Blanc SIMI 8 - Blanc - SIMI 8 - Chimie du solide, colloïdes, physicochimie

ORGANOCATALYZED POLYMERIZATIONS BY CARBENES: TOWARDS A MULTI-TASK CATALYTIC PLATFORM – CATAPULT

Catalytic Duos for Metal-free Polymer Synthesis

Development of a catalytic platform utilizing so-called Lewis pairs (acids and bases) for various polymerization reactions, and rationalizing the catalytic effects by combining theoretical calculations, molecular chemistry and macromolecular synthesis<br /> <br />

Novel Synthetic Pathways to Metal-free Polymeric Materials

One aim is to rationalize both electronic and steric effects of Lewis pairs towards the involved elementary reactions, and regarding the nature of the reaction intermediates. Another stake is to establish the polymerization conditions the best suited to achieve a good control over the dimensions and the overall structure of the as-prepared polymers. <br />Gathering experts in molecular chemistry (ISM), physico-chemists developing theoretical and modeling methods (IPREM), and a team with a competency in polymer chemistry (LCPO), should facilitate the selection of pairs that can serve such-or-such reaction. <br />We expect to provide more environmentally friendly synthetic alternatives to specialty polymers by substituting organic catalysts for metallic species that are traditionally employed to this end, while complying with the European REACH regulations. <br />

In order to rationalize the effects of Lewis pair catalysts, (meth)acrylic monomers have been selected as first substrates in the frame of Winnie Nzahou-Ottou PhD thesis work (partner 1). This consists in screening various NHC-type carbenes (as Lewis base) and a few silanes (as Lewis acids), employed alone or in a dual catalysis. The aim at this stage phase is to establish reactivity mapping of both Lewis acids and bases.
Model reactions utilizing equimolar amounts between the monomer substrate and the Lewis acid and/or base were first implemented and reaction products were analyzed by NMR spectroscopy. In case of specific combinations forming insoluble mono-adducts, analyses by mass spectrometry and X-Ray diffraction were required.
First observed trends were communicated to the two other partners, in particular to enable partner 2 (IPREM) to start DFT calculations concerning relevant combinations.
In the next step, most interesting catalysts (those having shown to activate the substrate) were evaluated for the polymerization of both acrylic and methacrylic monomers. Solvent and temperature effects were also taken into account. The working method also involved the thorough (macro)molecular characterization of the obtained polymers by usual techniques.

A first series of results has been obtained by reacting (meth)acrylic substrates in the presence of the carbene, in absence of the Lewis acid (Table1). The goal here is to figure out whether particular NHCs are capable, as such, to trigger the polymerization of (meth)acrylic monomers. It is important to note that X. Y. Chen (Univ. Colorado) et al. have recently published a paper (see Angew. Chem. Int. Ed. 2012, 51, 1) describing very similar works.
Table 1 shows that some monomer/NHC combinations directly lead, indeed, to the formation of polymers in high yields. Most promising results -which are also novel with regards to those reported by our competitors- have been obtained for the DMAEA/NHC 1 couple (Tableau 1). In this case, we have generated polymers with a molar mass that increases with the {DMAEA}/{NHC} molar ratio.
Interestingly, some monomers that cannot polymerize in the presence of the NHC alone form a polymer by adding a silane-type Lewis acid in the reaction mixture, establishing the proof of a Lewis pair-induced polymerization. That is, for instance, the case, of DMA as monomer which can be polymerized by NHC 3 only if (CH3)3OTf is employed as a Lewis acid.

