Filter and export

Publications

What is a Publication?
68 Publications visible to you, out of a total of 68
Switching Radical Stability by pH-Induced Orbital Conversion

Abstract (Expand)

In most radicals the singly occupied molecular orbital (SOMO) is the highest-energy occupied molecular orbital (HOMO); however, in a small number of reported compounds this is not the case. In the … present work we expand significantly the scope of this phenomenon, known as SOMO–HOMO energy-level conversion, by showing that it occurs in virtually any distonic radical anion that contains a sufficiently stabilized radical (aminoxyl, peroxyl, aminyl) non-π-conjugated with a negative charge (carboxylate, phosphate, sulfate). Moreover, regular orbital order is restored on protonation of the anionic fragment, and hence the orbital configuration can be switched by pH. Most importantly, our theoretical and experimental results reveal a dramatically higher radical stability and proton acidity of such distonic radical anions. Changing radical stability by 3–4 orders of magnitude using pH-induced orbital conversion opens a variety of attractive industrial applications, including pH-switchable nitroxide-mediated polymerization, and it might be exploited in nature.

Authors: Ganna Gryn'ova, David L. Marshall, Stephen J. Blanksby, Michelle L. Coote

Date Published: 1st Jun 2013

Publication Type: Journal

Which Side-Reactions Compromise Nitroxide Mediated Polymerization?

Abstract (Expand)

The mechanism of the nitroxide mediated polymerization (NMP) is well understood, however less is known about the side-reactions that interfere and in certain cases severely compromise it. Experimental … studies inevitably involve model fitting leading to at times contradictory conclusions as to which elementary side-reactions are behind the failure of a given NMP system. In the present work we use high-level quantum-chemical calculations to obtain the rate coefficients of the various side-reactions, both suggested previously and considered here for the first time, and first principles PREDICI kinetic simulations to identify the most deleterious side-reactions involved in the TEMPO, SG1 and DPAIO mediated polymerization of styrene, acrylate and methacrylate monomers. We show that the core mechanism for the thermal decomposition of alkoxyamines differs between the uni- and polymeric species, which often makes such experiments not suitable for modelling the NMP conditions. We also find that the main side-reaction responsible for the failure of TEMPO and SG1 in methacrylate homopolymerization is an intramolecular alkoxyamine decomposition (often referred to as ‘disproportionation’) via a Cope-type elimination, however in the case of SG1 the polymerization outcome is additionally affected by the equilibrium constant of alkoxyamine bond homolysis. On the basis of these findings, complemented by a thorough analysis of available experimental data, we define guidelines for minimising occurrence of the side-reactions and thus improving NMP. Finally, the accurate first principles rate parameters reported in this study should prove useful for subsequent kinetic modelling oriented at optimising different polymerization conditions.

Authors: Ganna Gryn'ova, Ching Yeh Lin, Michelle L. Coote

Date Published: 2013

Publication Type: Journal

Computational Evaluation of the Sulfonyl Radical as a Universal Leaving Group for RAFT Polymerisation

Abstract (Expand)

The present study investigates the performance of the sulfonyl radical, i.e. •SO2Ph, as a universal leaving group in reversible addition–fragmentation chain-transfer (RAFT) polymerisation. The sulfonyl … radical is widely used as a radical initiator and has already been proved successful as a leaving group in an atom-transfer radical polymerisation. Our results, obtained using high-level ab initio computational methodology under relevant experimental conditions, indicate superior performance of the sulfonyl compared with a reference cyanoisopropyl group in controlling RAFT of a wide range of monomers. Importantly, the presence of sulfonyl chain ends in the polymers so formed opens attractive possibilities for further functionalisation. Potential synthetic routes to the R-sulfonyl RAFT agents are discussed.

Authors: Ganna Gryn'ova, Tamaz Guliashvili, Krzysztof Matyjaszewski, Michelle L. Coote

Date Published: 2013

Publication Type: Journal

Use of Spin Trap Technique for Kinetic Investigation of Elementary Steps of RAFT-Polymerization

Abstract

Not specified

Authors: Anatoly Filippov, Elena Chernikova, Vladimir Golubev, Ganna Grynova, Ching Yeh, Michelle L.

