Applications of α, ω‐telechelic polydimethylsiloxane as cross‐linkers for preparing high‐temperature vulcanized silicone rubber

Multiamino‐terminated telechelic polydimethylsiloxane (MATTPS) was synthesized via aza‐Micheal reaction and amidation reaction and subsequently employed as cross‐linkers of polysiloxane containing γ‐chloropropyl groups (CPPS) for preparing a series of high‐temperature vulcanized silicone rubber (MCSR/MATTPS). The curing, mechanical, and thermal properties of MCSR/MATTPS were studied through rheometry, mechanical testing, and thermogravimetric analysis (TGA). MCSR/MATTPS exhibits optimal mechanical properties with a tensile strength of 9.52 MPa and tear strength of 45.4 kN/m when the molar ratio of [N–H]/[CH₂CH₂CH₂Cl] is of 1, which is attributed to the formation of concentrative cross‐linking in the three‐dimensional polymer networks. The thermal behaviors of MCSR/MATTPS display a two‐step weight loss process by heating in nitrogen whereas more than two weight loss process in air. TGA results indicate that the introduction of aliphatic long chain and carbonyl groups in the structure of MATTPS has little negative effect on the thermal stability of MCSR/MATTPS.     

Preparation of poly(alkylenebenzimidazole) by ruthenium complex catalyzed polycondensation of 3,3′‐diaminobenzidine with α,ω‐diols

The reactions of 3,3′‐diaminobenzidine with 1,12‐dodecanediol in 1 : 1–1:3 molar ratios in the presence of RuCl₂(PPh₃)₃ catalyst give poly(alkylenebenzimidazole), [ (CH₂)₁₁ O (CH₂)₁₁ Im / (CH₂)₁₀ Im ]_n (Im: 5,5′‐dibenzimidazole‐2,2′‐diyl) (I_a‐I_d) in 71–92% yields. The relative ratio between the [(CH₂)₁₁ O (CH₂)₁₁ Im ] unit (A) and the [‐ (CH₂)₁₀ Im ] unit (B) in the polymer chain varies depending on the ratio of the substrates used. The polymer I_a obtained from the 1 : 3 reaction contains these structural units in a 98 : 2 ratio. The polymers are soluble in polar solvents such as DMF (N,N‐dimethylformamide), DMSO (dimethyl sulfoxide), and NMP (N‐methyl‐2‐pyrrolidone) and have molecular weights M_n (M_w) of 4,200–4,800 (4,800–6,500) by GPC (polystyrene standard). The polymerization of the diol and 3,3′‐diaminobenzidine in higher molar ratios leads to partial cross‐linking of the resulting polymers I_e and I_f via condensation of imidazole NH group with CH₂OH group. Similar reactions of 3,3′‐diaminobenzidine with α,ω‐diols, HO(CH₂)_mOH (m = 4–10), in a 1 : 3 molar ratio give the polymers containing [ (CH₂)_m−1 O (CH₂) _m−1 Im ] and [ (CH₂) _m−2 Im ] units with partial cross‐linked structures. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1383–1392, 1999     

Synthesis and Properties of 2,3‐Dihydro‐1H‐corannuleno[2,3‐cd]pyridine (=2,3‐Dihydro‐1H‐dibenzo[1,10 : 6,7]fluorantheno[3,4‐cd]pyridine) Derivatives: Heterocyclic peri‐Anellated Corannulenes

The anellation of a 6‐membered ring to the 2,3‐position of corannulene (=dibenzo[ghi,mno]fluoranthene; 1 ) leads to curved aromatic compounds with a significantly higher bowl‐inversion barrier than corannulene (see Fig. 1). If the bridge is −CH₂−NR−CH₂−, a variety of linkers can be introduced at the N(2) atom, and the corresponding curved aromatics act as versatile building blocks for larger structures (see Scheme). The locked bowl, in combination with an amide bond (see 9 and 10 ), gives rise to corannulene derivatives with chiral ground‐state conformations, which possess the ability to adapt to their chiral environment by shifting their enantiomer equilibrium slightly in favor of one enantiomeric conformer. Rim annulation of corannulene seems to display a significantly lower electron‐withdrawing effect than facial anellation on [5,6]fullerene‐C₆₀‐I_h, as determined by an investigation of the basicity at the N‐atom of CH₂−NR−CH₂ (see 4 vs. 15 in Fig. 2).     

