Bulk properties of dendronized polymers with tailored end‐groups emanating from the same backbone

Dendronized polymers with a methacrylate backbone bearing pendant aliphatic polyester dendrons based on 2,2‐bis(methylol)propionic acid have been investigated by rheological measurements, differential scanning calorimetry (DSC), size exclusion chromatography (SEC), and ¹H NMR self‐diffusion techniques. The change in material properties due to the attachment of larger dendrons and/or different end‐groups to a backbone of the same length is investigated. Dendronized polymers of the second to fourth generation with hydroxyl, acetonide, or hexadecyl end‐group functionalities have been studied. DSC revealed that the glass transition temperature of the amorphous polymers increases with increasing size of the dendrons, and that the ability for the hexadecyl functional polymers to crystallize decreases with increasing size of dendrons. ¹H NMR self‐diffusion and longitudinal relaxation data are consistent with an elongated rod‐like model of the polymers in solution. Larger dendrons lead to a larger rod diameter that approximately double when increasing the generation of dendronized polymer from two to four. Rheological measurements demonstrated that the complex viscosity at low frequency increased with dendron size. Independently of the functionality, the second and third generation samples initially showed a Newtonian plateau, followed by a shear thinning region at higher frequencies. The fourth generation samples only showed shear thinning over the whole frequency region. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4496–4504, 2005     

A New Class of Tunable Dendritic Diphosphine Ligands: Synthesis and Applications in the Ru‐Catalyzed Asymmetric Hydrogenation of Functionalized Ketones

A series of tunable G₀–G₃ dendritic 2,2′‐bis(diphenylphosphino)‐1,1′‐binaphthyl (BINAP) ligands was prepared by attaching polyaryl ether dendrons onto the four phenyl rings on the P atoms. Their ruthenium complexes were employed in the asymmetric hydrogenation of β‐ketoesters, α‐ketoesters, and α‐ketoamides to reveal the effects of dendron size on the catalytic properties. The second‐ and third‐generation catalysts exhibited excellent enantioselectivities, which are remarkably higher than those obtained from the small molecular catalysts and the first‐generation catalyst. Molecular modeling indicates that the incorporation of bulky dendritic wedges can influence the steric environments around the metal center. In addition, the ruthenium catalyst bearing a second‐generation dendritic ligand could be recycled and reused seven times without any obvious decrease in enantioselectivity.     

Covalent functionalization of solid cellulose by divergent synthesis of chemically active dendrons

Covalent surface modification of solid cellulose with well‐defined and chemically reactive dendrons is introduced as a platform for cellulose grafting with functional materials. Surface functionalization with a first generation dendron is achieved by esterification employing bifunctional molecules based on 2,2‐bis(hydroxymethyl) propionic acid (bis‐MPA) under mild conditions and short reaction times. The activated cellulose surface displays hydrophobic properties and contains two reactive alkene end‐groups per graft, which are used for covalent binding to active agents, as demonstrated by selective functionalization of the modified cellulose with fluorescent dye via photopatterning. The number of active end‐groups on the surface of cellulose is multiplied by divergent solid‐state synthesis of second and third generation dendrons having four and eight reactive sites per dendron, respectively. The dendrons are assembled in only few hours by a sequence of thiol‐ene/esterification reactions. The ability to accurately control the number of binding sites on the surface of cellulose allows fine tuning of the surface properties, as shown by the attachment of hydrophobic small molecules to the dendronized cellulose. The first, second and third generation dendrons allow preparing surfaces with increasing hydrophobicities; second and third generation dendrons functionalized with small perfluoroalkyl molecule display superhydrophobic properties. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2103–2114     

