Substituent effect on azobenzene‐based liquid‐crystalline organophosphorus polymers

An Erratum has been published for this article in Journal of Polymer Science Part A: Polymer Chemistry (2003) 41(23) 3862 A new series of combined‐type, azobenzene‐based organophosphorus liquid‐crystalline polymers were synthesized, and their photoisomerization properties were studied. The prepared polymers contained azobenzene units as both the main‐chain and side‐chain mesogens. Various groups were substituted in the terminal of the side‐chain azobenzene mesogen, and the effects of the substituents were investigated. All the polymers were prepared at the ambient temperature by solution polycondensation with various 4‐substituted phenylazo‐4′‐phenyloxyhexylphosphorodichloridates and 4,4′‐bis(6‐hydroxyhexyloxy) azobenzene. The polymers were characterized with gel permeation chromatography, Fourier transform infrared, and ¹H, ¹³C, and ³¹P NMR spectroscopy. Thermogravimetric analysis revealed that all the polymers had high char yields. The liquid‐crystalline behavior of the polymers was examined with hot‐stage optical polarizing microscopy, and all the polymers showed liquid‐crystalline properties. The formation of a mesophase was confirmed by differential scanning calorimetry (DSC). The DSC data suggested that mesophase stability was better for electron‐withdrawing substituents than for halogens and unsubstituted ones. Ultraviolet irradiation studies indicated that the time taken for the completion of photoisomerization depended on the dipolar moment, size, and donor–acceptor characteristics of the terminal substituents. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3188–3196, 2003     

Synthesis and antibacterial activities of water‐soluble methacrylate polymers containing quaternary ammonium compounds

New water‐soluble methacrylate polymers with pendant quaternary ammonium (QA) groups were synthesized and used as antibacterial materials. The polymers with pendant QA groups were obtained by the reaction of the alkyl halide groups of a previously synthesized functional methacrylate homopolymer with various tertiary alkyl amines containing 12‐, 14‐, or 16‐carbon alkyl chains. The structures of the functional polymer and the polymers with QA groups were confirmed with Fourier transform infrared and ¹H and ¹³C NMR. The degree of conversion of alkyl halides to QA sites in each polymer was determined by ¹H NMR to be over 90% in all cases. The number‐average molecular weight and polydispersity of the functional polymer were determined by size exclusion chromatography to be 32,500 g/mol and 2.25, respectively. All polymers were thermally stable up to 180 °C according to thermogravimetric analysis. The antibacterial activities of the polymers with pendant QA groups against Staphylococcus aureus and Escherichia coli were determined with broth‐dilution and spread‐plate methods. All the polymers showed excellent antibacterial activities in the range of 32–256 μg/mL. The antibacterial activity against S. aureus increased with an increase in the alkyl chain length for the ammonium groups, whereas the antibacterial activity against E. coli decreased with increasing alkyl chain length. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5965–5973, 2006     

Synthesis,optical, and electrochemical properties of novel copolymers on the basis of benzothiadiazole and electron‐rich arene units

Novel alternating conjugated copolymers ( P1–P6 ) consisting of an electron‐deficient benzothiadiazole and a variety of electron‐rich thiophene‐arene‐thiophene units were synthesized by palladium‐catalyzed polycondensations (Stille and Suzuki reactions), aiming at processable materials with a reduced optical band gap. The structures of P1–P6 were confirmed by ¹H NMR and ¹³C NMR, and their molecular weights were determined by size exclusion chromatography. In the Suzuki polycondensation, the role of the catalyst [Pd(PPh₃)₄ and Pd(OAc)₂] on the resulting molecular weight was investigated. Pd(OAc)₂ enhances the molecular weight of the polymers for both thiophene and phenylene bis‐boronic esters as compared with Pd(PPh₃)₄. The optical properties of the polymers were examined in solution and the solid state. The polymers with n‐octyl substituents ( P1 , P4 , P5 , and P6 ) on the thiophene rings possessed less‐planar structures as a result of torsional steric hindrance, and their absorption spectra appeared blueshifted as compared with their unsubstituted analogues ( P2 and P3 ). The electrochemical properties of the polymers were studied using cyclic voltammetry. Although the alkyl substitution affects the oxidation potential, only marginal differences in the reduction potentials were observed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2360–2372, 2002     

