PDEGMA-b-PDIPAEMA copolymers via RAFT polymerization and their pH and thermoresponsive schizophrenic self-assembly in aqueous media

We report on a new doubly responsive polymeric system of amphiphilic diblock copolymers, namely poly(di-[ethylene glycol] methyl ether methacrylate)-b-poly(2-[diisopropylamino] ethyl methacrylate), PDEGMA-b-PDIPAEMA, obtained by the reversible addition-fragmentation chain transfer (RAFT) polymerization technique. Molecular characterization by size exclusion chromatography (SEC), nuclearmagnetic resonance (¹H-NMR) and infrared spectroscopy (FT-IR) confirms the successful synthesis of these novel block copolymers. The PDEGMA-b-PDIPAEMA block copolymers formed aggregates in aqueous media in response to solution pH and temperature changes, as evidenced by dynamic and static light scattering techniques, as well as fluorescence spectroscopy. Aggregates with PDEGMA core and PDIPAEMA corona domains are formed at elevated temperatures and low pH, whereas aggregates with PDIPAEMA cores and PDEGMA coronas are formed at neutral and high pH. Overall structural characteristics and solution behavior of the copolymers are affected by the copolymer composition. The obtained results provide valuable new information on the behavior and design guidelines for the construction of stimuli responsive, “schizophrenic” polymeric nanostructures with potential application in the biomedical field.     

Transient self-assembly of metal–organic complexes

Implementing transient processes in networks of dynamic molecules holds great promise for developing new functional behaviours. Here we report that trichloroacetic acid can be used to temporarily rearrange networks of dynamic imine-based metal complexes towards new equilibrium states, forcing them to express complexes otherwise unfavourable in their initial equilibrium states. Basic design principles were determined for the creation of such networks. Where a complex distribution of products was obtained in the initial equilibrium state of the system, the transient rearrangement temporarily yielded a simplified output, forcing a more structured distribution of products. Where a single complex was obtained in the initial equilibrium state of the system, the transient rearrangement temporarily modified the properties of this complex. By doing so, the mechanical properties of an helical macrocyclic complex could be temporarily altered by rearranging it into a [2]catenane.

Trichloroacetic acid was used to temporarily rearrange networks of imine-based metal complexes, enabling the expression of complexes unfavourable in the initial state of the networks. Basic design principles were determined for these networks.     

Isostructural self-assembly of 2'-deoxyguanosine derivatives in aqueous and organic media

We report the self-assembly of a hydrophilic 8-(m-acetylphenyl)-2'-deoxyguanosine (mAG) derivative into a discrete and thermally stable hexadecameric supramolecule in aqueous media. We demonstrate that this hexadecamer is isostructural to the one formed by a related lipophilic derivative in organic media. This mAG moiety represents a rare example of a small-molecule recognition motif that is capable of assembling isostructurally and with high fidelity in both organic and aqueous media.     

Controlled fabrication of organic nanotubes via self-assembly of non-symmetric bis-acylurea

We present novel non-symmetric bis-acylurea organogelators that self-assemble into hollow tubular nanostructures upon cooling in solutions. The bis-acylureas have aliphatic end groups of different lengths divided by a spacer group [?NHCONHCO?(CH₂)₅?CONHCONH?, C5] with two hydrogen bonding sites. Due to the intermolecular biaxial hydrogen bonding, the molecules crystallize into 2D thin layers at first, and then their wrapping ultimately results in nanotubes. On the contrary, symmetric bis-acylureas form multilayered nanosheets which are stabilized by the van der Waals interaction between the stacked layers. The size and shape of the nanotubes can be controlled by varying the difference of the alkyl chain lengths. When the difference is big, for example, eight methylene units (BuC5DD, butyl (Bu) and dodecyl (DD)), uniform nanotubes of 65-nm mean outer diameter are obtained, while a non-symmetric bis-acylurea with one methylene unit difference (UDC5DD, undecyl (UD)) forms a mixture of nanosheets and nanotubes. Template-assisted formation of nanotubes was successfully performed via gelation in inorganic nanopores. We also synthesized a thiol-functionalized non-symmetric bis-acylurea, HS-UDC5Bu (thiol (HS)), which was used as a tubular template for gold nanoparticles.     

Synthesis and structures of fulvene-bridged diruthenium complexes

Three diruthenium carbonyl complexes, namely (η ³:η ⁵-C₅H₄C(CH₂)₂)Ru₂(CO)₅ (1), (η ³:η ⁵-C₅H₄C(CHCH₂)(C₂H₅))Ru₂(CO)₅ (2), and (η ¹:η ⁵-C₅H₄C₅H₈)Ru₂(CO)₆ (3), were obtained from the reactions of C₅H₄C(Me)₂, C₅H₄C(Et)₂, and C₅H₄C(CH₂)₄, respectively, with Ru₃(CO)₁₂ in refluxing xylene. The complexes were characterized by elemental analysis, IR and ¹H NMR spectra. Single-crystal X-ray diffraction analysis for complexes 1 and 2 revealed that the fulvene ligands bridge two ruthenium atoms in η ³:η ⁵ fashion.     

