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Charge-Transfer Complexes as New Inhibitors/Photoinitiators for On-Demand Amine/Peroxide Redox Polymerization.

ABSTRACT: Redox free-radical polymerizations have widespread applications but still clearly suffer from poor time control of the reaction. Currently, the workability (delay of the gel time) in redox polymerization after mixing is possible thanks to two main types of inhibitors (radical scavengers): phenols and nitroxides. Out of this trend, we propose in this work an alternative strategy for time delaying of the redox polymerization, which is based on charge-transfer complexes (CTCs). Thanks to iodonium salt complexation, the amine (here 4-N,N-trimethylaniline) is proposed to be stored in a CTC equilibrium and is slowly released over a period of time (as a result of the consumption of free amines by peroxides). This alternative strategy allowed us to double the gel time (e.g., from 60 to 120 s) while maintaining a high polymerization efficiency (performance comparable to reference nitroxides). More interestingly, the CTCs involved in this retarding strategy are photoresponsive under visible LED@405 nm and can be used on demand as photoinitiators, allowing (i) spectacular increases in polymerization efficiencies (from 50 °C without light to 120 °C under mild irradiation conditions); (ii) drastic reduction of the oxygen-inhibited layer (already 45% C=C conversion at a 2 ?m distance from the top surface) compared to the nonirradiated sample (thick inhibited layer of more than 45 ?m); and (iii) external control of the redox polymerization gel time due to the possible light activation.

SUBMITTER: Garra P

PROVIDER: S-EPMC6644477 | biostudies-literature | 2018 Jun

REPOSITORIES: biostudies-literature

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Charge-Transfer Complexes as New Inhibitors/Photoinitiators for On-Demand Amine/Peroxide Redox Polymerization.

Garra Patxi P Morlet-Savary Fabrice F Dietlin Céline C Fouassier Jean-Pierre JP Lalevée Jacques J

ACS omega 20180625 6

Redox free-radical polymerizations have widespread applications but still clearly suffer from poor time control of the reaction. Currently, the workability (delay of the gel time) in redox polymerization after mixing is possible thanks to two main types of inhibitors (radical scavengers): phenols and nitroxides. Out of this trend, we propose in this work an alternative strategy for time delaying of the redox polymerization, which is based on charge-transfer complexes (CTCs). Thanks to iodonium s ...[more]

PMID: 31458852

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Project description:Amine-peroxide redox polymerization (APRP) is the prevalent method for producing radical-based polymers in the many industrial and medical applications where light or heat activation is impractical. We recently developed a detailed description of the APRP initiation process through a combined computational and experimental effort to show that APRP proceeds through SN2 attack by the amine on the peroxide, followed by the rate-determining homolysis of the resulting intermediate. Using this new mechanistic understanding, a variety of peroxides were computationally predicted to initiate APRP with fast kinetics. In particular, the rate of APRP initiation can be improved by radical and anion stabilization through increased π-electron conjugation or by increasing the electrophilicity of the peroxy bond through the addition of electron-withdrawing groups. On the other hand, the addition of electron-donating groups lowered the initiation rate. These design principles enabled the computational prediction of several new peroxides that exhibited improved initiation rates over the commonly used benzoyl peroxide. For example, the addition of nitro groups (NO2) to the para positions of benzoyl peroxide resulted in a theoretical radical generation rate of 1.9 × 10-9 s-1, which is ~150 times faster than the 1.3 × 10-11 s-1 radical generation rate observed with unsubstituted benzoyl peroxide. These accelerated kinetics enabled the development of a redox-based direct-writing process that exploited the extremely rapid reactivity of an optimized redox pair with a custom inkjet printer, capable of printing custom shapes from polymerizing resins without heat or light. Furthermore, the application of more rapid APRP kinetics could enable the acceleration of existing industrial processes, make new industrial manufacturing methods possible, and improve APRP compatibility with biomedical applications through reduced initiator concentrations that still produce rapid polymerization rates.

Project description:In any physicochemical process in liquids, the dynamical response of the solvent to the solutes out of equilibrium plays a crucial role in the rates and products: the solvent molecules react to the changes in volume and electron density of the solutes to minimize the free energy of the solution, thus modulating the activation barriers and stabilizing (or destabilizing) intermediate states. In charge transfer (CT) processes in polar solvents, the response of the solvent always assists the formation of charge separation states by stabilizing the energy of the localized charges. A deep understanding of the solvation mechanisms and time scales is therefore essential for a correct description of any photochemical process in dense phase and for designing molecular devices based on photosensitizers with CT excited states. In the last two decades, with the advent of ultrafast time-resolved spectroscopies, microscopic models describing the relevant case of polar solvation (where both the solvent and the solute molecules have a permanent electric dipole and the mutual interaction is mainly dipole-dipole) have dramatically progressed. Regardless of the details of each model, they all assume that the effect of the electrostatic fields of the solvent molecules on the internal electronic dynamics of the solute are perturbative and that the solvent-solute coupling is mainly an electrostatic interaction between the constant permanent dipoles of the solute and the solvent molecules. This well-established picture has proven to quantitatively rationalize spectroscopic effects of environmental and electric dynamics (time-resolved Stokes shifts, inhomogeneous broadening, etc.). However, recent computational and experimental studies, including ours, have shown that further improvement is required. Indeed, in the last years we investigated several molecular complexes exhibiting photoexcited CT states, and we found that the current description of the formation and stabilization of CT states in an important group of molecules such as transition metal complexes is inaccurate. In particular, we proved that the solvent molecules are not just spectators of intramolecular electron density redistribution but significantly modulate it. Our results solicit further development of quantum mechanics computational methods to treat the solute and (at least) the closest solvent molecules including the nonperturbative treatment of the effects of local electrostatics and direct solvent-solute interactions to describe the dynamical changes of the solute excited states during the solvent response.

Dataset Information

Charge-Transfer Complexes as New Inhibitors/Photoinitiators for On-Demand Amine/Peroxide Redox Polymerization.

Publications

Charge-Transfer Complexes as New Inhibitors/Photoinitiators for On-Demand Amine/Peroxide Redox Polymerization.

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