New Synthesis of Pyrrolidines via Reaction of γ‐Halocarbanions with Imines

γ‐Chlorocarbanions of proper nucleophilicity, generated from 3‐chloropropyl pentachlorophenyl sulfone (=pentachloro[(3‐chloropropyl)sulfonyl]benzene; 1 ; Ar=C₆Cl₅), add to electron‐deficient formal imines 3a – l to produce anionic adducts that enter intramolecular substitution leading to substituted pyrrolidines. This new and simple synthesis of pyrrolidines mimics a 1,3‐dipolar cycloaddition, although it proceeds in two distinct steps.     

PAMAM Dendrimers as Potential Agents against Fibrillation of α‐Synuclein,a Parkinson's Disease‐Related Protein

The effect of PAMAM dendrimers (generations G3, G4 and G5) on the fibrillation of α‐synuclein was examined by fluorescence and CD spectroscopy, TEM and SANS. PAMAM dendrimers inhibited fibrillation of α‐synuclein and this effect increased both with generation number and PAMAM concentration. SANS showed structural changes in the formed aggregates of α‐synuclein – from cylindrical to dense three‐dimensional ones – as the PAMAM concentration increased, on account of the inhibitory effect. PAMAM also effectively promoted the breaking down of pre‐existing fibrils of α‐synuclein. In both processes – that is, inhibition and disassociation of fibrils – PAMAM redirected α‐synuclein to an amorphous aggregation pathway.

     

Polymerization of higher linear α‐olefins with (CH3)2Si(2‐methylbenz[e]indenyl)2ZrCl2

Poly‐α‐olefins ranging from poly‐1‐pentene to poly‐1‐octadecene with narrow polydispersities were synthesized with (CH₃)₂Si(2‐methylbenz[e]indenyl)₂ZrCl₂ and methylaluminoxane at polymerization temperatures (T_p 's) ranging from −15 to 180 °C and were characterized by gel permeation chromatography, NMR spectroscopy, and differential scanning calorimetry. The molar masses of the homopolymers obtained with (CH₃)₂Si(2‐methylbenz[e]indenyl)₂ZrCl₂ were notably higher than those of poly‐α‐olefins synthesized with other zirconium‐based metallocenes under similar conditions. The temperature dependence of the molar mass distribution of the poly‐α‐olefins can be described by a common exponential decay function regardless of the investigated monomer. At T_p 's ranging from 20 to 100 °C, moderate isotacticity prevailed, but outside this temperature range, the polymers were less stereoregular. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2333–2339, 2000     

A three‐dimensional PbII coordination framework: poly[[μ4‐(E)‐2,2′‐(diazene‐1,2‐diyl)dibenzoato]dimethanollead(II)]

In the title coordination polymer, [Pb(C₁₄H₈N₂O₄)(CH₃OH)₂]_n, the asymmetric unit contains half of a Pb^II cation, half of a 2,2′‐(diazene‐1,2‐diyl)dibenzoate dianionic ligand (denoted L²⁻) and one methanol ligand. Each Pb^II centre is eight‐coordinated by six O atoms of chelating/bridging carboxylate groups from four L²⁻ ligands and two O atoms from two terminal methanol ligands, forming a distorted dodecahedron. The [PbL₂(MeOH)₂] subunits are interlinked via the sharing of two carboxylate O atoms to form a one‐dimensional [PbL₂(MeOH)₂]_n chain. Adjacent chains are further connected by L²⁻ ligands, giving rise to a two‐dimensional layer, and these layers are bridged by L²⁻ linkers to afford a three‐dimensional framework with a 4¹²6³ topology.     