Anionic polymerization of caprolactam in organic media. Morphological aspects

Crystalline entities formed during anionic polymerization of caprolactam (CL) in nonpolar solvents were examined, mainly by scanning electron microscopy and wide angle X-ray scattering. The morphological development of these entities is governed by the complex interaction between the competing polymerization and physical processes like phase separation and crystallization. The effect of the efficiency of catalytic systems on their interaction, the mechanisms of the phase separation and the crystal growth under topological restrictions set by the phase separation are discussed. Under reaction conditions, for these ones favouring high polymer yields, the final morphology of the polycaproamide (PCA) particles can be controlled by the efficiency of both catalytic species (activator and catalyst). Despite the large range of particle sizes two typical morphologies, namely – connected globules evolving to large blocks and macroporous powders are obtained using more or less efficient catalytic systems, respectively. An adequate selection of the catalytic pairs allows to control the particles’ size and their internal morphology, which is important for certain specific applications of PCA. Received: 1 August 1997 Accepted: 5 February 1998     

Aurophilic complexes as gold atom sources in organic media

The decomposition, either thermal or under H2, of [O(AuIPR3)3](CF3SO3) (R = Ph 1, R = Me 2) in organic solvents has been studied by 31P(1H) NMR, UV-vis spectroscopy and TEM; during the reaction, the phosphine acts as an efficient oxygen trap and gold nanoparticles are produced which may be stabilized by PVP in acetonitrile (mean diameter 4.5 nm) or oleylamine in toluene (mean diameter 9 nm).     

Enolic Schiff-base aluminum complexes and their application in lactide polymerization

A series of NNOO-tetradentate enolic Schiff-base ligands were prepared where ligand L₁ = bis(benzoylacetone)propane-1,2-diimine, L₂ = bis(acetylacetone)-propane-1,2-diimine, L₃ = bis-(acetylacetone)cyclohexane-1,2-diimine. Their further reaction with aluminum tris(ethyl) formed complexes LAlEt (1a, 2a and 3a). The solid structure of complexes 1a, 2a and 3a confirmed by X-ray single crystal analysis manifested that these complexes were all monomeric and five-coordinated with an aluminum atom in the center. The configurations of these complexes varied from trigonal bipyramidal geometry (tbp) to square pyramidal geometry (sqp) due to their different auxiliary ligand architectures. ¹H NMR spectra indicated that all these complexes retained their configuration in solution states. Their catalytic properties to polymerize racemic-lactide (rac-LA) in the presence of 2-propanol were also studied. The diimine bridging parts as well as the diketone segment substituents had very close relationship with their performance upon the polymerization process. All these complexes gave moderately isotactic polylactides with controlled molecular weight and very narrow molecular weight distributions.     

Controlled self-assembly of hexa-peri-hexabenzocoronenes in solution

Disc-shaped hexa-peri-hexabenzocoronenes (HBCs) peripherally substituted by flexible dodecyl chains (molecule 1) or rigid polyphenylene dendrons (molecules 2a,b and 3) were efficiently synthesized. Steric hindrance arising from the substituents, from less hindered dodecyl to bulky dendrons, was utilized to program the self-assembly of the HBC cores in solution. The high tendency of the hexadodecyl-substituted HBC 1 to aggregate was determined by concentration and temperature-dependent (1)H NMR spectroscopic measurements and nonlinear least-squares analysis of the experimental data. The rigid dendrons in molecule2a suppress the pi--pi interactions of the HBC cores to a certain extent, and a slow (with respect to the NMR time scale) monomer--dimer equilibrium is observed. This unique equilibrium was further controlled by temperature, concentration, and solvent to afford discrete monomeric or dimeric species. Further structural modifications such as the replacement of dodecyl groups in 2a with hydrogen atoms resulted in a stable dimer structure in 2b due to diminished steric hindrance, as supported by quantum chemical calculations. "Moving" the dendron arms closer to the HBC core gives molecule 3, which exists only as a nonaggregated monomer. UV-vis absorption and fluorescence spectra of these discrete species revealed obvious differences in their electronic and optoelectronic properties which can be explained by the existence or absence of pi--pi interactions.     