Synthesis and chiroptical properties of L‐valine‐containing poly(phenylacetylene)s with (a)chiral pendant terminal groups

Poly(phenylacetylene)s containing L ‐valine residues (P 1 ) with (a)chiral pendant terminal groups R^(*) [?(HC?C{C₆H₄CONHCH[CH(CH₃)₂]COO? R^(*)})_n?]; R^(*) = 1‐octyl (P 1 o), (1S,2R,5S)‐(+)‐menthyl [P 1 (+)], (1R,2S,5R)‐(?)‐menthyl [P 1 (?)] are designed and synthesized. The polymers are prepared by organorhodium catalysts in high yields (yield up to 88%) with high molecular weights (M_w up to ?6.4 × 10⁵). Their structures and properties are characterized by NMR, IR, TGA, UV, and circular dichroism analyses. All the polymers are thermally fairly stable (T_d ≥ 320 °C). The chiral moieties induce the poly(phenylacetylene) chains to helically rotate in a preferred direction. The chirality of the pendant terminal groups affects little the helicity of the polymers but their bulkiness stabilizes the helical conformation against solvent perturbation. The backbone conjugation and chain helicity of the polymers can be modulated continuously and reversibly by acid. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2117–2129, 2006     

Iridium(III) Complexes with Sulfonyl and Fluorine Substituents: Synthesis,Stereochemistry and Effect of Functionalisation on their Photophysical Properties

The synthesis and photophysical and electrochemical characterisation of new heteroleptic iridium complexes with electron‐withdrawing sulfonyl groups and fluorine atoms bound to phenylpyridine ligands are reported. The emission energy of these materials strongly depends on the position of the sulfonyl groups and on the number of fluorine substituents. A 90 nm wide tuning range of photoluminescence from the blue‐green (λ_em=468 nm) of iridium(III)bis[2‐(4′‐benzylsulfonyl)phenylpyridinato‐N,C2′][3‐(pentafluorophenyl)‐pyridin‐2‐yl‐1,2,4‐triazolate] to the orange (λ_em=558 nm) of iridium(III)bis[2‐(3′‐benzylsulfonyl)phenylpyridinato‐N,C2′](2,4‐decanedionate) has been achieved. Emission quantum yields ranging from 47 to 71 % have also been found for degassed solutions of the complexes, and a surprisingly high value of 16 % was recorded for iridium(III)bis[2‐(5′‐benzylsulfonyl‐3′,6′‐difluoro)phenylpyridinato‐N,C2′](2,4‐decanedionate) in air‐equilibrated dichloromethane. A unusual stereochemistry of the benzylsulfonyl‐substituted dimer and heteroleptic complexes has been detected by ¹H NMR spectroscopy, and is characterised by the mutual cis disposition of the pyridyl nitrogen atoms of the phenylpyridine ligands, which differs from the most common trans arrangement reported in the literature.     

1H, 13C and 15N NMR spectral assignments for new triazapentalene derivatives

Mesomeric heteropentalene betaines are conjugated fused polyheterocyclic structures that represent interesting intermediates for organic synthesis. Five such structures, containing at least four nitrogen atoms and various substituents, have been characterized by ¹H, ¹³C and ¹⁵N NMR. We report, apparently for the first time, nitrogen NMR data and coupling information on such systems. Inter‐ring long‐range correlations across five bonds with ¹⁵N (⁵J_HN) and up to seven bonds with ¹³C (⁶J_HC and ⁷J_HC) were observed in HSQC experiments. The incorporation of an electron‐withdrawing substituent such as NO₂ was observed to cause an increase in the magnitude of the remote couplings and deshielding of nearby protons, carbons and on all nitrogen atoms of the structure, including remote ones situated on other cycles. Copyright © 2009 John Wiley & Sons, Ltd.     