α‐Chymotrypsin‐Catalyzed Reaction Confined in Block‐Copolymer Vesicles

Herein the reactivity of the enzyme α‐chymotrypsin in the confinement of polystyrene‐block‐poly(acrylic acid) (PS‐b‐PAA) vesicles was investigated. Enzyme and substrate molecules were encapsulated in PS‐b‐PAA vesicles with internal diameters ranging from 26 nm to 165 nm during the formation of the vesicles. While the loading efficiencies of enzyme and substrate molecules were practically identical for vesicles of identical size, they were found to increase with decreasing vesicle size. The kinetics of the α‐chymotrypsin catalyzed hydrolysis of N‐succinyl‐Ala‐Ala‐Phe‐7‐amido‐4‐methylcoumarin (AMC) was evaluated following the increase of the absorption of the product 7‐amino‐4‐methylcoumarin by UV/Vis spectroscopy. The values of the catalytic turnover number obtained for reactions inside vesicles with different sizes showed an increase of up to fourteen times compared to the bulk value with decreasing vesicle volume, while the values of the Michaelis–Menten constant decreased, respectively. This increase in reactivity of α‐chymotrypsin is attributed to the effect of vesicle–wall interactions in the finite encapsulated space, where the reagents could diffuse, leading to enhanced collision frequencies.     

Functional Carbazole Liquid‐Crystal Block Codendrimers with Optical and Electronic Properties

The synthesis and characterisation of a family of block codendrimers consisting of highly versatile mesogenic and carbazole‐containing 2,2‐bis(hydroxymethyl)propionic acid (bis‐MPA) dendrons are reported. The liquid‐crystal behaviour was investigated by means of differential scanning calorimetry, polarised‐light optical microscopy and X‐ray diffraction. Depending on the chemical structure of the constituent dendrons, the codendrimers show lamellar or columnar mesophases. On the basis of the experimental results, models both at the molecular level and in the mesophase are proposed. The physical properties of the block codendrimers derived from the presence of the carbazole moiety in their structure were investigated: photoluminescence in solution and in the mesophase, electrochemical behaviour and hole transport. Electrodeposition of carbazole dendrons afforded a globular supramolecular conformation in which the mesogenic molecular side plays a key role.     

Dendronized polymers with tailored surface groups

A series of polymers tethered with bis‐MPA dendrons was synthesized by a combination of divergent growth and atom transfer radical polymerization (ATRP). Macromonomers of first and second generation were synthesized utilizing the acetonide protected anhydride of bis‐MPA as the generic esterfication agent. The macromonomers were polymerized in a controlled fashion by ATRP utilizing Cu(I)/Cu(II) and N‐propyl‐2‐pyridylmethanamine as the halogen/ligand system. The end‐groups of these polymers were further tailored to achieve hydroxyl, acetate, and aliphatic hexadecyl functionality. With this approach all polymers will emanate from the same backbone, enabling for an evaluation of both the generation and end‐group dependent properties. Furthermore, a dendronized tri‐block copolymer was synthesized. All materials were analyzed by ¹H and ¹³C NMR, as well as size‐exclusion chromatography (SEC). The SEC analysis revealed that the molecular weights of the divergently grown dendronized polymers increased with increasing generation while the polydispersity (PDI) was kept low. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3852–3867, 2005     

Divergent synthesis and self‐assembly of amphiphilic polymeric dendrons with selective degradable linkages

Enhancing the structural complexity and functionality of the building blocks allows the construction of supramolecular assemblies. In this work, we demonstrate a strategy for the design and synthesis of complex macromolecular architectures. We use atom transfer radical polymerization to produce well‐defined polymers with telechelic end‐group functionality, and “click” them together to form functional 3rd generation dendrons, and incorporated degradable linkages and certain functionality at the polymer chain‐ends of each generation. The 3rd generation polymeric dendrons consisted of homopolymer polystyrene (PSTY) with either four solketals or eight alcohols, diblock PSTY and poly(t‐butyl acrylate), and amphiphilic diblock. The peripheral ends consisting of alcohols create functionalization points for further chemical modification or chemical coupling and the cleavable linkages between the 2nd and 3rd generations all provide the first steps toward smart nanostructures. Importantly, we can synthesize these dendrons in pure form. The self‐assembly of the amphiphilic dendrons (the inner and outer generations consisting of PSTY and polyacrylic acid, respectively) in water produced micelles of uniform size with an aggregation number of 43 dendrons per micelle. The size of the micelles was small (D_H =20.7 nm) and comparable to the size found by transmission electron microscopy (14–18 nm). © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1533–1547, 2008     