Date Published: 12th Sep 2012

Publication Type: Book

New Insights into the Mechanism of Amine/Nitroxide Cycling during the Hindered Amine Light Stabilizer Inhibited Oxidative Degradation of Polymers

Abstract (Expand)

High-level ab initio molecular orbital theory calculations are used to identify the origin of the remarkably high inhibition stoichiometric factors exhibited by dialkylamine-based radical-trapping … antioxidants. We have calculated the free energy barriers and reaction energies at 25, 80, and 260 °C in the gas phase and in aqueous solution for a broad range of reactions that might, potentially, be involved in amine/nitroxide cycling, as well as several novel pathways proposed as part of the present work, including that of N-alkyl hindered amine light stabilizer activation. We find that most of the literature nitroxide regeneration cycles should be discarded on either kinetic or thermodynamic grounds; some are even inconsistent with existing experimental observations. We therefore propose a new mechanistic cycle that relies on abstraction of a β-hydrogen atom from an alkoxyamine (R1R2NOCHR3R4). Our results suggest that this cycle is energetically feasible for a range of substrates and provides an explanation for previously misinterpreted or unexplained experimental results. We also explore alternative mechanisms for amine/nitroxide cycling for cases where the alkoxyamines do not possess an abstractable β-hydrogen.

Authors: Ganna Gryn’ova, K. U. Ingold, Michelle L. Coote

Date Published: 24th Jul 2012

Publication Type: Journal

Computational Design of Cyclic Nitroxides as Efficient Redox Mediators for Dye-Sensitized Solar Cells

Abstract (Expand)

Cyclic nitroxide radicals represent promising alternatives to the iodine‐based redox mediator commonly used in dye‐sensitized solar cells (DSSCs). To date DSSCs with nitroxide‐based redox mediators … have achieved energy conversion efficiencies of just over 5 % but efficiencies of over 15 % might be achievable, given an appropriate mediator. The efficacy of the mediator depends upon two main factors: it must reversibly undergo one‐electron oxidation and it must possess an oxidation potential in a range of 0.600–0.850 V (vs. a standard hydrogen electrode (SHE) in acetonitrile at 25 °C). Herein, we have examined the effect that structural modifications have on the value of the oxidation potential of cyclic nitroxides as well as the reversibility of the oxidation process. These included alterations to the N‐containing skeleton (pyrrolidine, piperidine, isoindoline, azaphenalene, etc.), as well as the introduction of different substituents (alkyl‐, methoxy‐, amino‐, carboxy‐, etc.) to the ring. Standard oxidation potentials were calculated using high‐level ab initio methodology that was demonstrated to be very accurate (with a mean absolute deviation from experimental values of only 16 mV). An optimal value of 1.45 for the electrostatic scaling factor for UAKS radii in acetonitrile solution was obtained. Established trends in the values of oxidation potentials were used to guide molecular design of stable nitroxides with desired equation image, and a number of compounds were suggested for potential use as enhanced redox mediators in DSSCs.

Authors: Ganna Gryn'ova, Jonathan M. Barakat, James P. Blinco, Steven E. Bottle, Michelle L. Coote

Date Published: 11th Jun 2012

Publication Type: Journal

Reversible Addition–Fragmentation Chain Transfer Polymerization of Vinyl Chloride

Abstract (Expand)

Controlled/“living” radical polymerization (CLRP) of vinyl chloride (VC) via the reversible addition–fragmentation chain transfer (RAFT) process is reported for the first time. The cyanomethyl … methyl(phenyl)carbamodithioate (CMPCD) was found to be an efficient RAFT agent enabling the CLRP polymerization of VC monomer under certain experimental conditions. Two different radical initiators, having very distinct half-life times at room temperature, were employed in this study. The kinetic studies of RAFT polymerization of VC show a linear increase of the molecular weight with the monomer conversion and the lowest polydispersity (PDI) ever reported for poly(vinyl chloride) (PVC) synthesized with CLRP method (PDI ∼ 1.4). The resulting PVC was fully characterized using the matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS), 1H nuclear magnetic resonance spectroscopy (1H NMR), and gel permeation chromatography (GPC) techniques. The 1H NMR and MALDI-TOF-MS analysis of PVC prepared via RAFT polymerization method have shown the absence of structural defects and the presence of chain-end functional groups. The “livingness” of the PVC was also confirmed by a successful reinitiation experiment. The suitability of the RAFT agent was also confirmed via high-level ab initio molecular orbital calculations.

Authors: Carlos M. R. Abreu, Patrícia V. Mendonça, Arménio C. Serra, Jorge F. J. Coelho, Anatoliy V. Popov, Ganna Gryn’ova, Michelle L. Coote, Tamaz Guliashvili

Date Published: 27th Feb 2012

Publication Type: Journal

Powered by
 (v.1.14.2)
About HITS Publication SEEK | About HITS-Publication-SEEK | Funding and Programmes | Credits
Copyright © 2008 - 2023 The University of Manchester and HITS gGmbH