Synthesis and properties of Q‐silicon crosslinked siloxane networks: H3PO4‐catalyzed sol–gel dehydration/crosslinking of α,ω‐bis(hydroxy)oligodimethylsiloxanes with tetrakis(hydroxydimethylsiloxy)silane

Six silicate‐crosslinked oligodimethylsiloxane thin films were prepared by the phosphoric acid (1 mol %) catalyzed condensation of α,ω‐bis(hydroxy)oligodimethylsiloxane (P) and tetrakis(hydroxydimethylsiloxy)silane (Q). Other acid catalysts were evaluated. P and Q were prepared by the Pd‐catalyzed oxidation of the corresponding Si? H compounds with water. The starting materials were characterized by IR and ¹H, ¹³C, and ²⁹Si NMR. A thermal cure was achieved with H₃PO₄ in 24 h and with poly(phosphoric acid) in 3 h at 110–120 °C. Dynamic mechanical analysis was used to determine the glass‐transition temperatures and to evaluate the mechanical properties of the films. Their thermal stabilities (≥300 °C) in air and N₂ were determined by thermogravimetric analysis. Small amounts of non‐crosslinked P were recovered from the films by Soxhlet extractions with CH₂Cl₂ and analyzed by IR, gel permeation chromatography, and ²⁹Si NMR. The crosslink densities were evaluated by the CH₂Cl₂ absorption capacities of the films. The surface properties of the films were determined by static and dynamic contact‐angle measurements. Electrochemical impedance spectroscopy was carried out to evaluate the corrosion‐protective properties of the coatings on mild steel as a function of the exposure time to 0.5 N NaCl. The biofoul‐release properties of the films were evaluated with sporelings from mature Ulva linza plants and barnacles. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2237–2247, 2006     

Factors Affecting DNA–DNA Interstrand Cross‐Links in the Antiparallel 3′–3′ Sense: A Comparison with the 5′–5′ Directional Isomer

Reported herein is a study of the unusual 3′–3′ 1,4‐GG interstrand cross‐link (IXL) formation in duplex DNA by a series of polynuclear platinum anticancer complexes. To examine the effect of possible preassociation through charge and hydrogen‐bonding effects the closely related compounds [{trans‐PtCl(NH₃)₂}₂(μ‐trans‐Pt(NH₃)₂{NH₂(CH₂)₆NH₂}₂)]⁴⁺ (BBR3464, 1 ), [{trans‐PtCl(NH₃)₂}₂(μ‐NH₂(CH₂)₆NH₂)]²⁺ (BBR3005, 2 ), [{trans‐PtCl(NH₃)₂}₂(μ‐H₂N(CH₂)₃NH₂(CH₂)₄)]³⁺ (BBR3571, 3 ) and [{trans‐PtCl(NH₃)₂}₂{μ‐H₂N(CH₂)₃‐N(COCF₃)(CH₂)₄}]²⁺ (BBR3571‐COCF₃, 4 ) were studied. Two different molecular biology approaches were used to investigate the effect of DNA template upon IXL formation in synthetic 20‐base‐pair duplexes. In the “hybridisation directed” method the monofunctionally adducted top strands were hybridised with their complementary 5′‐end labelled strands; after 24 h the efficiency of interstrand cross‐linking in the 5′–5′ direction was slightly higher than in the 3′–3′ direction. The second method involved “postsynthetic modification” of the intact duplex; significantly less cross‐linking was observed, but again a slight preference for the 5′–5′ duplex was present. 2D [¹H, ¹⁵N] HSQC NMR spectroscopy studies of the reaction of [¹⁵N]‐ 1 with the sequence 5′‐d{TATACATGTATA}₂ allowed direct comparison of the stepwise formation of the 3′–3′ IXL with the previously studied 5′–5′ IXL on the analogous sequence 5′‐d(ATATGTACATAT)₂. Whereas the preassociation and aquation steps were similar, differences were evident at the monofunctional binding step. The reaction did not yield a single distinct 3′–3′ 1,4‐GG IXL, but numerous cross‐linked adducts formed. Similar results were found for the reaction with the dinuclear [¹⁵N]‐ 2 . Molecular dynamics simulations for the 3′–3′ IXLs formed by both 1 and 2 showed a highly distorted structure with evident fraying of the end base pairs and considerable widening of the minor groove.     

