Self-powered artificial nanomotors are currently attract-9 ing increased interest as mimics of biological motors but also as 10 potential components of nanomachinery, robotics, and sensing devices. 11We have recently described the controlled shape transformation of 12 polymersomes into bowl-shaped stomatocytes and the assembly of

A series of magnesium, aluminum, and zinc complexes of imidazol-2-ylidene-, imidazolin-2-ylidene-, and tetrahydropyrimidin-2-ylidene-derived N-heterocyclic carbenes (NHCs) was prepared. In addition, both symmetrical and unsymmetrical CO 2 -protected imidazol-2-ylidenes and imidazolin-2-ylidenes were prepared. Selected single-crystal Xray structures are reported. All compounds were investigated for their catalytic behavior in (poly)urethane (PUR) synthesis. Out of nine different compounds, the dimeric complex [Zn(CH 3 COO) 2 (1,3-dimesitylimidazol-2-ylidene)] 2 (7) proved to be the most active one, rivaling the industrially most relevant catalyst dibutyltin dilaurate in terms of catalytic ac-

Catalytic rivals: Both CO(2)-protected tetrahydropyrimidin-2-ylidene-based N-heterocyclic carbenes (NHCs) and Sn(II)-1,3-dimesitylimidazol-2-ylidene, as well as Sn(II)-1,3-dimesitylimidazolin-2-ylidene complexes (example displayed), have been identified as truly latent catalysts for polyurethane (PUR) synthesis rivaling all existing systems both in activity and latency.A series of CO(2)-protected pyrimidin-2-ylidenes as well as 1,3-dimesitylimidazol-2-ylidene and dimesitylimidazolin-2-ylidene complexes of Sn(II) have been prepared. Selected single-crystal X-ray structures are reported. The new compounds were investigated for their catalytic behavior in polyurethane (PUR) synthesis. All compounds investigated showed excellent catalytic activity, rivaling the industrially most relevant catalyst dibutyltin dilaurate. Even more important, all compounds displayed pronounced latent behavior, in selected cases rivaling and exceeding the industrially relevant latent catalyst phenylmercury neodecanoate both in terms of latency and catalytic activity. This allows for creating one-component PUR systems with improved pot lifetimes. Pseudo-second-order kinetics were found for both CO(2)-protected tetrahyropyrimidin-2-ylidenes and for [SnCl(2)(1,3-dimesityldihydroimidazol-2-ylidene)], indicating a fast pre-catalyst decomposition prior to polyurethane formation. 1,3-Di(2-propyl)tetrahydropyrimidin-2-ylidene was additionally found to be active in the cyclotrimerization of various isocyanates, offering access to a broad variability in polymer structure, that is, creating both urethane and isocyanurate moieties within the same polymer.

We describe the preparation of an injectable, biocompatible, and elastic segmented copolymer hydrogel for biomedical applications, with segmented hydrophobic bisurea hard segments and hydrophilic PEG segments. The segmented copolymers were obtained by the step growth polymerization of amino-terminated PEG and aliphatic diisocyanate. Due to their capacity for multiple hydrogen bonding within the hydrophobic segments, these copolymers can form highly stable gels in water at low concentrations. Moreover, the gels show shear thinning by a factor of 40 at large strain, which allows injection through narrow gauge needles. Hydrogel moduli are highly tunable via the physical cross-link density and the length of the hydrophilic segments. In particular, the mechanical properties can be optimized to match the properties of biological host tissues such as muscle tissue and the extracellular matrix.