Precision ADMET polyolefins containing dithiane: Synthesis,thermal properties,and macromolecular transformation

1,3‐Dithiane and its derivatives are widely used as powerful acyl anion equivalent to a range of useful transformations that are needed in the synthesis of natural products. In this work, a series of polyolefins containing pendant dithiane groups have been designed and synthesized via acyclic diene metathesis polymerization (ADMET) polymerization and subsequent hydrogenation. The structures of these polymers were characterized by ¹H NMR, ¹³C NMR, and FT‐IR, and successful incorporation of the dithiane groups was proved. With different contents of the dithiane moieties, these ADMET polymers exhibited distinct thermal properties different from each other as evidenced by differential scanning calorimetry and thermal gravimetric analysis. The dithiane units in the ADMET polymer with 20 methylene carbons between the adjacent dithiane groups were transformed into thiol groups via reaction with Bu₃SnH. This work provided a convenient route to synthesize polyethylene with pendant thiol groups that are evenly distributed in the chain. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2468–2475     

Synthesis and self‐catalyzed phase transfer reaction of novel methacrylate polymers

Ethyl α‐chloromethylacrylate was converted to an ester derivative using 5‐chlorovaleric acid in a single step. The homopolymerization of the new monomer (CEMA) and its copolymerization with methyl methacrylate were performed using photoinitiator Irgacure 651. The polymers were reacted with N,N‐dimethyldodecylamine to obtain polymers with pendant quaternary ammonium (QA) moieties. The polymers with pendant QA groups were used in self‐catalyzed phase transfer reactions with sodium phenoxide and 1‐dodecanethiol. The syntheses of the monomer and polymers were followed by FTIR, ¹H NMR, and ¹³C NMR. The average polymer molecular weights and polydispersities were determined by size exclusion chromatography. Thermal analysis was carried out using thermogravimetric analysis and differential scanning calorimetry. The copolymer composition, degree of quaternization, and degree of conversion in phase transfer reaction were determined by ¹H NMR. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5844–5854, 2005     

Amine‐blocked polyisocyanates. I. Synthesis of novel N‐methylaniline‐blocked polyisocyanates and deblocking studies using hot‐stage fourier transform infrared spectroscopy

A series of substituted N‐methylaniline‐blocked polyisocyanates based on 4,4′‐methylenebis(phenyl isocyanate) and poly(tetrahydrofuran) were prepared and characterized thoroughly with FTIR, ¹H NMR, and ¹³C NMR spectroscopy methods. Compared with unsubstituted N‐methylaniline, a blocking agent with an electron‐releasing substituent at the para position took a shorter time, whereas those with an electron‐releasing substituent at the ortho position or an electron‐withdrawing substituent at the ortho and para positions took longer times for the blocking reaction. The thermal dissociation reactions of blocked polyisocyanates were carried out with an FTIR spectrophotometer attached to hot‐stage accessories under dynamic and isothermal conditions. The dynamic method was used to determine the deblocking temperature, and the isothermal method was used to calculate the deblocking kinetics and activation parameters. The cure times of blocked polyisocyanates with hydroxyl‐terminated polybutadiene were also determined. The deblocking temperatures, the results of cure‐time studies, and the kinetic parameters revealed that the thermal dissociation of the N‐methylaniline‐blocked polyisocyanates was retarded by electron‐donating substituents and facilitated by electron‐withdrawing substituents. The action of N‐methylanilines as blocking agents for isocyanate was explained by the formation of a four‐center, intramolecularly hydrogen‐bonded ring structure during the thermal dissociation of the blocked polyisocyanates. The formation of such a hydrogen‐bonded ring structure was confirmed and supported by variable‐temperature ¹H NMR studies and entropy parameters, respectively. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1557–1570, 2007     

Novel thermotropic liquid crystalline‐cum‐photocrosslinkable polyvanillylidene alkyl/arylphosphate esters

