Mechanosynthesis of a Copper complex for redox initiating systems with a unique near infrared light activation

Abstract

The first use of a new mechanosynthesized copper complex (Cu(acac)(2dppba)) as a initiator for the redox and redox photoactivated polymerization of methacrylates under air is proposed. This paper (i) describes the mechanosynthesis of this complex, (ii) outlines the relative efficiency of the complex for redox polymerization (mechanosynthesized product vs. solvent synthesized product), (iii) follows the polymerization enhancement under a 405 nm light, and (iv) demonstrates the high performance of this complex in near infrared photoactivated redox polymerization where a completely colorless polymer is obtained (unprecedented under NIR irradiations, 785 nm, here). The light activated polymerization exhibit higher conversions, better time controls (activation control) and higher surface conversions than redox polymerization. The mechanosynthesis is well characterized by two solvent-free methods (visual color change and Electron Spin Resonance) and two solvent-based methods (high resolution-electrospray ionization-mass spectrometry (HR-ESI-MS) and UV-vis spectrometry). The involved mechanisms are discussed. Mechanosynthesis of copper complexes opens new perspectives for copper (photo)redox polymerization catalysts as the environmental impact and economical costs of the complex synthesis are significantly reduced.

Recent Advances on Copper Complexes as Visible Light Photoinitiators and (Photo) Redox Initiators of Polymerization

“…In 2018, mechanosynthesis was applied to the synthesis of photoinitiators activatable under near infrared light [69]. Indeed, on the basis of an interesting reaction discovered in 2016, where the grinding of copper (II) acetylacetonate (Cu(acac) 2 with 2-diphenylphosphinobenzoic acid (2-dppba) could give rise to a ligand exchange combined with a modification of the oxidation degree of Cu and the release of an acac• radical (see Figure 25) [70,71,[120][121][122], this approach based on a redox reaction was named metal acetylacetonate-bidentate ligand interaction (MABLI).…”

“…Prior to these works, the use of bis(amino)acid copper (II) complexes to initiate the polymerization of acrylamide, the use of Cu (I) or Cu (II) salts to photopolymerize tetrahydrofuran, or the use of borate salts to polymerize acrylates can be cited as works reported decades ago [66]. Very recently, in 2018, copper complexes have been involved in a new polymerization approach named metal acetylacetonate-bidentate ligand interaction (MABLI) enabling the formation of acetylacetonate (acac•) radicals by redox reaction with a phosphine and capable to initiate the free radical polymerization (FRP) of acrylates or to promote the free radical promoted cationic polymerization (FRPCP) of epoxides [67][68][69][70][71]. Contrarily to the aforementioned photosensitive complexes (i.e., Ir, Fe, and Zn) which are typically neutral complexes [72,73], copper complexes are generally positively charged so that a counter-anion has to be used.…”

Recent Advances on Visible Light Metal-Based Photocatalysts for Polymerization under Low Light Intensity

“…Copper complexes are metal complexes that have been widely used for thermal atom transfer radical polymerizations (ATRP) [76][77][78]. More recently, these complexes have also been used for a new mechanism of polymerization named MABLI (metal acetylacetonate-bidentate ligand interaction) involving the generation of acetylacetonate (acac•) radicals by a redox process enabling the initiation of the FRP of acrylates [79][80][81][82][83]. This point will be discussed later in this paragraph.…”

Redox two-component initiated free radical and cationic polymerizations: Concepts, reactions and applications

“…In redox photoactivated polymerization, the light activation of redox systems thanks to PIS is interesting since: i) the efficiency of the PIS should highly increase the surface conversion (where the light absorption is maximum); ii) an acceleration of the slow redox process is possible on-demand and iii) the global conversion can be improved (more active species generated). Additionally, in some specific cases, photoactivation enhanced the chemical bleaching which is of importance for dental cements [253].…”