Luminescence properties of soluble 2,2′,7,7′‐Tetrakis(2‐(9‐hexyl‐9H‐carbazol‐3‐yl)‐vinyl)‐9,9′‐spirobifluorene‐labeled dendrimers: Photoluminescence and electroluminescence

New light emitting dendrimers were synthesized by reacting 3,5‐bis‐(3,5‐bis‐benzyloxy‐benzyloxy)‐benzoic acid or 3,5‐bis‐[3,5‐bis‐(3,5‐bis‐benzyloxy‐benzyloxy)‐benzyloxy]‐benzoic acid with a carbazolyl vinyl spirobifluorene moiety. A blue‐emitting core dye was encapsulated by multibenzyloxy dendrons, and two dendrimers having different densities of dendrons were prepared. Photoluminescence (PL) studies of the dendrimers demonstrated that at the higher density of benzyloxy dendrons, the featureless vibronic transitions were improved, causing lesser excimer emission. The similarity of the solution and solid emission spectra of the larger dendrimer, 10 , revealed the suppression of molecular aggregation in the solid film, which is attributed to the presence of the bulky benzyloxy dendrons. The electroluminescence spectra of multilayered devices made using 10 predominantly exhibited blue emissions; similar emission was observed in the PL spectra of its thin film. The multilayered devices made using 3 , 9 , and 10 showed luminances of 1021 cd m^?2 at 5 V, 916 cd m^?2 at 6 V, and 851 cd m^?2 at 6.5 V, respectively. The largest dendrimer, 10 , bearing a greater number of benzyloxy dendrons, exhibited a blue‐like emission with CIE 1931 chromaticity coordinates of x = 0.16 and y = 0.13, which is due to the influence of a higher shielding effect. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 501–514, 2008     

Microwave‐Assisted Synthesis of Novel Bis(thiazoles) Incorporating Piperazine Moiety

Novel bis(hydrazinecarbothioamide) was prepared by the reaction of 1,4‐bis[(3‐formyl‐4‐hydroxyphenyl)methyl]piperazine with thiosemicarbazide. The bis(hydrazinecarbothioamide) was used as a building block for construction of a novel bis(thiazolyl)piperazines via its cyclocondensation with a series of each of α‐halocarbonyl compounds and hydrazonoyl halides in dimethylformamide in the presence of triethylamine as a catalyst under microwave irradiation. The structure of the newly bis(thiazolyl)piperazine derivatives is elucidated via the spectral data as well as elemental analyses.     

2,2′:3′,2′′‐Terthiophene‐Based all‐Thiophene Dendrons and Dendrimers: Synthesis,Structural Characterization,and Properties

The synthesis of generational dendritic oligothiophenes (DOTs) has been successfully achieved by a divergent/convergent approach that involves halogenation, boronation, and palladium‐catalyzed Suzuki coupling reactions. The key point in the presented synthetic approach is the use of trimethylsilyl (TMS) protecting groups, which allow for the core‐lithiation and subsequent boronation of the dendrons and for the peripheral ipso‐substitution with iodine monochloride or N‐bromosuccimide. In addition, the TMS protecting groups can be completely removed by using tetrabutylammonium fluoride, thus yielding only‐thiophene‐based dendrons and dendrimers. Due to their highly branched structure, all these synthesized DOTs are soluble in organic solvents. Chemical structures were confirmed by NMR spectroscopic, mass spectrometric, and elemental analysis. Concentration‐dependent ¹H NMR spectroscopic investigations revealed that the higher generation compounds tend to aggregate in solution. Such an aggregation behavior was further confirmed by measuring with MALDI‐TOF MS. Both MALDI‐TOF MS and gel‐permeation chromatography (GPC) analyses confirmed the monodispersity of the DOTs. Furthermore, GPC results revealed that these DOT molecules adopt a condensed globular molecular shape. Their optical and electronic properties were also investigated. The results indicated that these DOTs comprise various conjugated α‐oligothiophenes with different chain lengths, which results in the higher generation compounds showing broad and featureless UV/Vis absorption spectra and ill‐defined redox waves.     

