Amphiphilic core-cross-linked micelles functionalized with bis(4-methoxyphenyl)phenylphosphine as catalytic nanoreactors for biphasic hydroformylation

Core-cross-linked micelles (CCM) functionalized at the core with covalently linked bis(p-methoxyphenyl) phenylphosphine (BMOPPP) ligands have been synthesized by a three-step one-pot radical polymerization in emulsion, using the polymerization-induced self-assembly (PISA) strategy and reversible addition-fragmentation chain transfer (RAFT) as the controlling method. The CCM are obtained by chain extending in water poly(methacrylic acid-co-poly(ethylene oxide) methyl ether methacrylate) (P(MAA-co-PEOMA), degree of polymerization of 30, MAA/PEOMA units molar ratio of 50:50) synthesized in a first step by RAFT with a 95:5 M mixture of styrene and 4-[bis(p-methoxyphenyl)phosphino]styrene (BMOPPS) units. The resulting micelles exhibiting a core composed of P(S-co-BMOPPS) segments with a degree of polymerization of 300 are then crosslinked in a third step with a mixture of di(ethylene glycol) dimethacrylate (DEGDMA) and styrene. The resulting BMOPPP@CCM exhibit a narrow size distribution (PDI = 0.16) with an average diameter of 81 nm in water and swell in THF or by addition of toluene to the latex. The addition of [Rh(acac) (CO)2] to the toluene-swollen latex results in metal coordination to the phosphine ligands. 31P{1H} NMR spectroscopy shows that the Rh centers undergo rapid intraparticle phosphine ligand exchange. Application of these nanoreactors to the aqueous biphasic hydroformylation of 1-octene shows excellent activity and moderate catalyst leaching

Enhanced water barrier properties of surfactant-free polymer films obtained by macroRAFT-mediated emulsion polymerization

The presence of low-molar-mass surfactants in latex films results in detrimental effects on their water permeability, gloss, and adhesion. For applications such as coatings, there is a need to develop formulations that do not contain surfactants and have better water barrier properties. Having previously reported the synthesis of surfactant-free latex particles in water using low amounts ( < 2 wt %) of chains synthesized by controlled radical polymerization (Lesage de la Haye et al. Macromolecules 2017, 50, 9315-9328), here we study the water barrier properties of films made from these particles and their application in anticorrosion coatings. When films cast from aqueous dispersions of acrylate copolymer particles stabilized with poly(sodium 4-styrenesulfonate) (PSSNa) were immersed in water for 3 days, they sorbed only 4 wt % water. This uptake is only slightly higher than the value predicted for the pure copolymer, indicating that the negative effects of any particle boundaries and hydrophilic-stabilizing molecules are minimal. This sorption of liquid water is 5 times lower than what is found in films cast from particles stabilized with the same proportion of poly(methacrylic acid) (PMAA), which is more hydrophilic than PSSNa. In water vapor with 90% relative humidity, the PSSNa-based film had an equilibrium sorption of only 4 wt %. A small increase in the PMAA content has a strong and negative impact on the barrier properties. Nuclear magnetic resonance relaxometry on polymer films after immersion in water shows that water clusters have the smallest size in the films containing PSSNa. Furthermore, these films retain their optical clarity during immersion in liquid water for up to 90 min, whereas all other compositions quickly develop opacity ("water whitening") as a result of light scattering from sorbed water. This implies a remarkably complete coalescence and a very small density of defects, which yields properties matching those of some solvent-borne films. The latex stabilized with PSSNa is implemented as the binder in a paint formulation for application as an anticorrosive barrier coating on steel substrates and evaluated in accelerated weathering and corrosion tests. Our results demonstrate the potential of self-stabilized latex particles for the development of different applications, such as waterborne protective coatings and pressure-sensitive adhesives

Dynamic stratification in drying films of colloidal mixtures

In simulations and experiments, we study the drying of films containing mixtures of large and small colloidal particles in water. During drying, the mixture stratifies into a layer of the larger particles at the bottom with a layer of the smaller particles on top. We developed a model to show that a gradient in osmotic pressure, which develops dynamically during drying, is responsible for the segregation mechanism behind stratification

