Dear colleagues,
We would like to advertise a PhD position on:
In situ characterization of the evolution of the catalytic environment in the first polymerization steps by metallocene centers: dissolution-DNP and relaxometry NMR.
Location : Laboratoire des biomolécules - UMR 7203ENS/CNRS/SU - team « structure and dynamics of biomolecules »
Polyolefin polymerization, although widely developed and mastered, relies on mechanisms that are often poorly understood. In this respect, the role played by catalysts, such as metallocenes, is important and their nature determines the properties of the final polymer such as its flowability, processability, melting temperature, etc. It is difficult to characterize the initial polymerization stage, i.e., the transition from monomers to oligomers, from a kinetic point of view as well as in terms of identification and structure characterization of the first compounds. In this context, nuclear magnetic resonance spectroscopy (NMR) may seem the analytical method of choice, allowing to monitor chemical reactions in real time and to identify substrates and products simultaneously. However, a recurrent problem of NMR is its intrinsic lack of sensitivity that is due to the low polarization of nuclear spins. This low sensitivity leads to time consuming experiments and therefore limits the temporal resolution required for the characterization of fast chemical reactions. Dynamic Nuclear Polarization (DNP) methods provide NMR signal amplification by several orders of magnitude (~ x 104), thereby allowing to perform measurements impossible to perform under usual conditions.
The molecule to be polarized is put in solution containing stable free radicals (“polarizing agents”) and frozen at very low temperature (< 4.2 K). The microwave irradiation of the electron spins of the radicals allows for the transfer of this polarization to the nuclear spins. In the so-called “dissolution DNP”, or D-DNP, experiments, the hyperpolarized sample is then dissolved and transferred to an NMR spectrometer at room temperature for measurements.
The laboratory has two D-DNP systems, which will be suitable for organic phase polymerization experiments. In addition to state-of-the-art D-DNP systems, the laboratory is also equipped with a prototype relaxometer, adapted to study molecular dynamics at the nanosecond time scale. This system will be used to characterize the diffusion of monomers in porous nanoparticle support of catalysts during polymerization. The objective of the thesis is to take advantage of these innovative D-DNP and relaxometry techniques to study the catalysis of alkene polymerization in an organic phase. In particular, we will characterize in situ the evolution of the active sites of the catalysts of interest (new catalysts that are being developed, mixtures of conformers). The role of the monomer and activator on the polymerization kinetics will be also investigated. The diffusion of monomers into and at the surface of the porous nanoparticle support of the catalysts will be studied by high-resolution relaxometry to better understand the growth of polymer particle in the early stages of polymerization.
The candidate should preferably have a background in process chemistry, in physics, or in physical chemistry. He or she will be largely involved in instrumentation-oriented work for D-DNP, including the development of a fast, high-pressure, dissolution and transfer system dedicated to work in the organic phase; as well as the adaptation of the system for the gas phase on supported catalysts.
The thesis, in collaboration with the company TOTAL, will take place in part on the LBM site at ENS-Paris The LBM is equipped with three NMR spectrometers (400 MHz, 600 MHz and 800 MHz) equipped with several prototype accessories (including two polarizers dedicated to D-DNP and a high-resolution relaxometer), which makes the instrumentation versatile, allowing to tackle a wide selection of application areas, a unique configuration to date.
Contacts :
LBM: Daniel Abergel (daniel.abergel@ens.psl.eu), Mathieu Baudin, (mathieu.baudin@ens.psl.eu), Fabien Ferrage (fabien.ferrage@ens.psl.eu)
TOTAL : Lavinia UTIU (lavinia.utiu@total.com)
Daniel Abergel, MD, PhD
Laboratoire des biomolécules - UMR7203
Département de chimie
Ecole Normale Supérieure
24, rue Lhomond
75005 Paris
Tel.: +33 1 44 32 32 65
email: daniel.abergel@ens.psl.eu
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