A new class of linear unsaturated polyphosphate esters based on divanillylidene cyclohexanone possessing liquid crystalline‐cum‐photocrosslinkable properties have been synthesized from 2,6‐bis[n‐hydroxyalkyloxy(vanillylidene)]cyclohexanone [n = 6,8,10] with various alkyl/aryl phosphorodichloridates in chloroform at ambient temperature. The resultant polymers were characterized by intrinsic viscosity, FT‐IR, ¹H, ¹³C, and ³¹P‐NMR spectroscopy. All the polymers showed anisotropic behavior under hot stage optical polarized microscope (HOPM). The liquid crystalline textures of the polymers became more transparent with increasing spacer length. The thermal behavior of the polymers was studied by thermogravimetric analysis and differential scanning calorimetry. The T_g, T_m, and T_i of the polymers decreased with increasing flexible methylene chain. The photocrosslinking property of the polymer was investigated by UV light/UV spectroscopy; the crosslinking proceeds via 2π‐2π cycloaddition reactions of the divanillylidene exocyclic double bond of the polymer backbone. The pendant alkyloxy containing polymers show faster crosslinking than the pendant phenyloxy containing polymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5215–5226, 2004     

Novel fluorescent amphiphilic poly(allylamine) and their supramacromolecular self‐assemblies in aqueous media

Fluorescent amphiphilic polymers were produced by grafting different types and levels of hydrophobic pendant groups with intrinsic fluorescent properties (fluorenylmethoxy carbonyl (Fmoc), dimethylamino‐1‐naphthalenesulfonyl (Dansyl), and naphthalene (Naphth) to a water soluble homopolymer backbone, polyallylamine (PAA). Non‐fluorescent hydrophobic pendant group (cholesteryl moieties) were also grafted onto PAA. The polymers were characterized with elemental analysis, NMR and FTIR spectroscopy. All polymers formed self‐assemblies by probe sonication in water with sizes ranging from 120 to 199 nm and TEM images showed the presence of spherical particles. The critical aggregation concentration (CAC) varied from 0.093 to 1.5 mg ml^?1 depending on the type of hydrophobic pendant groups. The Cholesteryl and Dansyl polymers showed the presence of one CAC while the Fmoc and Naphth grafted polymers revealed the presence of two CACs. The first CAC observed was possibly due to intermolecular aggregation while the second CAC at the higher polymer concentration was the result of excimer formation revealed by their fluorescent spectra. We reasoned that Naphth and Fmoc aromatic pendant groups possess a flat stereochemistry, thus allowing π–π stacking at higher concentrations. The presence of the N‐dimethylamino group in the Dansyl moiety gives rise to a 3D structure, thus hindering any stacking. The understanding of the supramolecular assemblies formed by these fluorescent amphiphilic polymers will aid in the engineering of advanced materials with superior functionality for biomedical applications. Copyright © 2011 John Wiley & Sons, Ltd.     

Stereoselective Synthesis and Structure of New Types of Calix[4]resorcinarenes. Complexation of Tetrakis(O,O‐Phosphorus)Bridged‐Calix[4]resorcinarenes with Heavy Metal Atoms

The acid‐catalyzed (with HCl) condensation reactions of resorcinol ( 1 ) with 1‐naphthaldehyde ( 2 ) and isobutyraldehyde ( 3 ) furnished the tetrameric macrocyclic compounds 4 and 6 . Detailed NMR‐investigations of the acetylated tetrameric species 5 surprisingly support a structure not in agreement with the expected all‐cis conformation. The chair conformation (C_2h symmetry) of the acetylated derivative 5 was established through a crystal X‐ray diffraction study. The naphthyl substituents are arranged in trans position above and below the plane made up by the resorcinol units. The reaction of resorcinol 1 with isobutyraldehyde, in accord with expectation, led to the calix[4]resorcinaren ( 6 ). The ¹H NMR spectra of compound 6 and 7 appeared at room temperature as broad signals, indicating a conformation of C_2v symmetry. The reaction of the C‐methyl‐tetrakis‐P‐(chlorodioxaphosphocin)‐calix[4]resorcinarenes ( 8 ) and ( 10 ) with suitable N‐trimethylsilyl organic amines were conducted in tetrahydrofuran suspension, furnishing the P–N‐substituted calix[4]resorcinarenes ( 9 ) and ( 11 ). While in the complexation of C‐methyl‐tetrabromotetrakis‐P‐(dimethylaminodioxaphosphocin)‐calix[4]resorcinarene ( 13 ) with (tht)AuCl (tht = tetrahydrothiophene) the expected, neutral tetra‐substituted complex 15 was formed, the reaction of 13 with moist acetonitrile led to the anionic atomic framework 14 . X‐ray structure determinations of the complexes 14 and 15 show that both possess the cone conformation. In the gold complex 15 , the Au–Cl groups form a loose aggregate, with three Au…Cl contacts of 316–340 pm; one of the groups points towards the centre of the cone. The copper(I) complex 14 displays crystallographic mirror symmetry, with a central Cu₄Cl₅ unit involving tetrahedrally coordinated copper.     

