Study of the mechanism of alternating copolymerization by electron paramagnetic resonance method.
Electronic paramagnetic resonance (EPR)
Category: RC methods Magnetic resonance methods of research EPR is observed in solids (crystalline, polycrystalline and powdery), as well as liquid and gaseous. The most important condition for observing EPR is the absence of electroconductivity and macroscopic magnetization in the sample.
Under favorable conditions, the minimum number of spins, which can be fixed in the test sample, is 1010. The mass of the sample can be, from several micrograms to 500 milligrams. During the EPR study, the sample is not destroyed and can be used later for other experiments.
Electron spin resonance
The phenomenon of electron paramagnetic resonance (EPR) consists in the resonance absorption of electromagnetic radiation in the radio frequency range by substances placed in a constant magnetic field and caused by quantum transitions between the energy sublevels associated with the presence of a magnetic moment in electronic systems. EPR is also called electron spin resonance (ESR), magnetic spin resonance (MCP) and, among specialists working with magnetically ordered systems, ferromagnetic resonance (FMR).
The EPR phenomenon can be observed on:
• atoms and molecules that have an odd number of electrons in their orbitals - H, N, NO2, etc .;
• chemical elements in different charge states, in which not all electrons in external orbitals participate in the formation of a chemical bond - first of all, they are d- and f-elements;
• Free radicals - methyl radical, nitroxide radicals, etc .;
• electron and hole defects, stabilizing in the matrix of substances, O-, O2-, CO2-, CO23-, CO3-, CO33- and many others;
• molecules with an even number of electrons, the paramagnetism of which is due to quantum phenomena of the distribution of electrons along molecular orbitals-O2;
• nanoparticles-superparamagnets formed during dissolution or in alloys possessing a collective magnetic moment that behave like an electron gas.
Structure and properties of the EPR spectra
The behavior of magnetic moments in a magnetic field depends on the various interactions of unpaired electrons, both with each other and with the nearest environment. The most important of them are spin-spin and spin-orbit interactions, interactions between unpaired electrons and nuclei on which they are localized (hyperfine interactions), interactions with the electrostatic potential produced by ions of the nearest environment at the site of localization of unpaired electrons, and others. Most of these interactions lead to a regular splitting of the lines. In the general case, the EPR spectrum of the paramagnetic center is multicomponent.
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• Ultrafine structure (STS). The CTC lines from a particular isotope have approximately the same integral intensity and are practically equidistant. If the core of the PC has several isotopes, then each isotope gives its own set of CTC lines. Their number is determined by the spin I of the nucleus of the isotope, near which the unpaired electron is localized. The relative intensities of the HFS lines from the different isotopes of the PC are proportional to the natural abundance of these isotopes in the sample, and the distance between the lines of the STS depends on the magnitude of the magnetic moment of the nucleus of a particular isotope, the hyperfine interaction constant, and the degree of delocalization of unpaired electrons on this nucleus.
• Superhyperfine structure (CCTS). The number of lines of the CCTS depends on the number of nl equivalent ligands with which the unpaired spin density and the nuclear spin values of their isotopes interact.
Evaluate organometallic compounds as reversible spin traps and molecular weight regulators in processes of living radical polymerization.
Organometallic compounds as acceptors of growing radicals
Another interesting area of the controlled radical
polymerization is a compound of organic compounds
transition metals as reversible polymerization inhibitors. In th.
cases, metal-metal compounds can actually act on
relation to the growth radicals as reversible spin traps. When
this can accept radicals as paramagnetic metal complexes,
and diamagnetic compounds. This direction in the foreign
literature is commonly referred to as an organometallic mediated radicalPolymerization (OMRP). By and large this method is similar inits nature to reversible inhibition involving stable
radicals (SFRP, Scheme 1).
Back in the early 1990s, Boris Smirnov, based on the results
Studies of the polymerization of BA and some (meth) acrylic monomers in тhe presence of cobalt metal complexes was developed by the theoreticalmodel of radical polymerization under conditions of reversible inhibition in general form.
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In parallel with the works of Boris Smirnov, American scientists B. Wayland andM.Fryd et al. Found that cobalt tetramesyltinporphyrin initiates and controls the polymerization of acrylates with formation of homopolymers and block copolymers: a linear increase is observed molecular weight with monomer conversion and low values the coefficients of polydispersity (1.1-1.3). An exemplary scheme of controlled radical polymerization according tomechanism OMRP in the presence of organometallic complexes can be distributed as follows:
where Me n - a metal atom in the oxidation state "n", Y is a halogen atom (or another substitute); L - ligand; m is a monomer molecule. An active radical, formed either as a result of the disintegration of ordinary radical initiators, or directly in the splitting of the bond metal-carbon, reacts with the monomer molecule to form macroradical of growth, capable of interacting with a metal complex form a labile adduct that decomposes under certain conditions with regeneration of the same macroradical growth. This reversible reaction leads to alternation of the periods of "sleep" and "life" of the polymer radical:the monomer molecules (m) are successively attached to the macroradical, thus a consistent growth of the polymer chain takes place. The flow of the undesirable reaction of the bimolecular chain termination in As a result of dimerization of growing macroradicals, leading to spontaneous growth of molecular weight, is difficult.To date, polymerization by the mechanism of reversible inhibition with the participation of organometallic compounds has been studied in detail for cobalt and molybdenum complexes. Numerous literature data indicate that processes of controlled radical polymerization in the presence of MOC depending on the structure of the metal atom, which is part of its composition, and in a number cases and its ligand environment can really proceed as in ATRP mechanism, and OMRP:
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