Synthesis and Cytotoxicity Evaluation of Certain α‐Methylidene‐ γ‐butyrolactones Bearing Coumarin,Flavone, Xanthone,Carbazole, and Dibenzofuran Moieties

The cytotoxicities of α‐methylidene‐γ‐butyrolactones, which are linked to coumarins (see 15 and 16 ) and to potential DNA‐intercalating carriers such as flavones, xanthones, carbazole, and dibenzofuran (see 9a – e , 10a – e , 11 , and 12 ), were studied. These compounds were synthesized via alkylation of their hydroxy precursors followed by a Reformatsky‐type condensation (Scheme). These α‐methylidene‐γ‐butyralactones were evaluated in vitro against 60 human tumor cell lines derived from nine cancer cell types and demonstrated a strong growth‐inhibitory activity against leukemia cancer cells (Tables 1 and 2). For flavone‐ and xanthone‐containing α‐methylidene‐γ‐butyrolactones 9a – e and 10a – e , respectively, the overall potency (mean value) decreased on introduction of an electron‐withdrawing substituent at the γ‐phenyl substituent and increased with an electron‐donating substituent. Comparing the different chromophores established the following order of decreasing potency (log GI₅₀): dibenzofuran ( 12 , −6.17) > flavone ( 9a , −5.96) > carbazole ( 11 , −5.80) and xanthone ( 10a , −5.77) > coumarin ( 15 , −5.60; 16 , −5.65). Among them, the dibenzofuran derivative 12 showed not only strong inhibitory activities against leukemia cancer cell lines with an average log GI₅₀ value of −7.22, but also good inhibitory activities against colon, melanoma, and breast cancer cells with average log GI₅₀ values of −6.23, −6.31, and −6.39, respectively.     

Methyl 4‐O‐β‐D‐mannopyranosyl β‐D‐xylopyranoside

Methyl β‐D‐mannopyranosyl‐(1→4)‐β‐D‐xylopyranoside, C₁₂H₂₂O₁₀, (I), crystallizes as colorless needles from water, with two crystallographically independent molecules, (IA) and (IB), comprising the asymmetric unit. The internal glycosidic linkage conformation in molecule (IA) is characterized by a ϕ′ torsion angle (O5′_Man—C1′_Man—O1′_Man—C4_Xyl; Man is mannose and Xyl is xylose) of −88.38 (17)° and a ψ′ torsion angle (C1′_Man—O1′_Man—C4_Xyl—C5_Xyl) of −149.22 (15)°, whereas the corresponding torsion angles in molecule (IB) are −89.82 (17) and −159.98 (14)°, respectively. Ring atom numbering conforms to the convention in which C1 denotes the anomeric C atom, and C5 and C6 denote the hydroxymethyl (–CH₂OH) C atom in the β‐Xylp and β‐Manp residues, respectively. By comparison, the internal glycosidic linkage in the major disorder component of the structurally related disaccharide, methyl β‐D‐galactopyranosyl‐(1→4)‐β‐D‐xylopyranoside), (II) [Zhang, Oliver & Serriani (2012). Acta Cryst. C 68 , o7–o11], is characterized by ϕ′ = −85.7 (6)° and ψ′ = −141.6 (8)°. Inter‐residue hydrogen bonding is observed between atoms O3_Xyl and O5′_Man in both (IA) and (IB) [O3_Xyl...O5′_Man internuclear distances = 2.7268 (16) and 2.6920 (17) Å, respectively], analogous to the inter‐residue hydrogen bond detected between atoms O3_Xyl and O5′_Gal in (II). Exocyclic hydroxymethyl group conformation in the β‐Manp residue of (IA) is gauche–gauche, whereas that in the β‐Manp residue of (IB) is gauche–trans.     

A two‐dimensional mixed‐valence CuII/CuI coordination polymer constructed from 2‐(pyridin‐3‐yl)‐1H‐imidazole‐4,5‐dicarboxylate