Computational, 1H NMR,and X‐ray structural studies on 1‐arylurazole tetrazane dimers

Nitrogen‐centered urazole radicals exist in equilibrium with tetrazane dimers in solution. The equilibrium established typically favors the free‐radical form. However, 1‐arylurazole radicals bearing substituents at the ortho position favor the dimeric form. We were able to determine the structure of one of the dimers (substituted at both ortho positions with methyl groups), namely 1,2‐(2,4‐dimethylphenyl)‐2‐[2‐(2,4‐dimethylphenyl)‐4‐methyl‐3,5‐dioxo‐1,2,4‐triazolidin‐1‐yl]‐4‐methyl‐1,2,4‐triazolidine‐3,5‐dione, C₂₄H₂₈N₆O₄, via X‐ray crystallography. The experimentally determined structure agreed well with the computationally obtained geometry at the B3LYP/6‐311G(d,p) level of theory. The preferred syn conformation of these 1‐arylurazole dimers results in the two aromatic rings being proximate and nearly parallel, which leads to some interesting shielding effects of certain signals in the ¹H NMR spectrum. Armed with this information, we were able to decipher the more complicated ¹H NMR spectrum obtained from a dimer that was monosubstituted at the ortho position with a methyl group.     

Asymmetric anionic polymerizations of 7‐(o‐substituted phenyl)‐2,6‐dimethyl‐1,4‐benzoquinone methides: Electrostatic interaction and steric,inductive, and resonance effects of the ortho‐substituent on the optical activity

7‐(o‐Substituted phenyl)‐2,6‐dimethyl‐1,4‐benzoquinone methides which have an electron‐donating methoxy‐(o‐OMe, 2a ) and methyl‐ (o‐Me, 2b ) substituents or an electron‐withdrawing cyano‐ (o‐CN, 2c ) and trifluoromethyl‐ (o‐CF₃, 2d ) substituents at the ortho‐position of the aromatic ring and 7‐(m‐substituted phenyl)‐2,6‐dimethyl‐1,4‐benzoquinone methide with an electron‐withdrawing trifluoromethyl‐ (m‐CF₃, 2e ) substituent at the meta‐position of the aromatic ring were synthesized, and their asymmetric anionic polymerizations using the complex of lithium 4‐isopropylphenoxide with (?)‐sparteine were carried out in toluene at 0 °C. The polymers with negative optical activity were obtained for all of five monomers, and their specific rotation values largely changed depending upon the substituents of the monomers. On the basis of the comparison of various substituents effects, it was found that the specific rotation of obtained polymers is significantly affected by the electronic effects such as inductive and resonance effects rather than the steric and electrostatic effects of the substituent. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 1048–1058     

Study of Unusually High Rotational Barriers about S?N Bonds in Nonafluorobutane‐1‐sulfonamides: The Electronic Nature of the Torsional Effect

Slow rotation about the S? N bond in N,N‐disubstituted nonafluorobutane‐1‐sulfonamides 1 can easily be detected by NMR measurements at room temperature. This effect causes magnetic nonequivalence of otherwise identical geminal substituents in symmetrical staggered ground‐state conformation A . The torsional barriers determined (62–71 kJ?mol^?1) proved to be the highest ever observed for sulfonamide moieties. They are comparable to the values routinely measured for carboxylic acid amides or carbamates. The restricted rotation is interpreted as result (n_N? d_S)‐π and of n_N? σ interactions, which develop substantial S,N double‐bond character in sulfonamides 1 . The S,N binding interaction is increased by the highly electron‐withdrawing effect of the perfluorobutyl group. The anticipated symmetry of the ground‐state conformation A and the considerable shortening of the S? N bond (1.59 Å) compared to the mean value in sulfonamides (1.63 Å) are confirmed by single‐crystal X‐ray study of N,N‐dibenzylnonafluorobutane‐1‐sulfonamide ( 1c ).     

