Nickel(II) complexes bearing long alkyl chains are successfully obtained as stable solids by ethylene oligomerization using a Shell Higher Olefin Process (SHOP)-type anionic bidentate (P–O) ligand. In contrast to the salient stability of the alkyl-Ni(II) complexes in the solid state, their chemical derivatization implied the occurrence of non-negligible chain-transfer during the ethylene oligomerization process.
Post-polymerisation modification of commodity hydrocarbon-based polymers provides access to functional polymers not readily available through bottom-up synthesis methods. Here, we demonstrate the oxyfunctionalisation of different styrenic and rubbery (co-)polymers using a well-established and robust manganese-based homogeneous catalyst, MnTACN, a 1,4,7-trimethyl-1,4,7-triazacyclononane ligand-bearing di-nuclear tri-μ-oxo bridged Mn(IV) compound, and hydrogen peroxide as a green oxidant. Using various grades of polystyrene (PS) and polybutadiene (PBD), we successfully oxyfunctionalised the polymer backbones with alcohol (PS and PBD), ketone (PS) and epoxide (PBD) functional groups. Under optimised conditions, total functionalisation degrees up to 5% for PS and 18% for PBD can be achieved. Next to the homopolymers, we also show oxyfunctionalisation degrees as high as 11%, of the butadiene-derived part of a styrene–butadiene–styrene block-co-polymer (SBS). These results underscore the versatility of a single catalytic system for the oxyfunctionalisation of various C–H bonds as well as the CC bonds found in these commodity hydrocarbon polymers. Detailed analysis of the oxidised polymers before and after subsequent oxidative cleavage of the installed diol moieties on the PBD backbone suggest that the functional groups are randomly spaced along the polymer backbone. Moreover, this second oxidative cleavage also offers the possibility to selectively break down the polymer backbone after oxyfunctionalisation into a mixture of dialdehyde oligomers consisting of 4 up to 32 monomeric units. For PBD and low/mid Mw PS, oxyfunctionalisation coincided with minimal backbone cleavage or crosslinking, as evidenced by gel permeation chromatography (GPC). For the high molecular weight PS samples and SBS, GPC analysis suggests that backbone cleavage is in contrast more pronounced upon oxyfunctionalisation. The thermal properties of the oxyfunctionalised materials are largely unchanged, with decomposition temperatures decreasing with increasing functionalisation degrees, but overall remaining in the high thermal stability regime.
A special molecular association is π–π-stacking, driven by weak interactions within aromatic compounds. The π–π-stacking interactions can occur in either a sandwich-like or T-shaped manner. In this study, a method to recognise π–π-stacking from classical molecular dynamics trajectories is developed. By applying three criteria, the method is tested for simple lignin dimer, tetramer and octamer systems, with all G units and β-O4′ linkages. The criteria are geometric and based on distance between ring centroids, the angle between the planes of the two rings and the lateral displacement of the rings. In addition, a wide-angle X-ray scattering (WAXS) profile was calculated from a tetramer system, in agreement with previous experimental results. However, when the WAXS peak assigned to sandwich-shaped stacking was analysed in-depth, it was found to mainly be caused by other intramolecular structural motifs involving e.g. the α-carbon and ring carbons, rather than π–π-stacking. This finding is important for future analyses of WAXS profiles originating from lignin-based materials and shows the strength of combining X-ray scattering methods with molecular modelling.
Multimodal scanning transmission electron microscopy on vitrified frozen-hydrated specimens promises exceptional spatial resolution into the molecular mechanisms underlying the formation of organic crystals in both health and disease. Detection of crystalline volumes is essential for tracking and mapping nucleation and growth. We provide an analytical description of the low-dose detection limit in diffraction for a thin crystal embedded in a thick matrix, focusing on organic crystals and embedding matrices of low-Z elements such as vitrified ice. Numerical calculations refine our description by accounting for the effects of multiple scattering. Often underestimated, wide-angle tails associated with inelastic scattering play a crucial role for the detection of crystalline reflections in a thick ice matrix, common for cryo-electron microscopy. We show that guanine crystals as thin as a few nanometers can be detected with a fluence of just a few thousand electrons if the ice thickness is below one mean free path for inelastic scattering. The required fluence increases non-linearly with the embedding ice thickness, with a pronounced top–bottom effect regarding the location of the crystal in the sample. Energy-filtered recording significantly reduces the fluence needed for thicker samples. The low-dose simulations implemented here validate the analytical description while acknowledging its limitations due to abstraction from multiple scattering and beam spreading.
In this study, lignin from two different sources – organosolv pine and hydrolysis birch – were chemically modified through esterification of hydroxyl groups using octanoyl (C8), lauroyl (C12), and palmitoyl (C16) chlorides, as well as through chloromethylation followed by esterification with tetradecanoic acid (C14) and benzoic acid. Modification of lignin was confirmed by FTIR and NMR spectroscopy. The esterified lignin samples were loaded into polylactic acid (PLA) at loadings of 10%, 20%, and 30% using a solvent casting method. Thermal and mechanical properties of PLA/lignin composites revealed that esterification significantly affected the polymer matrix properties. PLA could sustain as much as 30% lignin ester loading without affecting the film integrity. Among the variations, hydrolysis lignin ester (HLE) and benzoic acid ester (BAEP) enhanced the heat stability of PLA, while esterification with palmitoyl chloride (OHLE_C16) increased its elasticity through plasticization.