Polyelectrolyte-induced self-assembly of positively charged alkynylplatinum(II)-terpyridyl complexes in aqueous media

Polyelectrolytes carrying multiple negative charges were found to induce the aggregation and self-assembly of the positively charged platinum(II)-terpyridyl complexes in aqueous media. The aggregation and self-assembly of the complexes were driven by electrostatic interactions between the polymer and the complex, and by terpyridine ligand pi-pi stacking and platinum-platinum (metal-metal) interactions. As a result, remarkable UV/Vis and emission spectral changes were observed. The spectroscopic property changes were related to the structural properties of the metal complexes as well as the polyelectrolytes. The induced self-assembly of the platinum complexes was also strongly affected by the solution properties of the aqueous media, for example, the solution pH, ionic strength, and the percentage of organic solvent added.     

Chiral-at-metal complexes and their catalytic applications in organic synthesis

This tutorial review provides an introduction to the synthesis and characterization of chiral-at-metal complexes and their catalytic application in organic transformations. The synthetic access to these architectures either via chiral resolution or by employment of chiral ligands is described, characterization techniques for the complexes are referenced and the application of the R/S nomenclature is explained. Racemization and epimerization processes are often observed for the title compounds; the article gives mechanistic insights to these processes and describes how to recognize and document them. Finally, key catalytic applications in organic synthesis are presented and how the molecular architectures of the chiral-at-metal complexes lead to stereodifferentiation and, thus to enantiomeric excesses in the products.     

Semiconductor nanoparticles for photoinitiation of free radical polymerization in aqueous and organic media

Photoinduced free radical polymerization of vinyl monomers by using semiconductor inorganic nanoparticles (NPs) is investigated. Zinc oxide and iron‐doped zinc oxide were used as photosensitive compounds to initiate the polymerization of acrylamide as a water‐soluble monomer in aqueous environment and methyl methacrylate as an oil‐soluble monomer in organic media under UV‐light irradiation. The method uses photochemically generated electrons and holes from the NPs to form initiating hydroxyl radicals in aqueous media, while tertiary amines and iodonium salt served as coinitiator in organic media. The initiation mechanism in organic media involves hydrogen abstraction or reduction processes via charge carriers, respectively. The kinetic of the polymerization in both environments was studied by means of a photo‐differential scanning calorimetry. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1500–1507     

Heat-set gel-like networks of lipophilic Co(II) triazole complexes in organic media and their thermochromic structural transitions

A novel class of thermally responsive supramolecular assemblies is formed from the lipophilic cobalt(II) complexes of 4-alkylated 1,2,4-triazoles. When an ether linkage is introduced in the alkylchain moiety, a blue gel-like phase is formed in chloroform, even at very low concentration (ca. 0.01 wt %, at room temperature). The blue color is accompanied by a structured absorption around 580-730 nm, which is characteristic of cobalt (II) in the tetrahedral (T(d)) coordination. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) of the gel-like phase confirms the formation of networks of fibrous nanoassemblies with widths of 5-30 nm. The observed widths are larger than a molecular length of the triazole ligand (ca. 2.2 nm) and they are consisted of aggregates of T(d) coordination polymers. Very interestingly, the blue gel-like phase turned into a solution by cooling below 25 degrees C. A pale pink solution is obtained at 0 degrees C, indicating the formation of octahedral (O(h)) complexes. The observed thermochromic transition is totally reversible. The formation of gel-like networks by heating is contrary to the conventional organogels, which dissolve upon heating. Temperature dependence of the storage and loss moduli (G' and G") shows minima around at 27 degrees C, at which temperature they gave comparable values. On the other hand, G' exceeds G" both in the gel-like phase (temperature above 27 degrees C) and in the solution phase (temperature below 25 degrees C). These observations indicate that T(d) complexes are present as low-molecular weight species around at 25-27 degrees C. They are self-assembled to polymeric T(d) complexes by heating and form gel-like networks. Upon cooling the solution below 25 degrees C, T(d) complexes are converted to O(h) complexes and they also self-assemble into oligomeric or polymeric species at lower temperatures. The observed unique thermochromic transition (pink solution --> blue gel-like phase) is accompanied by an exothermic peak in differential scanning calorimetry (DSC), and is shown to be an enthalpy-driven process. The lipophilic modification of one-dimensional coordination systems provides unique solution properties and it would be widely applicable to the design of thermoresponsive, self-assembling molecular wires.     

