the α-acidic monomers. Additionally, the polymerization studies proved a living behavior combined with significantly high activities, narrow molecular mass distributions, and the possibility of copolymerization.
ABSTRACT
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.
ABSTRACT
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.
ABSTRACT
(Phosphine-sulfonate)Pd(Me)(DMSO) catalyzed copolymerization of ethylene and 3,3,3-trifluoropropene (TFP) allows the synthesis of linear copolymers with high fluorine contents of up to 15 wt % (8.9 mol % TFP). 13C and 19F NMR analyses of the copolymers were performed, showing that most of the incorporated TFP is located in the polymer backbone. Copolymerization of ethylene-d4 with TFP revealed that TFP is inserted into Pd-D bonds in 1,2- as well as in 2,1-mode, although 1,2-insertion is slightly preferred. Chain transfer after TFP insertion is exclusively observed following 2,1-insertion. With higher TFP incorporation, an increase in the ratio of internal to terminal double bonds was detected in the 1H NMR spectra. This indicates that, in the case of 2,1-insertion of TFP, chain walking is facilitated relative to direct chain release after ß-H transfer to the palladium center.