In situ monitoring of latex film formation by small-angle neutron scattering: Evolving distributions of hydrophilic stabilizers in drying colloidal films

The distribution of hydrophilic species, such as surfactants, in latex films is of critical importance for the performance of adhesives, coatings and inks, among others. However, the evolution of this distribution during the film formation process and in the resulting dried films remains insufficiently elucidated. Here, we present in situ (wet) and ex situ (dry) SANS experiments that follow the film formation of two types of latex particles, which differ in their stabilizer: either a covalently bonded poly(methacrylic acid) (PMAA) segment or a physically adsorbed surfactant (sodium dodecyl sulfate, SDS). By fitting the experimental SANS data and combining with gravimetry experiments, we have ascertained the hydrophilic species distribution within the drying film and followed its evolution by correlating the size and shape of stabilizer clusters with the drying time. The evolution of the SDS distribution over drying time is being driven by a reduction in the interfacial free energy. However, the PMAA-based stabilizer macromolecules are restricted by their covalent bonding to core polymer chains and hence form high surface-area disc-like phases at the common boundary between particles and PMAA micelles. Contrary to an idealized view of film formation, the PMAA does not remain in the walls of a continuous honeycomb structure. The results presented here shed new light on the nanoscale distribution of hydrophilic species in drying and ageing latex films. We provide valuable insights into the influence of the stabilizer mobility on the final structure of latex films

Mass transfer assessment and kinetic investigation of biphasic catalytic systems

Efficient catalyst recovery and recycling is still a major challenge for the development of homogeneous catalysis. In the 80’s, the concept of biphasic catalysis, in which the catalyst is confined into a solvent immiscible with the products, has opened new perspectives for transition metal complex driven homogeneous catalysis, after the industrial success of the Ruhrchemie/Rhone-Poulenc process operating the rhodium-catalyzed hydroformylation of propene in water. However, the low solubility of long-chain a-olefins has limited the scope of hydrosoluble catalysts for this reaction. To overcome this problem, various strategies have been developed since then, which consist in replacing water by a more suitable solvent or using additives/ligands able to increase the substrate solubility or create a favorable microenvironment in the aqueous phase. Apart from the screening/tailoring of solvent and ligand, the determination of an adequate kinetic model and the assessment of the mass transfer role is of great importance for the design and optimization of the multiphase reaction system. This presentation gives an account of collaborative works between chemical engineering and chemistry teams to address these issues for two different biphasic catalysis approaches: catalyst immobilization in ionic liquids and use of amphiphilic polymer ligands. The hydroformylation of oct-1-ene by rhodium complexes was selected as model reaction for the developed methodology. This includes the thermodynamic study of the complex reaction medium (gas-liquid and liquid-liquid equilibria), the thorough investigation of the effect of process parameters to evaluate the location of the catalytic act and the interfacial mass transfer resistance, the discrimination and identification of intrinsic kinetic models (derived from elementary reaction steps) and their coupling with (gas-liquid) mass transfer under low stirring conditions. In the first example, the role of the ionic liquids as solvents for biphasic catalysis was specified, by characterizing the solubility of both gaseous and organic substrates, and a detailed kinetic model was able to accurately describe the time-concentration profiles of reactants and products (1-octene,internal octenes, n-nonanal and branched aldehydes) measured in the organic phase. TOF values could be further improved (up to 560 h-1) by supporting the ionic liquid phase onto a silica gel support. In the second example, the proof of concept of cross-linked micelles as efficient supports for aqueous phase catalysis was established, demonstrating that the reaction occurs within the nano-objects with fast exchange with the organic phase. The study also provided clues for their optimization: a low functionalization degree and a nanogel structure embedding the phosphine moieties were proved to improve the catalytic activity and reduce the metal leaching, respectively. These innovative ligands yielded TOF in the range of 350 to 650 h-1 and linear/branched aldehyde ratios between 3 and 6. The Rh loss could be reduced to 0.1 ppm with adequate pH and temperature conditions