Coordination polymers are a thriving class of functional solid‐state materials and there have been noticeable efforts and progress toward designing periodic functional structures with desired geometrical attributes and chemical properties for targeted applications. Self‐assembly of metal ions and organic ligands is one of the most efficient and widely utilized methods for the construction of CPs under hydro(solvo)thermal conditions. 2‐(Pyridin‐3‐yl)‐1H‐imidazole‐4,5‐dicarboxylate (HPIDC²⁻) has been proven to be an excellent multidentate ligand due to its multiple deprotonation and coordination modes. Crystals of poly[aquabis[μ₃‐5‐carboxy‐2‐(pyridin‐3‐yl)‐1H‐imidazole‐4‐carboxylato‐κ⁵N¹,O⁵:N³,O⁴:N²]copper(II)dicopper(I)], [Cu^IICu^I₂(C₁₀H₅N₃O₄)₂(H₂O)]_n, (I), were obtained from 2‐(pyridin‐3‐yl)‐1H‐imidazole‐4,5‐dicarboxylic acid (H₃PIDC) and copper(II) chloride under hydrothermal conditions. The asymmetric unit consists of one independent Cu^II ion, two Cu^I ions, two HPIDC²⁻ ligands and one coordinated water molecule. The Cu^II centre displays a square‐pyramidal geometry (CuN₂O₃), with two N,O‐chelating HPIDC²⁻ ligands occupying the basal plane in a trans geometry and one O atom from a coordinated water molecule in the axial position. The Cu^I atoms adopt three‐coordinated Y‐shaped coordinations. In each [CuN₂O] unit, deprotonated HPIDC²⁻ acts as an N,O‐chelating ligand, and a symmetry‐equivalent HPIDC²⁻ ligand acts as an N‐atom donor via the pyridine group. The HPIDC²⁻ ligands in the polymer serve as T‐shaped 3‐connectors and adopt a μ₃‐κ²N,O:κ²N′,O′:κN′′‐coordination mode, linking one Cu^II and two Cu^I cations. The Cu cations are arranged in one‐dimensional –Cu1–Cu2–Cu3– chains along the [001] direction. Further crosslinking of these chains by HPIDC²⁻ ligands along the b axis in a –Cu2–HPIDC²⁻–Cu3–HPIDC²⁻–Cu1– sequence results in a two‐dimensional polymer in the (100) plane. The resulting (2,3)‐connected net has a (12³)₂(12)₃ topology. Powder X‐ray diffraction confirmed the phase purity for (I), and susceptibilty measurements indicated a very weak ferromagnetic behaviour. A thermogravimetric analysis shows the loss of the apical aqua ligand before decomposition of the title compound.     

Dendrimers with 1, 3, 5‐Trisilacyclohexane as Core Unit

1, 1, 3, 3, 5, 5‐Hexavinyl‐1, 3, 5‐trisilacylohexane [Si(CH=CH₂)₂CH₂]₃ was synthesized and hydrosilylated with trichlorosilane to afford the first generation of a dendrimer. Conversion of this molecule with 18 Si–Cl functions on its surface with an excess of vinylmagnesium bromide yielded the 18‐fold vinylated dendrimer. The new compounds were identified by elemental analyses, multi‐nuclear NMR spectroscopy, and mass spectrometry. Crystal structures were obtained for [Si(CH=CH₂)₂CH₂]₃ and [Si(CH₂–CH₂SiCl₃)₂CH₂]₃.     

A novel two‐dimensional (4,4) network based on dinuclear cadmium secondary building units: poly[(μ5‐benzene‐1,4‐diacetato)[μ2‐1,4‐bis(1,2,4‐triazol‐1‐yl)butane]cadmium(II)]

In the title mixed‐ligand metal–organic polymeric compound, [Cd(C₁₀H₈O₄)(C₈H₁₂N₆)]_n or [Cd(PBEA)(BTB)]_n [H₂PBEA is benzene‐1,4‐diacetic acid and BTB is 1,4‐bis(1,2,4‐triazol‐1‐yl)butane], the asymmetric unit contains one Cd^II ion, one BTB molecule and one PBEA²⁻ anion. The Cd^II ion is in a slightly distorted pentagonal–bipyramidal geometry, coordinated by five carboxylate O atoms from three distinct PBEA²⁻ anions and by two BTB N atoms. There are two coordination patterns for the carboxylate groups of the PBEA²⁻ ligand, one being a μ₁‐η¹:η¹ chelating mode and the other a μ₂‐η²:η¹ bridging mode, while the BTB molecule shows a trans–trans–trans conformation. The crystal structure is constructed from the secondary building unit (SBU) [Cd₂(CO₂)₄N₂O₂], in which the two metal centres are held together by two PBEA²⁻ linkers. The SBU is connected by BTB and PBEA²⁻ bridges to form a two‐dimensional grid‐like (4,4) layer with meshes of dimensions 14.69 × 11.28 Å.     

