Enhanced synthesis of multiblock copolymers via acid-triggered RAFT polymerization.

The synthesis of multiblock copolymers has emerged as an efficient tool to not only reveal the optimal way to access complex structures and investigate polymer properties but also to ascertain the end-group fidelity of a given polymerization methodology. Although reversible addition-fragmentation chain-transfer (RAFT) polymerization is arguably the most dominant strategy employed, its success is often hampered by the unavoidable and excessive use of radical initiators which results in increased termination and loss of end-group fidelity. In this work, we employ acid in RAFT polymerization to enhance the synthesis of multiblock copolymers. By the addition of a small amount of acid, a 4-fold decrease in the overall required radical initiator concentration was achieved, enabling the synthesis of a range of well-defined multiblock copolymers with various degrees of polymerization (DP) per block. The acid enhances the propagation rate, minimizing the initiator concentration. In all cases, near-quantitative monomer conversion was obtained (>97%) for every iterative block formation step. Notably, and in contrast to conventional RAFT approaches, the tailing to low molecular weight was significantly suppressed and the dispersity was maintained nearly constant (i.e. in most cases D = 1.1-1.2), thus indicating minor termination events and side reactions during acid-enhanced synthesis. The possibility to synthesize multiblocks consisting of methacrylates, acrylates, and acrylamides was also demonstrated. This work presents an advancement in the synthesis of well-defined multiblock copolymers and more broadly, RAFT polymers with high end-group fidelity.

Chemical recycling of bromine-terminated polymers synthesized by ATRP.

Chemical recycling of polymers is one of the biggest challenges in materials science. Recently, remarkable achievements have been made by utilizing polymers prepared by controlled radical polymerization to trigger low-temperature depolymerization. However, in the case of atom transfer radical polymerization (ATRP), depolymerization has nearly exclusively focused on chlorine-terminated polymers, even though the overwhelming majority of polymeric materials synthesized with this method possess a bromine end-group. Herein, we report an efficient depolymerization strategy for bromine-terminated polymethacrylates which employs an inexpensive and environmentally friendly iron catalyst (FeBr2/L). The effect of various solvents and the concentration of metal salt and ligand on the depolymerization are judiciously explored and optimized, allowing for a depolymerization efficiency of up to 86% to be achieved in just 3 minutes. Notably, the versatility of this depolymerization is exemplified by its compatibility with chlorinated and non-chlorinated solvents, and both Fe(ii) and Fe(iii) salts. This work significantly expands the scope of ATRP materials compatible with depolymerization and creates many future opportunities in applications where the depolymerization of bromine-terminated polymers is desired.

Concurrent control over sequence and dispersity in multiblock copolymers.

Controlling monomer sequence and dispersity in synthetic macromolecules is a major goal in polymer science as both parameters determine materials' properties and functions. However, synthetic approaches that can simultaneously control both sequence and dispersity remain experimentally unattainable. Here we report a simple, one pot and rapid synthesis of sequence-controlled multiblocks with on-demand control over dispersity while maintaining a high livingness, and good agreement between theoretical and experimental molecular weights and quantitative yields. Key to our approach is the regulation in the activity of the chain transfer agent during a controlled radical polymerization that enables the preparation of multiblocks with gradually ascending (Ɖ = 1.16 → 1.60), descending (Ɖ = 1.66 → 1.22), alternating low and high dispersity values (Ɖ = 1.17 → 1.61 → 1.24 → 1.70 → 1.26) or any combination thereof. We further demonstrate the potential of our methodology through the synthesis of highly ordered pentablock, octablock and decablock copolymers, which yield multiblocks with concurrent control over both sequence and dispersity.

Near-zero surface pressure assembly of rectangular lattices of microgels at fluid interfaces for colloidal lithography.

Understanding and engineering the self-assembly of soft colloidal particles (microgels) at liquid-liquid interfaces is broadening their use in colloidal lithography. Here, we present a new route to assemble rectangular lattices of microgels at near zero surface pressure relying on the balance between attractive quadrupolar capillary interactions and steric repulsion among the particles at water/oil interfaces. These self-assembled rectangular lattices are obtained for a broad range of particles and, after deposition, can be used as lithography masks to obtain regular arrays of vertically aligned nanowires via wet and dry etching processes.

Self-templating assembly of soft microparticles into complex tessellations.

Encoding Archimedean and non-regular tessellations in self-assembled colloidal crystals promises unprecedented structure-dependent properties for applications ranging from low-friction coatings to optoelectronic metamaterials1-7. Yet, despite numerous computational studies predicting exotic structures even from simple interparticle interactions8-12, the realization of complex non-hexagonal crystals remains experimentally challenging13-18. Here we show that two hexagonally packed monolayers of identical spherical soft microparticles adsorbed at a liquid-liquid interface can assemble into a vast array of two-dimensional micropatterns, provided that they are immobilized onto a solid substrate one after the other. The first monolayer retains its lowest-energy hexagonal structure and acts as a template onto which the particles of the second monolayer are forced to rearrange. The frustration between the two lattices elicits symmetries that would not otherwise emerge if all the particles were assembled in a single step. Simply by varying the packing fraction of the two monolayers, we obtain not only low-coordinated structures such as rectangular and honeycomb lattices, but also rhomboidal, hexagonal and herringbone superlattices encoding non-regular tessellations. This is achieved without directional bonding, and the structures formed are equilibrium structures: molecular dynamics simulations show that these structures are thermodynamically stable and develop from short-range repulsive interactions, making them easy to predict, and thus suggesting avenues towards the rational design of complex micropatterns.

Microgels Adsorbed at Liquid-Liquid Interfaces: A Joint Numerical and Experimental Study.

Soft particles display highly versatile properties with respect to hard colloids and even more so at fluid-fluid interfaces. In particular, microgels, consisting of a cross-linked polymer network, are able to deform and flatten upon adsorption at the interface due to the balance between surface tension and internal elasticity. Despite the existence of experimental results, a detailed theoretical understanding of this phenomenon is still lacking due to the absence of appropriate microscopic models. In this work, we propose an advanced modeling of microgels at a flat water/oil interface. The model builds on a realistic description of the internal polymeric architecture and single-particle properties of the microgel and is able to reproduce its experimentally observed shape at the interface. Complementing molecular dynamics simulations with in situ cryo-electron microscopy experiments and atomic force microscopy imaging after Langmuir-Blodgett deposition, we compare the morphology of the microgels for different values of the cross-linking ratios. Our model allows for a systematic microscopic investigation of soft particles at fluid interfaces, which is essential to develop predictive power for the use of microgels in a broad range of applications, including the stabilization of smart emulsions and the versatile patterning of surfaces.