Structural and dynamic study of chemically modified polyHIPE by solid‐state 13C NMR spectroscopy

The structure of laboratory‐made polyHIPEs was successfully characterized by cross‐polarity/magic‐angle spinning, solid‐state ¹³C NMR experiments. The signals of vinyl groups appeared in the spectrum of the polyHIPE precursor PH? CH?CH₂, which was prepared by the polymerization of the divinylbenzene continuous phase from a highly concentrated reverse emulsion. This material was chemically modified by the regioselective free‐radical addition of thiols to the pendant vinyl groups. Spectra of materials modified by the grafting of C₈ and C₁₂ alkyl chains, PH? SC₈ and PH? SC₁₂, respectively, were produced. The signals of the vinyl groups disappeared in favor of methylene groups. This experiment clearly established that the alkyl chains were covalently bound to the polymer. To elucidate the dynamic aspect of long chains in polyHIPE, we measured the ¹³C spin–lattice relaxation times (T₁) of PH? SC₁₂ from 25 to 100 °C with variable‐temperature, solid‐state, high‐resolution ¹³C NMR spectroscopy, revealing a strong variation in T₁ along the alkyl side chain. Furthermore, the crystallinity of a wide range of chemically modified polyHIPEs, including PH? SC₁₂, was studied with pulse ¹H NMR. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 956–963, 2001     

Degradation of poly(acrylates) under SF5+ primary ion bombardment studied using time‐of‐flight secondary ion mass spectrometry. 3. Poly(hydroxyethyl methacrylate) with chemical derivatization

Molecular depth profiling of polymers by secondary ion mass spectrometry (SIMS) has focused on the use of polyatomic primary ions due to their low penetration depth and high damage removal rates in some polymers. This study is the third in a series of systematic characterizations of the effect of polymer chemistry on degradation under polyatomic primary ion bombardment. In this study, time‐of‐flight SIMS (ToF‐SIMS) was used to assess 5 keV SF₅⁺‐induced damage of ～90 nm thick spin‐cast poly(2‐hydroxyethyl methacrylate) (PHEMA) and ～130 nm thick trifluoroacetic anhydride‐derivatized PHEMA (TFAA‐PHEMA) films. The degradation of these polymers under extended SF₅⁺ bombardment (～2 × 10¹⁴ ions cm^?2) was compared to determine the effect of the pendant group chemistry on their degradation. The sputter rate and ion‐induced damage accumulation rate of PHEMA were similar to a poly(n‐alkyl methacrylate) of similar pendant group length, suggesting that the addition of a terminal hydroxyl group to the alkyl pendant group does not markedly change the stability of poly(n‐alkyl methacrylates) under SF₅⁺ bombardment. The sputter rate and ion‐induced damage accumulation rate of TFAA‐PHEMA were much higher than a poly(n‐alkyl methacrylate) of similar pendant group length, suggesting that derivatization of the terminal hydroxyl group can significantly reduce degradation of the polymer under SF₅⁺ bombardment. This result is in good agreement with the literature on the thermal and radiation‐induced degradation of fluorinated poly(alkyl methacrylates), which suggests that the electron‐withdrawing fluorinated pendant group increases the probability of depolymerization. Copyright © 2004 John Wiley & Sons, Ltd.     

Structures and Properties of Molecular Torsion Balances to Decipher the Nature of Substituent Effects on the Aromatic Edge‐to‐Face Interaction

