Improved fluorinated polymers through organometallic mediated radical polymerization – FLUPOL

The project requires several skills: ligand synthesis, polymer synthesis, coordination chemistry, theoretical calculations. The project is structured in 6 tasks: coordination/conception task (T0) and 5 scientific tasks, grouped in three “Work Packages”: coordination chemistry (WP1, task 1), polymerization (WP2, tasks 2, 3 et 4) and theoretical chemistry (WP3, task 5).
The synthesis of complexes (T1) will take place in the first 2 years. Other syntheses will take place if the polymerizations and the theoretical studies suggest needed improvements for the metal coordination sphere. As soon as the complexes are available, they will be investigated experimentally and computationally in terms of the metal-carbon bond homolytic dissociation.
The compounds prepared within task 1 will be used as initiators for the OMRP studies (T2-4). These complexes will first be applied to the homopolymerization f fluorinated monomers (T2) by paying particular attention to growth slowdown or stopping phenomena dues to head-head additions and to the possibility of catalytic chain transfer. The polymerizations will be investigated in both possible controlling modes: dissociative (reversible termination) and associative (degenerative transfer). At the same time, statistical copolymerizations between fluorinated monomers and other monomers will also be investigated under the same conditions in order to understand the limits of control and also to generate new copolymer architectures with wanted dimensions. The systems that insure a sufficient level of control in homopolymerization and/or copolymerization will then be applied to the development or more complex macromolecular architectures, starting from simple block copolymers (task 4).

The investigations have started with the computational exploration of the Mt-RF bond strength for two Mt systems: Co(acac)2 and Mn(CO)5. They have shown that the bond energies for the Mt-PVDF bond reactivation are quite similar for the two types of terminations, giving hope of a good control of VDF by OMRP as proposed in the project.
The synthesis of new complexes was initially oriented toward cobalt systems with a new tetra-oxygen ligand but it has met with difficulties and has been abandoned. Other Co(II) complexes have then been synthesized and tested in VAc polymerization. The preliminary results show similar behavior to Co(acac)2, suggesting that the targeted RF-CoIII bonds have insufficient strength to allow isolation. This objective has therefore set aside in favor of the synthesis of new manganese complexes RF-Mn(CO)5 with stronger bonds (RF = CF3, CHF2 and CH2CF3). More recently, new complexes with a mixed N/O/S coordination sphere have been synthesized and their exploration as controlling agents for less reactive monomers has started.
The polymerizations of fluorinated monomers have started with the use of the already well-known Co(acac)2 complex. The first tests have involved the copolymérization VAc/CH2=C(CF3)(COOtBu). An alternating copolymer has been obtained with good control. The investigations have been continued with the copolymerization of VAc and VDF and with the homopolymerization of the latter according to the initial plans, yielding very encouraging results.

Fundamental investigations: (i) experimental verification of the bond strengths for the (CO)5Mn-RF system in order to compare with the theoretical les predictions; (ii) use of theoretical calculations for the coordination sphere optimization of cobalt complexes in order to equalize the bond strengths with the “head” and “tail” chain ends of PVFD.
Synthesis of organometallic complexes: (i) new (CO)5Mn-R complexes with non fluorinated alkyls and study of their Mn-C bond strengths; (ii) synthesis of other complexes of Co, Fe and Cu with different coordination spheres.
Polymerization reactions: (i) application of the already available cobalt complexes to the VAc-VDF copolymerization, then to the homopolymerization of VDF, then to the statistical incorporation of other comonomers into PVDF; (ii) investigation of the PVDF initiation by (CO)5Mn-RF complexes.

One monopartner publication, plus two submitted articles and two more in preparation; two oral presentation in international meetings and one in a national meeting in 2016. Four abstracts submitted to a meeting abroad in 2017.

Radical polymerization is one of the most important processes for the synthesis of plastics. The recently introduced controlled radical polymerization (CRP) methods offer tremendous improvement in terms of microstructure design, molecular weight control, low dispersity and access to complex architectures for new applications. However, the success of CRP is mostly restricted to the more reactive monomers. The control of the less reactive fluorinated olefins has been achieved so far only by ITP and MADIX but is still insufficient for the generation of polymers with the desired properties. Developing methods for efficiently controlling the radical polymerization of fluorinated olefins and their copolymerization with other more reactive monomers is a scientifically challenging goal with the potential, which is at reasonable reach, to lead to a breakthrough in polymer technology. This project proposes to explore Organometallic Mediated Radical Polymerization (OMRP) as a toolbox for the development of controlled fluorinated polymer architectures. It is a fundamental knowledge based project, having the challenging goal of achieving a satisfactory polymerization control in this area, but its success opens the way to the generation of improved materials, notably for application in the energy area (novel membranes for direct methanol fuel cells, polyelectrolytes for lithium-ion batteries, ferroelectric relaxors, piezoelectric devices, etc.). The LCC Team is the World leader in OMRP, particularly providing an understanding of the reactivity of transition metal complexes with organic radicals. The ICG Team is World leader in the polymerization of fluorinated monomers. Therefore, the challenges addressed by this project can only be addressed by the combined effort of the two Teams. The participation of the international partner (CERM, Liège) is justified by its strong know-how in cobalt-mediated polymerization for the control of less reactive monomers and is guaranteed by the past successful collaboration with the LCC team.
The main bottleneck of the proposed research is the reactivation of the more stable dormant species that forms after an inverted head-to-head monomer addition. This is currently a major limitation of CRP success for the less reactive monomers, which are characterized by significant degree of inversion in the radical chain growth. The exploration of the metal-alkyl bond strength for the two types of dormant species and the proposed coordination sphere engineering aimed at removing this bottleneck are the project most original and innovative characters. The project is divided into 5 scientific tasks. The first one is the synthesis of organometallic compounds with metal-carbon bonds susceptible to reversible homolytic cleavage, and ligand engineering for modulation of these bond strengths. Low cost and toxicity metals such as Fe and Cu will be privileged. Particular attention will be devoted to the bond strengths of the regular “head” and inverted “tail” dormant species. A second task deals with the use of the synthesized organometallic compounds as mediators for the OMRP of fluorinated monomers. Task 3 will look at the controlled statistical copolymerization of fluorinated olefins with other comonomers, including simple olefins. Task 4 aims at synthesizing block copolymers, joining together blocks of all types of polymers developed in the previous tasks. Finally, Task 5 addresses all the relevant fundamental phenomena in this project from the computational point of view with density functional theory calculations: metal-carbon bond strengths, ligand effects, activation/deactivation pathways. All these studies will result in the development of previously inaccessible added-value polymers but especially to improved understanding of how to incorporate unreactive monomers into well-defined polymer architectures by controlled radical polymerization.