两亲嵌段树枝状分子Gx-PBE-b-Gx-PMPA(OH)x2的合成及多重氢键作用

合成了1~3代的嵌段树枝状分子聚苄醚-聚脂肪酯(Gx-PBE-b-Gx-PMPA, x=1,2,3)和两亲嵌段树枝状分子聚苄醚-周边含羟基的聚脂肪酯[Gx-PBE-b-Gx-PMPA(OH)x2, x=1,2,3]. PMPA(OH)x2-树枝片(Dendron)段周边的羟基数目分别是2, 4和8. 通过1H NMR, 13C NMR, FTIR和基质辅助激光解吸附电离飞行时间质谱(MALDI-TOF)(或场解析电离质谱)技术表征了Gx-PBE-b-Gx-PMPA和Gx-PBE-b-Gx-PMPA(OH)x2的结构. 同时, 采用变温FTIR光谱研究了在两亲嵌段树枝状分子中形成的氢键模式. 结果表明, 随着树枝片代数的增加, 两亲嵌段树枝状分子内趋向于形成作用较弱的分子内氢键, 说明形成3代两亲嵌段树枝状分子的三维结构削弱了羟基形成分子间氢键的能力.     

Influence of Amine‐Based Cationic Gemini Surfactants on Catalytic Activity of α‐Chymotrypsin

The α‐chymotrypsin activity was tested in aqueous media with the presence of novel cationic amine–based gemini surfactant, with different spacer chain lengths and head group size, and also compared with the cationic cetyltrimethylammonium bromide (CTAB) and cetyltriphenylphosphonium bromide (CTPB) surfactants and aqueous buffer only. The p‐nitrophenyl acetate (PNPA) hydrolysis rate was monitored in the presence of the surfactant concentration at 30°C. Most of these gemini surfactants gave higher catalytic activity as compared to cationic CTAB and CTPB. The highest superactivity was measured in the presence of gemini 16‐12‐16, [dodecanediyl‐1,12‐bis(cetyldimethylammonium bromide)] surfactant at pH 7.5. The catalytic reaction follows the Michaelis–Menten mechanism. The catalytic rate constants, k_cat, show the same profile that the catalytic affinity; K_M being enhanced with increasing space chain length. The results are favorable for considering that the amine‐based gemini surfactant influences more than both the aqueous and cationic micellar media.     

Multifunctional Supramolecular Dendrimers with an s‐Triazine Ring as the Central Core: Liquid Crystalline,Fluorescence and Photoconductive Properties

Novel liquid crystal (LC) dendrimers have been synthesised by hydrogen bonding between an s‐triazine as the central core and three peripheral dendrons derived from bis(hydroxymethyl)propionic acid. Symmetric acid dendrons bearing achiral promesogenic units have been synthesised to obtain 3:1 complexes with triazine that exhibit LC properties. Asymmetric dendrons that combine the achiral promesogenic unit and an active moiety derived from coumarin or pyrene structures have been synthesised in order to obtain dendrimers with photophysical and electrochemical properties. The formation of the complexes was confirmed by IR and NMR spectroscopy data. The liquid crystalline properties were investigated by differential scanning calorimetry, polarising optical microscopy and X‐ray diffractometry. All complexes displayed mesogenic properties, which were smectic in the case of symmetric dendrons and their complexes and nematic in the case of asymmetric dendrons and their dendrimers. A supramolecular model for the lamellar mesophase, based mainly on X‐ray diffraction studies, is proposed. The electrochemical behaviour of dendritic complexes was investigated by cyclic voltammetry. The UV/Vis absorption and emission properties of the compounds and the photoconductive properties of the dendrons and dendrimers were also investigated     

Cationic Reverse Micelles Create Water with Super Hydrogen‐Bond‐Donor Capacity for Enzymatic Catalysis: Hydrolysis of 2‐Naphthyl Acetate by α‐Chymotrypsin

