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Daniel Taton1

1, University of Bordeaux, Pessac, , France

Both catalyst immobilization and use of environmentally benign solvents are paramount for the development of sustainable catalytic processes. In recent years, we have devised modular synthetic strategies polymer-supported N-Heterocyclic carbenes (NHCs) to catalyze benchmark reactions of molecular organic and organometallic chemistry (e.g. benzoin condensation, transesterification, or the Suzuki and the Heck cross-coupling reactions). For this purpose, imidazolium-based (co)poly(ionic liquid)s (coPILs) featuring non-innocent counter-ions, such as acetate or hydrogen carbonate, have been specifically synthesized. This has been achieved either by free-radical polymerization (FRP) or by controlled radical polymerization proceeding by the reversible addition fragmentation chain transfer (RAFT) process. The organic and basic anions can eventually interact with the C2-proton of imidazolium units to in situ generate (co)polymer-supported NHCs. Different air-stable, recyclable and thermally switchable styrenic-type coPIL precursors have thus been rationally designed, including:

i) statistical coPILs made by FRP,[1,2]
ii) single chain nanoparticles (SCNPs) with imidazolium-type crosslinks obtained by folding of a RAFT-derived parent copolymer bearing antagonist groups,[3]
iii) star-like core-crosslinked polymeric nanoreactors with supported Pd(II)-NHC2 catalytic sites inside their core.[4]

In the latter case, catalytic entities have been segregated into the cross-linked hydrophobic core of star-like polymeric micelles, the hydrophilic shell ensuring water-solubility. A parent diblock copolymer, consisting of a hydrophilic PEO block and a statistical PS-co-poly(4-vinylbenzylethylimidazolium chloride) hydrophobic block, has been first synthesized by RAFT copolymerization. Addition of Pd(OAc)2 followed by nanoprecipitation in water has driven the metal-ligand coordination core crosslinking reaction, forming robust polymeric micelles with internal Pd(II)-NHC2 crosslinks. An outstandingly high catalytic activity is observed at low loadings both for the Pd-catalyzed Suzuki and Heck reactions in pure water, in comparison to molecular and non-self-assembled analogues.

References.
[1] P. Coupillaud, J. Vignolle, D. Mecerreyes, D. Taton, Polymer 2014, 55, 3404-3414
[2] R. Lambert, P. Coupillaud, A.-L. Wirotius, J. Vignolle, D. Taton, Macromol. Rapid Commun. 2016, 37, 1143-1149.
[3] R. Lambert, A.-L. Wirotius, D. Taton, ACS Macro Let. 2017, 6, 489-494.
[4] D. Taton et al. Manuscript in preparation.

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