Controlled/living polymerization of methacrylamide in aqueous media via the RAFT process

The polymerization of methacrylamide (MAM) was performed in aqueous media via reversible addition fragmentation chain transfer (RAFT) polymerization with the dithiobenzoate chain‐transfer agent (CTA) 4‐cyanopentanoic acid dithiobenzoate (CTP) and 4,4′‐azobis(4‐cyanopentanoic acid) (V‐501) as initiator. The polymerization in unbuffered water at 70 °C with a CTP/V‐501 ratio of 1.5 was controlled for the first 3 h, after which the molecular weight distribution broadened and a substantial deviation of the experimental from the theoretical molecular weight occurred, presumably because of a loss of CTA functionality at longer polymerization times. Conducting the polymerization in an acidic buffer afforded a well‐defined homopolymer (M_n = 23,800 g/mol, M_w/M_n = 1.08). To demonstrate the controlled/living nature of the system, a block copolymer of MAM and acrylamide was successfully prepared (M_n = 33,800 g/mol, M_w/M_n = 1.25) from a polymethacrylamide macro‐CTA. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3141–3152, 2005     

Self-assembling molecular wires of halogen-bridged platinum complexes in organic media. Mesoscopic supramolecular assemblies consisting of a mixed valent Pt(II)/Pt(IV) complex and anionic amphiphiles

A novel class of supramolecular assemblies in organic media consisting of a molecular wire of a halogen-bridged platinum complex [Pt(en)2][PtCl2(en)2]4+ (en = 1,2-diaminoethane) and anionic amphiphiles is developed. When double-chained phosphates or sulfonates are employed, the resultant [Pt(en)2][PtCl2(en)2](4+)-lipid complexes displayed intervalence charge transfer (CT) absorption bands in the crystalline state. They are soluble in organic solvents because of the amphiphilic superstructure, in which the solvophobic one-dimensional platinum complex is surrounded by solvophilic alkyl chains. CT absorption bands of halogen-bridged linear complexes are maintained in organic media, with varied colors that depend on the chemical structure of constituent amphiphiles. Monoalkylated phosphates failed to form colored, halogen-bridged ternary complexes probably because of their coordination to the axial position of PtII(en)2. Formation of mesoscopic supramolecular assemblies in organic media was confirmed for the [Pt(en)2][PtCl2(en)2] complexes by electron microscopy. Interestingly, a supramolecular complex consisting of dihexadecyl sulfosuccinate and [Pt(en)2][PtCl2(en)2]4+ displayed clear, indigo solutions that are distinct from the yellow color observed for those of [Pt(en)2][PtCl2(en)2]/dialkyl phosphate complexes. The indigo color of the former complex disappeared upon heating the solution to 60 degrees C, whereas it reappeared reversibly by cooling the solution to room temperature. In electron microscopy, rodlike nanostructures with a minimum width of 18 nm and lengths of 700-1700 nm were observed after cooling, though not at elevated temperatures. Apparently, the lipid-[Pt(en)2][PtCl2(en)2]4+ complex undergoes reversible dissociation and reassembly processes in chloroform, and it becomes better dispersed after the reassembling process. The present finding opens a general route to solution chemistry of low-dimensional inorganic complexes and enables rational design and control of self-assembling inorganic molecular wires.     

Bimetallic nickel complexes of macrocyclic tetraiminodiphenols and their ethylene polymerization

Schiff’s base condensation of 2,6-diformyl-4-R-phenol and affords 34-membered macrocyclic tetraiminodiphenol compounds, (R = H and R′ = iPr, 1; R = Me and R′ = iPr, 2; R = F and R′ = iPr, 3; R = Me and R′ = Et, 4; R = F and R′ = Et, 5) in good yields (47-62%), from which dinuclear nickel complexes, (R = H and R′ =  iPr, 6; R = Me and R′ = iPr, 7; R = F and R′ = iPr, 8) are prepared. Molecular structures of 2, dipotassium salt of 1, and 7 were confirmed by X-ray crystallography. Addition of B(C₆F₅)₃ to a toluene solution of 6-8 gives insoluble precipitates which show good activity for ethylene polymerization.     

Controlled synthesis of organic two-dimensional nanostructures via reaction-driven,cooperative supramolecular polymerization

The bottom-up approach of supramolecular polymerization is an effective synthetic method for functional organic nanostructures. However, the uncontrolled growth and polydisperse structural outcome often lead to low functional efficiency. Thus, precise control over the structural characteristics of supramolecular polymers is the current scientific hurdle. Research so far has tended to focus on systems with inherent kinetic control by the presence of metastable state monomers either through conformational molecular design or by exploring pathway complexity. The need of the hour is to create generic strategies for dormant states of monomers that can be extended to different molecules and various structural organizations and dimensions. Here we venture to demonstrate chemical reaction-driven cooperative supramolecular polymerization as an alternative strategy for the controlled synthesis of organic two-dimensional nanostructures. In our approach, the dynamic imine bond is exploited to convert a non-assembling dormant monomer to an activated amphiphilic structure in a kinetically controlled manner. The chemical reaction governed retarded nucleation–elongation growth provides control over dispersity and size.

We report the kinetically controlled supramolecular polymerization of organic two-dimensional charge-transfer nanostructures via a chemical reaction (imine)-driven approach.