Synthesis,characterization, and cross‐linking of a library of statistical copolymers based on 2‐“soy alkyl”‐2‐oxazoline and 2‐ethyl‐2‐oxazoline

The synthesis of statistical copolymers consisting of 2‐ethyl‐2‐oxazoline (EtOx) and 2‐“soy alkyl”‐2‐oxazoline (SoyOx) via a microwave‐assisted cationic ring‐opening polymerization procedure is described. The majority of the resulting copolymers revealed polydispersity indices below 1.30. The reactivity ratios (r_EtOx 1.4 ± 0.3; r_SoyOx = 1.7 ± 0.3) revealed a clustered monomer distribution throughout the polymer chains. The thermal and surface properties of the pEtOx‐stat‐SoyOx copolymers were analyzed before and after UV‐curing demonstrating the decreased chain mobility after cross‐linking. In addition, the cross‐linked materials showed shape‐persistent swelling upon absorption of water from the air, whereby as little as 5 mol % SoyOx was found to provide efficient cross‐linking. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5371,–5379, 2007     

Methyl 4‐O‐β‐d‐galactopyranosyl α‐d‐mannopyranoside methanol 0.375‐solvate

Methyl β‐d ‐galactopyranosyl‐(1→4)‐α‐d ‐mannopyranoside methanol 0.375‐solvate, C₁₃H₂₄O₁₁·0.375CH₃OH, (I), was crystallized from a methanol–ethanol solvent system in a glycosidic linkage conformation, with ϕ′ (O5_Gal—C1_Gal—O1_Gal—C4_Man) = −68.2 (3)° and ψ′ (C1_Gal—O1_Gal—C4_Man—C5_Man) = −123.9 (2)°, where the ring is defined by atoms O5/C1–C5 (monosaccharide numbering); C1 denotes the anomeric C atom and C6 the exocyclic hydroxymethyl C atom in the βGalp and αManp residues, respectively. The linkage conformation in (I) differs from that in crystalline methyl α‐lactoside [methyl β‐d ‐galactopyranosyl‐(1→4)‐α‐d ‐glucopyranoside], (II) [Pan, Noll & Serianni (2005). Acta Cryst. C 61 , o674–o677], where ϕ′ is −93.6° and ψ′ is −144.8°. An intermolecular hydrogen bond exists between O3_Man and O5_Gal in (I), similar to that between O3_Glc and O5_Gal in (II). The structures of (I) and (II) are also compared with those of their constituent residues, viz. methyl α‐d ‐mannopyranoside, methyl α‐d ‐glucopyranoside and methyl β‐d ‐galactopyranoside, revealing significant differences in the Cremer–Pople puckering parameters, exocyclic hydroxymethyl group conformations and intermolecular hydrogen‐bonding patterns.     

Synthesis and NMR Analysis in Solution of Oligo(3‐hydroxyalkanoic acid) Derivatives with the Side Chains of Alanine,Valine, and Leucine (β‐Depsides): Coming Full Circle from PHB to β‐Peptides to PHB

Oligomers of 3‐hydroxyalkanoic acids that contain two, three, and six residues with and without O‐terminal (tBu)Ph₂Si and C‐terminal PhCH₂ protection have been synthesized in such a way that the side chains on the oligoester backbone were those of the proteinogenic amino acids Ala (Me), Val (CHMe₂), and Leu (CH₂CHMe₂). The enantiomerically pure 3‐hydroxyalkanoates were obtained by Noyori hydrogenation of the corresponding 3‐oxo‐alkanoates with [Ru((R)‐binap)Cl₂](binap=2,2′bis(diphenylphosphanyl)‐1,1′‐binaphthalene)/H₂ (Scheme 1), and the coupling was achieved under the conditions (pyridine/(COCl)₂, CH₂Cl₂, −78°) previously employed for the synthesis of various oligo(3‐hydroxybutanoic acids) (Schemes 2 and 3). The Cotton effects in the CD spectra of the new oligoesters provided no hints about chiral conformation (cf. a helix) in MeOH, MeCN, octan‐1‐ol, or CF₃CH₂OH solutions (Figs. 1 and 2). Detailed NMR investigations in CDCl₃ solution (Figs. 3–6, and Tables 1–5) of the hexa(3‐hydroxyalkanoic acid) with the side chains of Val (HC), Ala (HB), Leu (HH), Val, Ala, Leu (from O‐ to C‐terminus; 3 ) gave, on the NMR time‐scale, no evidence for the presence of any significant amount of a 2₁‐ or a 3₁‐helical conformation, comparable to those identified in stretched fibers of poly[(R)‐3‐hydroxybutanoic acid], or in lamellar crystallites and in single crystals of linear and cyclic oligo[(R)‐3‐hydroxybutanoic acids], or in the corresponding β‐peptide(s) (the oligo(3‐aminoalkanoic acid) analogs; 1 – 3 ). Thus, the extremely high flexibility (averaged or ‘random‐coil' conformation) of the polyester chain (CO−O rotational barrier ca. 13 kcal/mol; no hydrogen bonding), as compared to polyamide chains (CO−NH barrier ca. 18 kcal/mol; hydrogen bonding) has been demonstrated once again. The possible importance of this structural flexibility, which goes along with amphiphilic properties, for the role of PHB in biology, in evolution, and in prebiotic chemistry is discussed. Structural similarities of natural potassium‐channeling proteins and complexes of oligo(3‐hydroxybutanoates) with Na⁺, K⁺, or Ba²⁺ are alluded to (Figs. 7–9).     