Reverse micelles (RMs) are very good nanoreactors because they can create a unique microenvironment for carrying out a variety of chemical and biochemical reactions. The aim of the present work is to determine the influence of different RM interfaces on the hydrolysis of 2‐naphthyl acetate (2‐NA) by α‐chymotrypsin (α‐CT). The reaction was studied in water/benzyl‐n‐hexadecyldimethylammonium chloride (BHDC)/benzene RMs and, its efficiency compared with that observed in pure water and in sodium 1,4‐bis‐2‐ethylhexylsulfosuccinate (AOT) RMs. Thus, the hydrolysis rates of 2‐NA catalyzed by α‐CT were determined by spectroscopic measurements. In addition, the method used allows the joint evaluation of the substrate partition constant K_p between the organic and the micellar pseudophase and the kinetic parameters: catalytic rate constant k_cat, and the Michaelis constant K_M of the enzymatic reaction. The effect of the surfactant concentration on the kinetics parameters was determined at constant W₀=[H₂O]/[surfactant], and the variation of W₀ with surfactant constant concentration was investigated. The results show that the classical Michaelis–Menten mechanism is valid for α‐CT in all of the RMs systems studied and that the reaction takes place at both RM interfaces. Moreover, the catalytic efficiency values k_cat/K_M obtained in the RMs systems are higher than that reported in water. Furthermore, there is a remarkable increase in α‐CT efficiency in the cationic RMs in comparison with the anionic system, presumably due to the unique water properties found in these confined media. The results show that in cationic RMs the hydrogen‐bond donor capacity of water is enhanced due to its interaction with the cationic interface. Hence, entrapped water can be converted into “super‐water” for the enzymatic reaction studied in this work.     

Bis‐Lactam Peptide [i,i + 4]‐Stapling

Even though the bis‐lactam peptide stapling with the [i, i + 7] and the [i, i + 11] systems has been known to be able to afford % α‐helicity values up to 100% (25°C), the performance of the bis‐lactam peptide stapling with the [i, i + 4] system in current literature has been mediocre (% α‐helicity ≦40%, 25°C). In the current study, we found that high % α‐helicity is also obtainable with the bis‐lactam [i, i + 4]‐stapling by demonstrating with our model peptide sequence that the bis‐lactam [i, i + 4]‐stapling with N^ε‐para‐phenylenediacetyl‐lysine was able to afford a % α‐helicity value of ~64.1% (25°C). Therefore, the bis‐lactam [i, i + 4]‐stapling could also be an efficacious peptide stapling mode that can be employed for biomedical applications.     

Comparison of facially amphiphilic biaryl dendrimers with classical amphiphilic ones using protein surface recognition as the tool

Facially amphiphilic biaryl dendrimers are compared with the more classical benzyl ether amphiphilic dendrimers for molecular recognition, using protein binding as the probe. The protein used for the proposed study is chymotrypsin (ChT). A generation-dependent binding affinity was observed with the benzyl ether dendrimers, while the affinities were independent of generation in the case of the biaryl dendrimers. Similarly, although the ligands incorporated in both dendrons are the same, the biaryl dendrimers are able to bind more proteins compared to the benzyl ether dendrimers. For example, G3-dendron of biaryl dendrimer can bind six molecules of chymotrypsin, whereas G3-analogue of benzyl ether dendrimers can bind only three molecules of chymotrypsin. This result is consistent with our hypothesis that the internal layers of the facially amphiphilic biaryl dendrons are solvent-exposed and accessible for recognition. In addition, the systematic size differences in dendrons were also used to gain insights into the substrate selectivity that the enzyme gains upon binding to a ligand scaffold.     

Bispalladacycle‐Catalyzed Michael Addition of In Situ Formed Azlactones to Enones

The development and further evolution of the first catalytic asymmetric conjugate additions of azlactones as activated amino acid derivatives to enones is described. Whereas the first‐generation approach started from isolated azlactones, in the second‐generation approach the azlactones could be generated in situ starting from racemic N‐benzoylated amino acids. The third evolution stage could make use of racemic unprotected α‐amino acids to directly form highly enantioenriched and diastereomerically pure masked quaternary amino acid products bearing an additional tertiary stereocenter. The step‐economic transformations were accomplished by cooperative activation by using a robust planar chiral bis‐Pd catalyst, a Brønsted acid (HOAc or BzOH; Ac=acetyl, Bz=benzoyl), and a Brønsted base (NaOAc). In particular the second‐ and third‐generation approaches provide a rapid and divergent access to biologically interesting unnatural quaternary amino acid derivatives from inexpensive bulk chemicals. In that way highly enantioenriched acyclic α‐amino acids, α‐alkyl proline, and α‐alkyl pyroglutamic acid derivatives could be prepared in diastereomerically pure form. In addition, a unique way is presented to prepare diastereomerically pure bicyclic dipeptides in just two steps from unprotected tertiary α‐amino acids.     