Abstract:The synthesis of a resin-supported, carbon dioxide-protected N-heterocyclic carbene (NHC) and its use in organocatalysis and organometallic catalysis are described. The resin-bound carbon dioxide-protected NHC-based catalyst was prepared via ring-opening metathesis copolymerization of 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo,endo-dimethanonaphthalene (DMNH6) with 3-(bicycloA C H T U N G T R E N N U N G [2.2.1]hept-5-en-2-ylmethyl)-1-(2-propyl)-3,4,5,6-tetrahydropyrimidin-1-ium-2-carboxylate (M1), using the well-defined Schrock catalyst

The importance of scorpionate ligands in modern coordination chemistry continues to increase, because of their outstanding versatility, tunability and user-friendliness. Herein, we provide a short overview of recent developments in the classes of scorpionate ligands, derived from pyrazoles, triazoles, imidazoles, oxazolines, thioimidazoles and other similar systems, followed by an in-depth discussion of a new type of ro- Classes of Scorpionate LigandsScorpionate ligands [1] are facially coordinating tridentate chelates, which have evolved into one of the most useful and [a]

We report on the synthesis of poly(homo-isobutylene) and poly(homo-R-methylstyrene) and the thermal properties of these new polymers. Based on the ring-opening metathesis polymerization (ROMP) of 3,3-dimethylcyclopropene and 3-methyl-3-phenyl-cyclopropene, the respective ring-opened polymers were generated. Several catalytic systems (first-generation (I), second-generation (II), third-generation Grubbs-type (IV) and the Schrock-type initiator Mo(N-2,6-iPr 2 C 6 H 3 )(CHCMe 2 Ph)(OCMe 3 ) 2 (III) were used. Particularly II, III and IV offered access to living polymerization reactions in the case of 3-methyl-3-phenyl-cyclopropene, however, not with 3,3-dimethylcyclopropene. The obtained ROMP-polymers were hydrogenated using tosylhydrazide, furnishing poly(homo-isobutylene) and poly(homo-R-methylstyrene) in high yields. Thermal-measurements (DSC-measurements) revealed T g -and T m -values located between those of poly(isobutylene), poly(styrene), and high-density poly(ethylene).

Cross-link density is an important parameter for the macroscopic mechanical properties of hydrogels. Increasing network density leads to an increase in the storage and loss moduli of the gel and can be accomplished by either increasing the concentration of cross-linkers, or by reducing the fraction of mechanically inactive cross-links. Mechanically inactive crosslinks consist of loops in the network, which do not contribute to the mechanical properties. Suppression of loop formation is demonstrated in a system of semiflexible supramolecular rods of poly(ethylene glycol)−bis(urea) bolaamphiphiles. Use of a cross-linker which, due to self-sorting of its hydrophobic segments, preferentially connects different rods, increases the modulus of a hydrogel by a factor of 15 compared to a system without self-sorting. By using statistical-mechanical calculations, it is shown that this increase can be explained by the increased tendency of the cross-linkers to form bridges between the semiflexible rods and thus increasing the cross-link density in the supramolecular hydrogel. ■ INTRODUCTIONHydrogels are cross-linked materials that absorb a substantial amount of water. They are of enormous economical importance due to their use as food additives, in the oil industry, and for biomedical applications. 1−3 In all applications, their mechanical behavior is of paramount importance, and it is determined to a large extent by cross-link density. In physical hydrogels, the cross-links are reversible, and control over mechanical behavior can be obtained by tuning chemical structure to create well-defined and specific cross-linking interactions. Cross-links are formed by specific parts of the components through aggregation by noncovalent interactions, such as ion complexation and hydrophobic interactions. 4 Specificity and additional strength of aggregation may be obtained by additional physical interactions such as hydrogen bonding. The combination of multiple noncovalent interactions gives highly desirable mechanical properties to natural hydrogelators such as collagen or actin. 5,6 The recent realization that mechanical properties and forces play an important role in the behavior of cells has opened up new markets for materials with tunable mechanical properties, with considerable potential for use in, for instance, tissue engineering. Despite their attractive mechanical properties, the use of natural hydrogelators in biomedical applications is limited by biocompatibility issues. With the aim of gaining full control over properties of biocompatible hydrogels, several synthetic approaches to physical hydrogels have been reported. Amino acids are popular building blocks in synthetic hydrogelators, both in engineered polypeptides 7 and synthetic peptide amphiphiles 8 because the chemical diversity of amino acids allows tuning of hydrophobicity and creates the possibility to engineer specific recognition motifs. Alternatively, hydrogelators have been developed with biocompatible poly(ethylene oxide) (PEO) as hydrophilic component and pep...