Organometallic mediated radical polymerization of less reactive monomers <br /> <br />
This project has four scientific objectives: (1) extend the utility of organometallic mediated radical polymerization to control the chain growth of less reactive monomers such as alpha-olefins and vinyl ethers; (2) develop a general understanding of the criteria that favour a reversible radical trapping with controlled chain growth relative to hydrogen atom transfer leading to chain transfer catalysis; (3) explore and validate the new concept of alkene activation toward radical addition by coordination to Lewis acids; (4) apply the alkene activation concept to the efficient radical cyclization of bromoalkenes. Objective 1 represents a significant challenge, the major obstacle being the strong bonds formed by the active radicals and their tendency to lead to H atom transfer processes. Weakening the metal-carbon bonds by choosing metals that naturally form weak bonds with alkyl groups and engineering the metal coordination sphere is predicted to overcome this obstacle and a proof of concept has already been obtained by the proponent team in previous work. As a technical objective, we wish to gain access to copolymer materials with elaborated and controlled architecture containing both the less reactive monomers to more reactive ones.
The project is organized in seven scientific tasks: 1. DFT studies, where all fundamental processes needed to realize the project objectives (reversible homolytic cleavage of metal-carbon bonds with analysis of the bond energetics, olefin coordination and activation toward radical addition, and competitive H atom transfer processes) will be analyzed giving feedback to ligand engineering. 2. Synthesis of all Mtn and Mtn+1 complexes needed for the study (complexes of copper and other metals). 3. Experimental studies of Mtn+1-R bond energetics and OMRP activation barriers (both kinetic and thermodynamic studies). 4. Experimental investigation of chain transfer processes, whenever the developed metal systems are found to promote competitive H atom transfer processes. 5. Experimental studies of radical addition to coordinated olefins, defining the extent of activation of alkenes by metal coordination. 6: controlled polymerization studies. Task 7: Halo-alkene cyclization studies. The computational (Task 1) and experimental mechanistic (Tasks 3-5) studies on complexes isolated in Task 2 will give feedback for optmizing the metal coordination sphere for developing metal complexes able to promote controlled polymer growth for the less reactive monomers (Task 6).
The controlled polymerization of vinyl acetate by cobalt bis(acetylacetonate) is now well established. Thorough investigations of the same process mediated by iron bis(acetylacetonate), motivated by the biocompatibility and low cost of this metal, have unfortunately revealed a weak level of control for this monomer. The application of Co(acac)2 to the polymerization of other less reactive monomers (vinyl amides: N-vinylcaprolactame, N-vinylpyrrolidone, N-vinyl-N-methyl acetamide) have resulted in a well controlled polymerization. The different polymerization rates have been rationalized by DFT calculations. Several copper complexes have been synthesized but the first tests of controlled polymerization of vinyl acetate have provided disappointing results. However, the unprecedented principle of a reversible trapping of radical chains by copper(I) to yield organometallic copper(II) dormant species has been validated.
More copper complexes will be synthesized and tested. Calculations of Cu(II)-R bond strength will be carried out and guide us to the best coordination sphere and monomer combination likely to lead to a controlled polymerization process. Olefin coordination studies and then activation studies will also be carried out in the near future.
Articles in refereed journals
P1. S. Gulli, J.-C. Daran, R. Poli, Eur. J. Inorg. Chem. 2011, 1666-1672.
P2. Y. Champouret, S. Gulli, J.-C. Daran, R. Poli, Eur. J. Inorg. Chem. 2012, 1672-1679.
P3. Z. Xue, J.-C. Daran, Y. Champouret, R. P
Controlled radical polymerization (CRP) has literally exploded in the last decade, offering a tremendous improvement over conventional radical processes in the preparation of polymers with controlled molecular weights and relatively low polydispersity. In addition, it provides new routes to block copolymers and other macromolecular architectures, including functionalities for targeted applications. One limitation of CRP is the range of polymerizable monomers, which comprise acrylates, methacrylates, styrenes, acrylamides, and other monomers that generate stabilized radicals. Unfortunately, controlled polymerization of less reactive monomers such as alpha-olefins, vinyl ethers, vinyl chloride and vinyl acetate is still a challenge, although advances have been made for the last two monomers. The controlled polymerization of these monomers would provide new materials with high added value.
The current project involves the development of metallic systems capable of filling this gap by the organometallic route, the control being assured by the reversible deactivation of the growing polymer chain via formation of a metal-carbon bond. Two American partners will also participate in this research (with their own funding). Partner 2 is an expert on atom transfer radical processes for organic synthetic applications while Partner 3 is a World leading scientist in controlled radical polymerization. The three partners have already extensively collaborated, demonstrating the synergy of their approach.
Previous work with ANR funding by the proponent laboratory has establish the proof of concept and has developed system capable of controlling the polymerization of vinyl acetate. Engineering of the metal coordination sphere can tune the homolytic metal-carbon bond dissociation energy to a suitable range for controlled polymer growth. Successful design of a suitable coordination sphere leading to controlled chain growth needs full understanding of how various parameters regulate the competing hydrogen atom transfer from the radical growing chain to the metal leading to catalytic chain transfer (CCT) on one side, and reversible radical trapping on the other side. Monomers that have a very low reactivity toward the active radical, such as the alpha-olefins, may need activation by metal coordination. Thus, the alkene coordination process to the metal complex and its activation toward radical addition, on which very little is known, will also be investigated. The major effort will be directed toward the use of copper, in which Partner 2 has extensive experience, but other metals will also be investigated.
The ultimate objectives of the project include (a) the achievement of the polymerization of alpha-olefins and vinyl ethers for the preparation of homopolymers with targeted molecular weight and narrow polydispersities; (b) the caracterization of the microstructure of these polymers; (c) the use of these “living” polymers as macroinitiators for the development of block-type and more complex macromolecular architectures with the inclusion of other monomers. Fundamental knowledge that will be gained along the way will serve not only to develop new polymeric materials, but also to reach other equally useful goals such as new efficient chain transfer catalysts in radical polymerization and to new efficient radical addition and cyclization reactions in small molecule organic synthesis. Thus, the project is highly multidisciplinary in nature, requiring know-how in basic synthetic organic and organometallic chemistry, physical characterization of paramagnetic compounds, polymer synthesis and kinetic studies, as well as computational investigations. The three proponents have the collective expertise needed to achieve the project objectives make major advances in this area.
Monsieur Rinaldo POLI (CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE MIDI-PYRENEES) – Rinaldo.Poli@lcc-toulouse.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.
LCC CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE MIDI-PYRENEES
Help of the ANR 210,000 euros
Beginning and duration of the scientific project: - 36 Months