Effect of pH and generation of dendron on single‐step synthesis of gold nanoparticles using PEGylated polyamidoamine dendron in aqueous medium

Three types of PEGylated polyamidoamine (PAMAM) dendrons were synthesized through PEGylation of primary amines at the periphery of second, third, and fourth generation dendrons. Au(III) precursors and the synthesized PEGylated PAMAM dendrons were mixed at various pHs to evaluate the effect of pH on gold nanoparticle (Au NP) synthesis by monitoring the change in surface plasmon resonance. The Au NP synthesis reaction was controlled by pH through the balance between protonated and deprotonated tertiary amines and the reactivity of Au(III) precursors. By using PEGylated PAMAM dendrons with higher generation, the obtained Au NPs had narrow size distribution with small average size because of the limitation of intermolecular space among PEGylated PAMAM dendrons for the growth to Au NP. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1391–1398, 2010     

Synthesis,Structural Characterization,and Thermoresponsivity of Hybrid Supramolecular Dendrimers Bearing a Polyoxometalate Core

A series of cationic dendrons bearing triethylene glycol monomethyl ether terminal groups of different generations have been synthesized and used to encapsulate an inorganic polyanionic cluster [K_12.5Na_1.5(NaP₅W₃₀O₁₁₀)] through electrostatic interactions. The resulting dendritic cation–encapsulated polyoxometalate (POM) complexes, cluster–dendrimers, are soluble in water and exhibit lower critical solution temperatures (LCST). The thermoresponsivities of these complexes in aqueous solutions were studied by turbidimetry and variable‐temperature ¹H NMR spectroscopy. The observed cloud points show a remarkable dependence on the generation of the dendrons. Complexes composed of first‐generation dendrons exhibit no obvious thermoresponsive properties, but for complexes bearing second‐generation dendrons, the LCST decreases as the number of dendritic cations around the POM cluster increases. Complexes composed of third‐generation cations underwent reversible aggregation and disaggregation upon heating and cooling, respectively. This thermally induced self‐aggregation was characterized by DLS and TEM. In addition, the effects of salt and solvent on the LCST were investigated. This research demonstrates a new type of thermoresponsive dendritic organic–inorganic hybrid complex and provides a general route to the endowment of POMs with temperature‐sensitive properties through electrostatic interactions.     

Supramolecular micellization and pH‐inducible gelation of a hydrophilic block copolymer by block‐specific threading of α‐cyclodextrin

A poly(ethylene glycol)‐b‐poly(L ‐lysine) diblock copolymer (PEG‐b‐PLL) was synthesized. Micellization of this hydrophilic copolymer due to the block‐specific threading of α‐cyclodextrin (α‐CD) molecules onto the polyethylene glycol (PEG) block yielded supramolecular‐structured nanoparticles, which undergoes pH‐inducible gelation in aqueous media. The pH‐inducible gelation of supramolecular micelle in water appeared to be completely reversible upon pH changes. The synergetic effect of selective complexation between PEG block and α‐CD and the pH‐inducible hydrophobic interaction between PLL blocks at pH 10 was believed to be the driving force for the formation of the supramolecular hydrogel. ¹H NMR and wide angle X‐ray diffraction (WAXD) were employed to confirm the inclusion complexation between α‐CD and PEG block. Meanwhile, the morphology of the micellized nanoparticles was investigated by transmission electron microscopy (TEM). The thermal stability of inclusion complexes (ICs) was investigated and the rheologic experiment was conducted to reveal the micelle‐gel transition. Such pH‐induced reversible micelle‐gel transition of the supramolecular aggregates may find applications in several fields, for example as advanced biomedical material possessing stimulus‐responsiveness. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 782–790, 2008