Silicon-based polymers by catalysed dehydropolymerisations – POLCADE

Task 1 consisted in identifying and preparing high performance alkaline earth (Ae, with Ae = Ca-Ba) catalysts for targeted NH/HSi and NH/HP/HSi dehydrocoupling and dehydropolymerisation reactions. It involved the design of reactive Ae-hydrides and amides, and that of bulky ancillary ligands that warranted the formation of heteroleptic Ae complexes stable against ligand scrambling in solution. A range of new barium complexes, many of them unknown to this point, has been developed. In task 2, the PH/HSi and especially NH/HSi (multiple) dehydrocoupling reactions were performed. These reactions afforded new molecules that constituted the seeds to the preparation of the targeted P-functionalised macromolecules. The polymerisation studies performed in task 3 aimed at generating polycarbosilazanes and polyphosphacarbosilazanes unknown to date, using alkaline earth catalysts. It was expected that the presence of phosphorus atoms in variable quantities in these new materials would display good properties as flame retardants. This took us to task 4, the study of the properties of the new Si/N/P materials as flame-retardant component of polymer matrixes.

POLCADE is a project focusing on fundamental research and coordinated by Jean-François Carpentier (ISCR Rennes, UMR 6226). It involved Yann Sarazin (ISCR), François Ganachaud (IMP @ INSA Lyon, UMR 5223) as well as Rodolphe Sonnier and Claire Longuet (Mines d´Alès). The project started in March 2017 and lasted 42 months. It has received a financial support of 380 k€ from ANR, for a total cost of around 776 k€.
Major highlights include progress in the general understanding of the organometallic chemistry of the large alkaline-earth metals, and, at the forefront, barium, the most reactive of these metals. New nitrogen ancillary ligands have been designed, allowing access to previously unknown stable complexes. Among these, amide, phosphide, stannylide and hydroborate complexes deserve special mention. In catalysis, these molecular compounds have allowed for the production materials of P-functionalised polycarbosilazanes. This new type of polymers showed original flame-retardant properties, improving on the existing state-of-the-art.

Several of the initial objectives have been fulfilled, namely:
(i) A better understanding of the parameters that govern the stability and the reactivity of Ae-based (and among these, most notably barium) complexes,
(ii) The design of efficient catalytic systems for dehydrocoupling of P-functionalised substrates (amines and silanes) for the formation of polycarbosilazanes with improved flame-retardant properties,
(iii) The investigation on the flame-retardants properties of these materials as polymers or polymer additives,
(iv) New approaches to fire resistance of silicone through new additives, either commercial or recycled from silicones.
It is however that among the initial targets, we have met serious and unexpected difficulty in synthesising the sought P-functionalised amines and hydrosilanes, or sometimes their reactivity proved entirely uncontrollable; also, we have so far failed in devising an efficient catalytic system for the initially targeted dehydrocoupling of phosphines and hydrosilanes. Moreover, these difficulties have led to substantial loss of time and resources.
Despite these initial setbacks, the results achieved in organometallic Ae chemistry, Ae-promoted dehydrocoupling catalysis and material science are undeniably very promising, and open up several avenues for future investigations. Each of these areas were virtually unexplored prior to POLCADE, and therefore the project can be considered a main success in this respect. It is our ambition to pursue these leads, and as a matter of fact, we are currently devoting time and resources in our laboratories to further build on these results. The data gathered through POLCADE have helped us define new targets, that involve in particular specific ligand frameworks for barium chemistry, the quest for stable barium-hydrides, investigation of the reactivity of the new barium complexes obtained through the project towards specific substrates, and new opportunities of recycling silicones for enhanced fire-targeted materials.

The results obtained through POLCADE have so far resulted in several publications in international peer-reviewed journals, including, for parts of it, in the most highly rated journals of the discipline. Seven articles have already been published, and three others are in preparation. The published results are related to the organometallic chemistry of the large alkaline-earth metals (Ca, Sr and Ba) and to their use in homogeneous catalysis. The data have also been presented on multiple occasions as invited lectures and oral presentations in national and international conferences.

Publications in international peer-reviexed journals
1 Bis(imino)carbazolate: a master key for barium chemistry. P. M. Chapple, S. Kahlal, J. Cartron, T. Roisnel, V. Dorcet, M. Cordier, J.-Y. Saillard,* J.-F. Carpentier, Y. Sarazin,* Angew. Chem. Int. Ed. 2020, 59, 9120-9126.
2 A versatile nitrogen ligand for alkaline-earth chemistry. P. M. Chapple, M. Cordier, V. Dorcet, T. Roisnel, J.-F. Carpentier, Y. Sarazin,* Dalton Trans. 2020, 49, 11878-11889.
3 Contemporary Molecular Barium Chemistry. P. M. Chapple, Y. Sarazin,* Eur. J. Inorg. Chem. 2020, 3321-3346.
4 Metal-metal bonded alkaline-earth distannyls. P. M. Chapple, J. Cartron, G. Hamdoun, S. Kahlal, M. Cordier, H. Oulyadi, J.-F. Carpentier, J.-Y. Saillard,* Y. Sarazin,* Chem. Sci. 2021, 12, 7098-7114
5 Bis(imino)carbazolate lead(II) fluoride and related halides. P. M. Chapple, G. Hamdoun, T. Roisnel, J.-F. Carpentier, H. Oulyadi, Y. Sarazin,* Dalton Trans. 2021, 50, 9021-9025
6 Bonding analysis in ytterbium(II) distannyl and related tetryls. Peter M. Chapple, Julien Cartron, Ghanem Hamdoun, Marie Cordier, Samia Kahlal, Hassan Oulyadi, Jean-François Carpentier, Jean-Yves Saillard,* Yann Sarazin,* Dalton Trans. 2021, 50, 14273-14284
7 Heteroleptic Carbazolato-Barium Hydroborates and a Related Separated Ion Pair. Peter. M. Chapple, Thierry Roisnel, Marie Cordier, Jean-François Carpentier and Yann Sarazin,* Polyhedron, under evaluation at the time of writing.