The chemical recycling of post-consumer polylactic acid by microwave-assisted alcoholysis promoted by a phenoxy-imine pyridine zinc is investigated. Different alcohols and diols are used to achieve, in all cases, outstanding activity and selectivity toward the targeted products. Thanks to the robustness of the catalyst the reactions can be performed in the air, with technical grade reagents and without the use of additional solvents, thus ensuring the full sustainability of the procedure.
The pressing need to replace petroleum-based plastics with renewable and biodegradable alternatives has sparked growing interest in biopolymers derived from lignocellulosic biomass as sustainable solutions. Among these, nanocellulose stands out as a versatile product capable of forming strong, transparent, and flexible films. However, these films lack active properties like antioxidant, antibacterial and UV-shielding capacity, essential for applications such as food packaging. To address this, incorporating lignin, a byproduct of lignocellulosic biorefineries, offers a promising route to enhance the functionality of nanocellulose films. In line with this idea, this work studies the incorporation of kraft lignin into nanocellulose films by two different protocols: the first protocol involves directly mixing a cellulose nanofiber (CNF) suspension with an aqueous lignin suspension; the second protocol uses lignin dissolved in acetone : water (9 : 1) which is transformed into lignin nanoparticles (LNPs) via solvent shifting when mixed with the aqueous CNF suspension. The resulting suspensions of CNFs and lignin were subsequently used to produce casting films. It was found that incorporating lignin into the CNF film not only conferred UV-shielding capacity, but also enhanced barrier properties without compromising the mechanical properties, particularly when lignin was introduced as LNPs (even at 10–20% LNP content). However, adding bulk lignin at a high concentration (20%) negatively affected water vapor permeability and mechanical properties. Antioxidant and antibacterial capacities correlated with lignin content, showing greater enhancement when lignin was present as nanoparticles compared to bulk lignin. These results indicate that forming LNPs in situ within the CNF suspension is a more effective approach to optimize the properties of nanocellulose films. Thus, the obtained films presented good active properties with mechanical properties comparable to those of traditional plastics, but significantly lower barrier properties.
The production of functional (nano)materials based on lignin as a renewable starting material depends on the thorough understanding of lignin’s physico-chemical properties, among which self-assembly and agglomeration/aggregation are the most important. Nevertheless, the knowledge about the structure–property relations for lignin is still in its infancy and needs further in-depth investigations. In this context, this works focuses on the study of the size and the colloidal stability of lignin nanoparticles (LNPs) prepared from softwood kraft lignin (SKL) using the solvent–antisolvent method. Conformational rearrangements of lignin chains were found to contribute significantly to the formation of the first lignin nuclei. The slow addition of ethylene glycol and THF into water caused the formation of nuclei with low aggregation numbers, minimizing the hydrodynamic volume of the final LNPs. On the other hand, a quick addition of these organic solvents created spatially and temporally higher lignin concentrations, yielding nuclei with high aggregation numbers and larger hydrodynamic volumes. Molecular dynamics simulations revealed the major role of intramolecular and intermolecular hydrogen bonds in this process, together with the contribution from π–π stacking interactions. The superficial concentration of phenolic and condensed guaiacyl units was found to strongly influence the corresponding LNPs’ zeta-potential values. Altogether, these results shed further light on the properties of colloidal lignin with a view to enabling its full potential as a key material for technological applications.
This paper summarizes a series of ideas and/or concepts, most of which were the object of the Faraday Discussion on ‘Structural and functional asymmetry of plasma membranes’. A historical review is provided of the early symmetrical and asymmetrical models for membrane structure. Membrane asymmetry in the framework of evolution is suggested as a possible field of study. Functional membrane asymmetry in experimental models is briefly discussed, and the hypothesis that functional asymmetry preceded structural asymmetry in evolution is proposed. Lipid asymmetry and lipid scrambling in bilayers are presented as two complementary aspects of the same process. The use of sphingomyelinases in experimental studies of asymmetry is criticized, due to the lipid scrambling properties of the sphingomyelinase end-product ceramide. The paper ends with a note on the (apparently?) cyclical nature of scientific research.
Poly(lactide-co-valerolactone) copolymers were prepared via the one-pot copolymerisation of rac-lactide or S,S-lactide with δ-valerolactone at ambient temperature, mediated by bis(dimethylsilyl)amido lanthanum complexes supported by ligands derived from a salan framework (salan = N,N′-bis(o-hydroxy, m-di-tert-butylbenzyl)-1,2-diaminoethane), which incorporate either benzyl or 2-pyridyl groups at the tertiary amine moieties. Poly(δ-valerolactone)s were also prepared by the ring-opening polymerisation of δ-valerolactone and high molecular weight polymers (up to 83.6 kg mol−1) with narrow dispersities were obtained. The poly(lactide-co-valerolactone) copolymers were fully characterized using 1H and 13C NMR spectroscopy, gel-permeation chromatography and differential scanning calorimetry. Both the reaction solvent (toluene or THF) and the number of 2-pyridyl groups the complex possesses affect the complex activity and copolymer microstructure. The use of a non-coordinating solvent and presence of at least one 2-pyridyl group is required for high conversion of both monomers. Variation of the monomer feed ratio allowed copolymers across the full compositional range to be prepared. The copolymers are formed via a transesterification mechanism whereby all of the lactide undergoes rapid polymerisation in the early stages of the reaction and the δ-valerolactone is subsequently incorporated into the polymer. The rate and extent of δ-valerolactone polymerisation increases with the number of 2-pyridyl groups in the catalyst in toluene and is more rapid in non-coordinating solvent (toluene) than coordinating solvent (THF). Only low levels of the TII mode of transesterification occur, with the TI transesterification mode dominating, leading to the formation of copolymers with intact lactidyl units.
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