Metal complexes for polymerization mechanism switching – POLYSWITCH

Synthetic polymers have found applications in many areas, ranging from most “basic” ones such as packaging to high-tech fields such as energy storage, aeronautics or biomedicine. The need of complex and precise architectures with specific monomer composition and enchainment requires living or controlled polymerization techniques, and several breakthroughs have already been achieved in that direction. Block or segmented copolymers combining biodegradable polyester or polycarbonate segments, made by ring-opening polymerization (ROP) of bio-based lactones or by ring-opening co-polymerization (ROcoP) of epoxides and CO2, respectively, and poly(vinyl monomer) segments, made by controlled radical polymerization (CRP), are of particular interest for biomedical applications. These polymeric structures are not attainable using a single polymerization technique and have so far been assembled by either: 1) preparing different blocks and covalently linking them together via various strategies (e.g. “Click” chemistry), or 2) growing the different chains at each end of a specific linker with adequate end groups using the appropriate method. These approaches require several catalytic systems or multi-steps syntheses, and access to multi-block copolymers may become very difficult. Moreover, statistical copolymers do not seem accessible following these strategies.
The present proposal aims at using coordination and organometallic chemistry to overpass this limitation by the development of compounds that would mediate both organometallic-mediated radical polymerization (OMRP) of vinyl monomers and RO(co)P of cyclic esters/carbonates or epoxides and CO2, and switch from one mechanism to the other either randomly or “on demand”.
The choice of these two polymerization techniques to access such partially unprecedented polymeric architectures is based on a carefully analysis of their mechanisms. The OMRP mechanism involves reversible trapping of a radical active species (propagating chain) by a metal complex to form an organometallic dormant species. The propagation step in a coordination-insertion RO(co)P mechanism is based on the activation of the monomer by the metal center, followed by the attack of the nucleophile ligand linked to the metal (growing polymer chain). Indeed, both mechanisms make use of similar species of type [(L)Mt-polymer], the dormant species of OMRP having a Mt-C bond and the propagating species of RO(co)P having a Mt-O bond. Therefore, the key to succeed in our highly ambitious project is to develop compounds with a homolytically weak Mt-O bond, able at the same time to generate oxygen-based radicals to initiate OMRP and to activate a cyclic ester/carbonate/epoxide monomers to initiate and sustain RO(co)P. If this reactivity is achieved, the polymerization mechanism switch may be established sequentially, by successive additions of monomers to form block copolymers of the type polyester/polycarbonate-b-poly(vinyl monomer), or randomly on a pool of various monomers (one-pot reaction) to produce statistical polyesters/polycarbonate-co-poly(vinyl monomer) copolymers.
Cobalt(III) and iron(III) alkoxide complexes have been identified as the best candidates for this purpose, because of their well-established or promising performances in both techniques. However, none of the known complexes have so far been applied to both OMRP and ROP. The present consortium possesses all the required know-how in coordination and organometallic chemistry and in OMRP and RO(co)P processes to achieve the challenging objectives of this proposal and to access novel bio-based high-performance materials.