Various recent computational studies initiated this systematic re‐investigation of substituent effects on aromatic edge‐to‐face interactions. Five series of Tröger base derived molecular torsion balances (MTBs), initially introduced by Wilcox and co‐workers, showing an aromatic edge‐to‐face interaction in the folded, but not in the unfolded form, were synthesized. A fluorine atom or a trifluoromethyl group was introduced onto the edge ring in ortho‐, meta‐, and para‐positions to the C?H group interacting with the face component. The substituents on the face component were varied from electron‐donating to electron‐withdrawing. Extensive X‐ray crystallographic data allowed for a discussion on the conformational behavior of the torsional balances in the solid state. While most systems adopt the folded conformation, some were found to form supramolecular intercalative dimers, lacking the intramolecular edge‐to‐face interaction, which is compensated by the gain of aromatic π‐stacking interactions between four aryl rings of the two molecular components. This dimerization does not take place in solution. The folding free enthalpy ΔG_fold of all torsion balances was determined by ¹H NMR measurements by using 10 mM solutions of samples in CDCl₃ and C₆D₆. Only the ΔG_fold values of balances bearing an edge‐ring substituent in ortho‐position to the interacting C?H show a steep linear correlation with the Hammett parameter (σ_meta) of the face‐component substituent. Thermodynamic analysis using van′t Hoff plots revealed that the interaction is enthalpy‐driven. The ΔG_fold values of the balances, in addition to partial charge calculations, suggest that increasing the polarization of the interacting C?H group makes a favorable contribution to the edge‐to‐face interaction. The largest contribution, however, seems to originate from local direct interactions between the substituent in ortho‐position to the edge‐ring C?H and the substituted face ring.     

Synthesis and thermal crosslinking of benzophenone‐modified poly(dimethylsiloxane)s

Dihydridocarbonyltris(triphenylphosphine)ruthenium catalyzes the regiospecific anti‐Markovnikov addition of an ortho C? H bond of benzophenone across the C? C double bonds of α,ω‐bis(trimethylsilyloxy)copoly(dimethylsiloxane/vinylmethylsiloxane) (99:1), α,ω‐bis(vinyldimethylsilyloxy)poly(dimethylsiloxane), and 1,3‐divinyltetramethyldisiloxane to yield α,ω‐bis(trimethylsilyloxy)copoly[dimethylsiloxane/2‐(2′‐benzophenonyl)ethylmethylsiloxane]), α,ω‐bis[2‐(2′‐benzophenonyl)ethyldimethylsilyloxy]poly(dimethylsiloxane), and 1,3‐bis[2‐(2′‐benzophenonyl)ethyl]tetramethyldisiloxane, respectively. These materials have been characterized with ¹H, ¹³C, and ²⁹Si NMR and IR spectroscopy. Their molecular weight distributions have been determined by gel permeation chromatography. The thermal stability of the polymers has been measured by thermogravimetric analysis, and their glass‐transition temperatures (T_g's) have been determined by differential scanning calorimetry. The molecular weight distribution, thermal stability, and T_g's of the modified polysiloxanes are similar to those of the precursor polymers. The molecular weights of these materials can be significantly increased via heating to 300 °C for 1 h. This may be due to crosslinking, by pyrocondensation, of pendant anthracene groups, which are produced by the pyrolysis of the attached ortho‐alkyl benzophenones. UV spectroscopy of the pyrolysate of 1,3‐bis[2‐(2′‐benzophenonyl)ethyl]tetramethyldisiloxane has confirmed the presence of pendant anthracene groups. Thermal crosslinking by the pyrocondensation of pendant anthracene groups has been verified by the pyrolysis of α,ω‐bis(trimethylsilyloxy)copoly[dimethylsiloxane/2‐(9′‐anthracenyl)ethylmethylsiloxane] (97:3). © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5514–5522, 2004     

Preparation of 5,5′‐pyrilidene and 5,5′‐quinolidene bis‐barbituric acid derivatives

NMR reaction following experiments were used to find optimal conditions for the barbituric acid double addition to aromatic and heteroaromatic carboxaldehydes. It was established that aromatic aldehydes with electron‐donating substituents such as hydroxy, methoxy, and dimethylamino produce only the single addition barbituric acid adduct (barbituric acid benzylidenes). If these electron‐donating substituents are transformed into electron‐withdrawing substituents by virtue of protonation (NMe₂ to NHMe₂⁺) then the double barbituric acid adduct becomes the sole product of the reaction. This is also true regardless of the reaction media if strong electron‐withdrawing substituents (such as a nitro group) are present. Considering that the reactive species for nitrogen containing aromatic heterocycles are actually the conjugated acids (electron deficient molecule) only the double barbituric acid adducts are isolated. All synthetic procedures presented are applicable to multi‐gram scale preparations of double barbituric acid adducts.