Oral presentations in international conferences/symposia
1 Cluj Academic Research Days, Cluj, Romania, Oct. 2021 (webinar) – Yann Sarazin (invited lecture)
2 100th Anniversary of the foundation of University Babes-Bolyai, Cluj, Romania, Oct. 2019 – Yann Sarazin (invited lecture)
3 International Conference on Heteroatom Chemistry (ICHAC): Prague, Rép. Tchèque, June-July. 2019 – Yann Sarazin (invited lecture)
4 EuCheMS International Organometallic Conference XXIII, Helsinki, Finland, June 2019 – Yann Sarazin
5 7th EuCheMS Conference on N-ligands, Lisbon, Portugal, September 2018 – Yann Sarazin

oral presentations in national conferences/symposia
1 Université de Haute-Alsace Mulhouse, LIMA UMR 7042 CNRS, Nov 21th, 2019 – Yann Sarazin (invited lecture)
2 Université de Strasbourg, LIMA UMR 7042 CNRS, Nov 20th, 2019 – Yann Sarazin (invited lecture)
3 Congress of the SCF, SCF2018, Montpellier, July 2018 – Yann Sarazin
4 Polymeris, Solaize, 9/12/2021 – Rodolphe Sonnier

POLCADE aims at developing atom-efficient catalytic processes for Si–N and Si–P bond formations, to yield original silazane-based oligomers and polymers with good hydrolysis resistance or to be used as flame retardants. At its core lies the design of synthetic procedures towards such materials by alkaline earth-catalysed multiple dehydrocoupling reactions of hydrosilanes with primary/secondary amines and phosphines, and the characterisation and exploitation of their applicative physico-chemical properties. The project will implement atom-efficient and cost-effective routes to materials of technological significance relevant to the widespread industry of silicon:
- Polycarbosilazanes, useful as coating agents;
- Original Si/N/P hybrid cooligomers and copolymers, embedded with phosphorus-based moieties that shall be used as fire-retardants for thermoplastic matrices and/or as scale resistant materials.

In this context, three main aspects will be specifically investigated:

1) Alkaline-earth mediated catalysis. The Si-N(-P) polymers will be synthesised by catalysed dehydropolymerisations of hydrosilanes with amines and/or phosphines. We will use alkaline earth (Ae = Ca, Sr, Ba) complexes, in particular hydrides, which are ideal candidates for such processes, to catalyse the envisaged dehydropolymerisations. These catalysts will be prepared and their performance in the simple cross-dehydrocoupling of hydrosilanes and phosphines, and in the step-growth formation of Si/N, Si/P and Si/N/P macromolecules, will be assessed prior to the ultimately targeted syntheses of the polymer materials.

2) Resistance to hydrolysis. Poly(carbo)silazanes are usually very sensitive to hydrolysis. In POLCADE, we will seek to stabilise these polymers by adapting the structure of the macromolecular precursors. In particular, the dehydropolymerisation of aromatic amines and hydrosilanes is anticipated to yield new polycarbosilazanes of greater resistance to hydrolysis.

3) Flame retardancy. Silicones exhibit good thermal and fire-resistant properties owing to their very stable Si-O bonds and to the formation of a protective silica layer during burning. P- and N-functionalised Si-based oligomers and polymers will be evaluated as flame retardants in different thermoplastic model matrixes (e.g. polyesters, polyamides, polymethylmethacrylate etc.) at various contents.

Three teams combining their specific sets of competences, all essential to the success of POLCADE, will work in a collaborative framework. The Rennes partners have a large experience in catalyst design, including with alkaline earths, and in homogeneous catalysis for polymerisation and fine chemicals. The team in Lyon is well-known for its competences in polymer chemistry, especially silicones and silicon-containing polymers. The collaborators in Alès are experts in flame-retardant additives and associated mechanisms.

The work program spreads over 3 years. The recruitment of one PhD student (Rennes) and one post-doc research assistant (Lyon) is planned. During the course of the project, these two collaborators will stay during short-term periods in Alès to study fire resistance experiments, and in the other partners’ labs to ensure optimal synergy and transfer of competences. Funding towards the acquisition of multiple nuclei NMR probe dedicated to the analysis of polymers (Lyon) and an in situ ReactIR instrument (Rennes) is solicited. An overall budget line of € 380,495.00 is requested for POLCADE.

The programme bears strong potential for the production of marketable silicon-based materials, and as such it has been endorsed by the "pôle de compétitivité" PLASTIPOLIS, and by two major companies in the silicone industry, BOSTIK and BLUESTAR Silicones.