Patxi Garra

New copper(I) complex based initiating systems in redox polymerization and comparison with the amine/benzoyl peroxide reference

Novel copper complex based initiating systems for redox free radical polymerization (FRP) of methacrylate resins under mild conditions are proposed. Remarkably, the FRP performance of the Cu(I)/ascorbic acid (Vitamin C – VitC)/benzoyl peroxide (BPO) redox system is noticeably more significant than that of the amine/BPO reference especially at the top surface where the oxygen inhibition is particularly deleterious. The Cu(I)/BPO interaction leads to a decomposition of the peroxide and generates a benzoyl radical and Cu(II) as a byproduct. The ascorbic acid acts as a reducing agent and allows the regeneration of Cu(I) from Cu(II). Hydroperoxides formed at the surface through the radical/O2 interaction are also decomposed by the new proposed Cu complex leading to additional polymerization initiating species. The structure/reactivity relationship of the new proposed Cu(I) complexes in the Cu(I)/VitC/BPO systems is studied. The use of Cu(I)/VitC/BPO/iodonium salt systems in redox photoactivated FRP is also explored.

Mechanosynthesized copper(I) complex based initiating systems for redox polymerization: towards upgraded oxidizing and reducing agents

Significant improvements of the recently proposed Cu(I)/vitamin C + water/dibenzoyl peroxide (BPO) system for the redox free radical polymerization of methacrylates in air are presented here, the goal being to avoid the presence of water and to eliminate the unstable BPO compound. Additionally, the use of mechanosynthesized Cu(I) complexes bearing cheap triphenylphosphine and methylpyridine ligands, allowed us to reduce the economical and synthesis costs significantly. Water-free reducing agents such as 6-O-palmitoyl-L-ascorbic acid, organotin or hydrazine derivatives are evaluated. Cumene hydroperoxide and tert-butyl perbenzoate are proposed as a replacement for less stable BPO. Most of these novel combinations are more efficient in air than the usual amine/benzoylperoxide system (4-N,N-TMA/BPO) reference especially at the sample surface. Among them, Cu(I)/organotin/cumene hydroperoxide appears as an efficient and competitive alternative to the conventional amine/BPO couple.

Photopolymerization processes of thick films and in shadow areas: a review for the access to composites

The photopolymerization processes are currently associated with thin samples for which the light penetration is good enough to activate the photoinitiator or the photoinitiating system for the entre samples thickness. The photopolymerization of very thick films and in shadow areas where the light penetration is inhibited (e.g. in filled, pigmented, and dispersed samples) remains a huge challenge (e.g. for the access to composites). In the present paper, an overview of the different strategies for the photopolymerization of thick samples is reported. First, strategies based on the optimization of the photonic (light intensity, excitation wavelength, etc.) or chemical (efficiency/reactivity/bleaching of the photoinitiating systems, etc.) parameters are presented that result in a full temporal and spatial control. Then, the main strategies based on propagation/diffusion mechanisms of latent species for the curing beyond the irradiated areas are given (partial loss of spatial resolution and access to shadow areas). Also, dual systems (thermal/photochemical or photochemical/redox) are described. The state of the art for the access to thick samples by photopolymerization processes as well as some perspectives are provided.

Metal Acetylacetonate–Bidentate Ligand Interaction (MABLI) as highly efficient free radical generating systems for polymer synthesis

Metal Acetylacetonate–Bidentate Ligand Interaction (MABLI) is presented here as a new chemical mechanism for the highly efficient generation of free radicals for polymer synthesis. This MABLI process involves simultaneous ligand exchange and a change of the metal oxidation degree and is associated with the efficient release of free radicals. In conventional redox two-component radical generating systems, two criteria are required to be efficient: (1) oxidizing agents must exhibit a low bond dissociation energy (BDE) i.e. they are usually unstable (e.g. peroxides) and (2) a small difference must exist between the oxidation potential of the reducing agent and the reduction potential of the oxidation agent. In contrast, here, the criteria for efficient MABLI radical generation were energetic and geometric for both bidentate ligands and metal acetylacetonates. The strength of this approach is to use stable compounds in 2-components free radical initiating systems and to generate carbon centered radicals. Mechanistic investigations demonstrated the formation of new metal adducts by means of high-resolution mass spectroscopy (HR-ESI-MS) as well as UV-vis spectrometry. As a result of its high radical generating rate, the potential of MABLI was illustrated on the methacrylate free radical polymerization under mild conditions (room temperature, in air) and initiated with a small amount of metal acetylacetonate though it opens new perspectives for acac-like additions in organic chemistry.

Stable copper acetylacetonate-based oxidizing agents in redox (NIR photoactivated) polymerization: an opportunity for the one pot grafting from approach and an example on a 3D printed object

The use of new stable copper acetylacetonates in combination with specific bidentate ligands (e.g. 2dppba) is proposed for redox free radical polymerization (FRP) under mild reaction conditions (under air and at room temperature). These systems show high performances so that they can compete with the well-established reference redox initiating systems based on the amine/benzoylperoxide combination. Remarkably, some of our systems are also highly efficient when excited in the near infrared region (NIR), demonstrating that our redox systems can also be activated by light. Using this strategy, the initial redox free radical polymerization could be converted into a redox photoactivated one. More interestingly, a stable copper methacryloyloxy-ethylacetoacetate (Cu(AAEMA)2) bearing a reactive methacrylate function and a copper acetylacetonate moiety could be used to polymerize a first polymer layer onto which a second polymerization could be carried out. Precisely, the second polymerization was initiated by a redox process using mild reaction conditions (at room temperature and under air), different from the process used to form the underlayer. To the best of our knowledge, this re-initiation of polymerization from a pre-polymerized layer by a second polymerization technique is highly original and is of crucial industrial/academic interest as no high energy consumption/harmful wavelengths are required to reinitiate the polymerization. The efficiency of this strategy is illustrated by grafting from a 3D printed object.