Influence of structure and solubility of chain transfer agents on the RAFT control of dispersion polymerisation in scCO2

Reversible addition-fragmentation chain transfer (RAFT) dispersion polymerisation of methyl methacrylate (MMA) is performed in supercritical carbon dioxide (scCO2) with 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) present as chain transfer agent (CTA) and surprisingly shows good control over PMMA molecular weight. Kinetic studies of the polymerisation in scCO2also confirm these data. By contrast, only poor control of MMA polymerisation is obtained in toluene solution, as would be expected for this CTA which is better suited for acrylates. In this regard, we select a range of CTAs and use them to determine the parameters that must be considered for good control in dispersion polymerisation in scCO2. A thorough investigation of the nucleation stage during the dispersion polymerisation reveals an unexpected “in situtwo-stage” mechanism that strongly determines how the CTA works. Finally, using a novel computational solvation model, we identify a correlation between polymerisation control and degree of solubility of the CTAs. All of this ultimately gives rise to a simple, elegant and counterintuitive guideline to select the best CTA for RAFT dispersion polymerisation in scCO

Core-shell nanoreactors for efficient aqueous biphasic catalysis

Water-borne phosphine-functionalized core-cross-linked micelles (CCM) consisting of a hydrophobic core and a hydrophilic shell were obtained as stable latexes by reversible addition-fragmentation chain transfer (RAFT) in water in a one-pot, three-step process. Initial homogeneous aqueous-phase copolymerization of methacrylic acid (MAA) and poly(ethylene oxide) methyl ether methacrylate (PEOMA) is followed by copolymerization of styrene (S) and 4-diphenylphosphinostyrene (DPPS), yielding P(MAA-co-PEOMA)-b-P(S-co-DPPS) amphiphilic block copolymer micelles (M) by polymerization-induced self-assembly (PISA), and final micellar cross-linking with a mixture of S and diethylene glycol dimethacrylate. The CCM were characterized by dynamic light scattering and NMR spectroscopy to evaluate size, dispersity, stability, and the swelling ability of various organic substrates. Coordination of [Rh(acac)(CO)2 ] (acac=acetylacetonate) to the core-confined phosphine groups was rapid and quantitative. The CCM and M latexes were then used, in combination with [Rh(acac)(CO)2 ], to catalyze the aqueous biphasic hydroformylation of 1-octene, in which they showed high activity, recyclability, protection of the activated Rh center by the polymer scaffold, and low Rh leaching. The CCM latex gave slightly lower catalytic activity but significantly less Rh leaching than the M latex. A control experiment conducted in the presence of the sulfoxantphos ligand pointed to the action of the CCM as catalytic nanoreactors with substrate and product transport into and out of the polymer core, rather than as a surfactant in interfacial catalysis

Core phosphine-functionalized amphiphilic nanogels as catalytic nanoreactors for aqueous biphasic hydroformylation

Amphiphilic phosphine-functionalized nanogel particles were synthesized by aqueous polymerization induced self-assembly insuring a well-defined architecture as well as a narrow size distribution (average diameter of ca. 90 nm in water). They were successfully applied as ligands for the biphasic hydroformylation of 1-octene catalyzed by rhodium, yielding TOFs in the 350–650 h-1 range and a linear to branched aldehyde ratio of 3.5. Embedding the phosphine ligands within a cross-linked structure did not strongly impede mass transfer toward the active centers, as proved by fast metal coordination and a catalytic activity tantamount to that of higher chain mobility micelles or core-cross-linked micelles that have phosphine moieties located on flexible linear arms. However, this extended cross-linking reduced particle swelling and transfer to the organic phase, affording a significantly lowered Rh loss. For all the architectures, a low functionalization degree was preferable to achieve high activity, the selectivity remaining essentially unchanged

Polymerization-Induced Self-Assembly

International audienc

RAFT Miniemulsion Polymerization: Influence of the Structure of the RAFT Agent

International audienc