Next months will be devoted to the design of novel organic Lewis acids Lewis (partner 2- ISM). The reactivity of the latter catalysts towards (meth)acrylic substrates will be evaluated, either as such or used in duo with various NHCs.
NHC-induced polymerization reactions, in absence or in presence of the Lewis acid, will be optimized for relevant combinations and exemplified by using other selected NHC/monomer combinations, with and without the Lewis acid (partner 1- LCPO). Special efforts will be made towards the functionalization of zwitterionic polymer chain ends using appropriate quenching agents, which has never been reported before. Here also, model reactions involving the (meth)acrylic substrate and equimolar amounts of the Lewis base NHC and/or Lewis acid will be investigated.
Finally, DFT calculations will be performed to get a better insight into the mechanism of elementary reactions (partner 3- IPREM).

A joint article between IPREM and LCPO partners has been published in Journal of American Chemical Society 2012, 134, 6776. Results described in this paper relate to the synthesis and the facile manipulation of novel NHC precursors. Although not being, as such, at the core of the CATAPULT project, these aspects have been briefly addressed in our proposal. Hence the ANR has been acknowledged in this paper, since while writing this proposal, exchanges between the two partners (1 & 3) have opened avenues beyond the scope of the CATAPULT project.

This research project proposes innovations in metal-free polymer synthesis utilizing an organocatalytic platform based on carbenes as main catalysts for various polymerization reactions. On one hand, carbenes will be used as single component catalysts of “first generation” in order to get a better understanding into the mechanism(s) operating in the so-called group transfer polymerization (GTP) of alkyl (meth)acrylates. Since the structure of the carbene has a dramatic impact on the nature of intermediates formed in GTP, this has important implications on the final properties of the GTP-derived polymers. Therefore, a rationalization of both the electronic and steric effects of the carbene catalysts over the GTP mechanism will be undertaken. Next, the potential of the associative mechanism will be investigated to control the diastereoselectivity of the GTP process, so as to synthesize highly syndiotactic poly(meth)acrylics. Another expected advantage of the associative mechanism is that block copolymers could be prepared by sequential GTP, regardless of the order of addition of the monomers, in contrast to most examples of block copolymer synthesis by “controlled/living” sequential polymerization.
On the other hand, we propose to develop a “second generation” of organocatalysts where the carbene Lewis base (LB) is associated with a Lewis acid (LA) co-catalyst such as a silane (SiR1R2R3R4) or a borane (BR’3). This bicomponent organocatalytic platform based on a “LA/LB pair” should result in a dual electrophilic/nucleophilic activation and allows the polymerization of a wide range of monomers, including (meth)acrylates, oxiranes, aziridines and poorly reactive (non-polar) alkene derivatives that cannot be readily activated by a carbene alone (e.g. styrene or butadiene). In this part, the “proof of concept” of such a dual activation through model reactions involving the catalytic LA/LB pair in the presence of equimolar amounts of the targeted monomer substrates will be first established. This will be further applied to the polymerization of these monomers.
This program will be implemented in a collaborative framework between organic chemists (F. Robert and Y. Landais from Institut des Sciences Moléculaires, ISM, Bordeaux) possessing undisputed proficiency in the synthesis of catalysts and in molecular chemistry in general, physico-chemists (K. Miqueu and J.-M. Sotiropoulos from Institut Pluridisciplinaire de Recherche sur l’Environnement et les Matériaux, IPREM, Pau) specialized in the determination of the structure and electronic properties of compounds involving new bonding situations, using both theoretical expertise and experimental skills - UV Photoelectron Spectroscopy -, and the coordinator's team (D. Taton and J. Vignolle at the Laboratoire de Chimie des Polymères Organiques, LCPO à Bordeaux) who has extensively contributed to the field of macromolecular engineering and, more recently, to the field of polymerization reactions organocatalyzed by N-heterocyclic carbenes.

Project coordination

Daniel Taton (CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION AQUITAINE LIMOUSIN) – taton@enscbp.fr

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

UMR 5629 CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION AQUITAINE LIMOUSIN
UMR 5255 UNIVERSITE BORDEAUX I
UMR 5254 CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION AQUITAINE LIMOUSIN

Help of the ANR 475,000 euros
Beginning and duration of the scientific project: October 2011 - 48 Months

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