A new cross‐linked system of silicone rubber based on silicone‐polyurea block copolymer

A new cross‐linked system of silicone rubber (SR) was obtained from silicone‐polyurea block copolymers that was synthesized with aminopropyl terminated polydimethylsiloxane and (4‐isocyanatocyclohexyl)‐methane. SR possessed self‐reinforced and physical cross‐linked structure. It had better mechanical properties that the hardness, the tensile strength, and the elongation at break could reach 65 Shore A, 3.78 MPa, and 458% with the polyurea segment content ranging from 2.01% to 9.13% by weight . The hydrogen bond that led to the physical cross‐linked structure was proved byFourier transform infrared spectroscopy. The microphase separated structure that caused the self‐reinforcement was illustrated by scanning electron microscopy, X‐ray diffraction analysis, and dynamic mechanical analysis. Fourier transform infrared spectroscopy results showed the hydrogen bond formation between the polyurea units. Scanning electron microscopy, dynamic mechanical analysis, and X‐ray diffraction analysis results proved the microphase separation existed between polyurea units and ―Si―O―Si― chains. The increase of polyurea contents enhanced the binding of hydrogen bond and improved the extent of microphase separation. Accordingly, it decreased the thermal properties and lowered the glass transition temperature (T_g) from −108°C to −114°C. Also, the increase of polyurea contents increased the hydrophobicity of SR that the surface free energy could reach to −24.81 mN/m.     

The σ‐donating and π‐accepting properties of ortho‐Si(CH3)3 phosphinine macrocycles

A theoretical investigation of both the ortho‐Si(CH₃)₃ phosphinine and some silacalix[n]phosphinines was performed. The optimized geometries agree well with those reported from X‐ray analysis and other structural studies. The silacalix[n]phosphinine macrocyle is very flexible because of the C Si C bridges. This, in turn, makes possible the formation of strained configurations in solid packed structures. In the silacalix[3]phosphinine, a P P bonding interaction that is presumably responsible for its structural and electronic features seems to exist. The molecular orbital calculations corroborate that both the π‐accepting properties and the σ‐donating capacities of the phosphinine unit may be enhanced by ortho‐Si(CH₃)₃ substitution. These features satisfy the proposal of the synthesizers as regards the production of macrocyclic phosphorus compounds, with good π‐accepting properties and strong σ‐donating capacities, which are sufficiently flexible as to encapsulate metals with coordination spheres of different geometries. © 2003 Wiley Periodicals, Inc. Heteroatom Chem 14:160–169, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10118     

Selective and Tunable Near‐Infrared and Visible Light Transmittance of MoO3−x Nanocomposites with Different Crystallinity

In this Communication, we report MoO_3−x nanocomposites in which the near‐infrared and visible light transmittance can be selectively modulated through the crystallinity. The MoO_3−x nanocomposites were fabricated by a hydrothermal method, and their optical properties were characterized by UV‐Vis spectrometer. The obtained results proved the possibility to tune the nanocomposite's optical properties in the UV/Visible spectral region: crystalline MoO₃ mainly regulates the near‐infrared range (800–2600 nm), and amorphous MoO_3−x mainly changes the visible range from 350 nm to 800 nm and MoO_3−x , with semi‐crystalline structures mainly modulating around 800–1000 nm. These kinds of optical modulations could be attributed to small polar absorption, free electron absorption and plasmon absorption according to different crystallinity. Our work may create new possibilities for future applications such as photochromism, photocatalysis, and electrochromism.