Molecular dynamics simulation of semirigid polymers

The chain rigidity of poly(p-hydroxybenzoate) was estimated through the theoretical evaluation of its persistence length (L_p). A non-Brownian molecular dynamics (MD) simulation of an isolated chain with 20 monomeric units was performed. The sampled conformational population was analyzed and the orientational correlation function between monomeric units along the chain was calculated. An algorithm based on the worm-like chain model was applied to evaluate the persistence length. The results were compared with those obtained from equilibrium models like the freely-rotating-chain and the rotational-matrix method with fluctuations. Equilibrium models give different results depending on the degree of accuracy used in describing the monomeric unit. The inclusion of thermal fluctuations is crucial to obtain realistic results. These coincide with those given by MD simulation when only nearest-neighbour orientational correlations are taken into account: inclusion of higher-order correlation terms leads to lower values of the persistence length. The origin of this discrepancy was investigated. The MD simulation results are characterized by an overrepresentation of conformations with a short end-to-end distance resulting from an anomalous energy concentration in the first bending mode of the chain. In analogy with previous simulation results from systems characterized by a week coupling amoung their degrees of freedom, failure in the energy equipartition is proposed as a likely explanation of the anomalous dynamical behaviour.     

Synthesis of new polymeric rhodium carborane complexes serving as homogeneous catalysts for the hydrosilylation of 1-hexene

The coordination of Rh(PPh₃)₃Cl on a dicarbaundecaborate polyamide gives a new polymeric rhodium hydride complex with catalytic activity in the hydrosilylation of 1-hexene by triethylsilane. The rhodium derivative of 7,9-dicarbanido-undecaborate(11)-7,9-dicarboxyanilide was synthesized as a model of the monomeric unit. Co^III and Ni^IV bis(dicarbollyl) complexes also display catalytic activity in hydrosilylation reactions.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 960–962, April, 1991.     

X-ray structure determination of isotactic poly(2-vinylpyridine)

The structure of isotactic poly(2-vinylpyridine) has been determined from x-ray fiber data utilizing rigid-body least-squares refinement techniques. The polymer exists as a threefold helix with three chains passing through a hexagonal unit cell having the dimensions a = b = 15.49 Å and c = 6.56 Å (fiber axis) and space group P3₁ or P3₂. The asymmetric unit consists of three monomeric units (one in each chain), resulting in three crystallographically independent chains in the unit cell. The results show that two of the chains are interlocked through pyridine ring interactions whereas the third chain is less restricted. A statistical structure is proposed in which any two chains in the unit cell can be interlocked, provided they have the same sense. Such a situation permits a completely statistical structure in which half the chains in any crystallite are oriented upward and the other half downward.     

Poly(phosphoramidites)-Polycondensation Products of Phosphorous Triamides with Aliphatic Diols and Dibasic Phenols

Phosphorous triamides react with aliphatic diols and dibasic phenols: 1,6-hexanediol, 2,2'-(4-hydroxyphenyl)propane, 1,4-hydroquinone, and 4,4'-dihydroxybiphenyl in the temperature range 80-110°C in diethylene glycol dimethyl ether or without a solvent to afford phosphorus-containing regular high-molecular products having an O₂P(NR₂) moiety in the monomeric unit. The resulting poly(phosphoramidites) are capable of swelling in organic solvents, and they can be used as macroligands in the catalytic hydrogenation of styrene over rhodium(I) complexes.     

Photoluminescence of biphenyl-containing polymers

Poly(4-vinylbiphenyl) and copolymers of methyl methacrylate and 4-vinylbiphenyl show both monomeric (λ_max = 325 nm) and excimer (λ_max = 380 nm) fluorescence. The quantum yield of excimer emission increases and the monomeric emission decreases with increase in the fraction of vinylbiphenyl units in the copolymer. The decrease of the monomeric emission is closely related to a decrease in singlet lifetime. These results are interpreted in terms of a kinetic controlled excimer formation. Comparison of the emission in the homo and copolymers with that of the dimeric model compound shows that excimer formation in the polymer strongly depends upon the possibility of energy migration along sequences of vinylbiphenyl units. This conclusion is considered as of particular relevance due to the change in geometry of the biphenyl unit upon excitation.     

Synthesis,Characterization, and Catalysis of Water-Soluble Trimeric and Monomeric Palladium Complexes of 8-Aminoquinolines

New water soluble neutral and cationic palladium complexes were synthesized using 8-aminoquinoline (8-AQ) and 2-methyl 8-aminoquinoline (2-Me 8-AQ) ligands and their catalytic properties were evaluated. The neutral trimeric complexes having a Pd₃N₃ core were found to form when Pd(OAc)₂ was reacted with 8-AQ or 2-Me 8-AQ irrespective of the stoichiometry between the 2 reagents. Controlled addition of triflic acid to the neutral trimeric complex resulted in the formation of a trimeric cationic palladium complex as well as a monomeric cationic complex. A cationic palladium complex having two units of 2-Me-8AQ ligand was also synthesized from the cationic monomeric complex. Crystal structures of the new palladium complexes are reported in this study. The water-soluble neutral palladium complex showed catalytic activity for the oxidation of benzyl alcohols to benzaldehydes, while the cationic palladium complexes were found to be efficient catalysts for the oxidation of styrenes to methyl ketones.     

Drei stereoisomere quadratisch-pyramidale [Ma3b2]-Komplexe: Untersuchungen zur Existenz und zur Struktur kristalliner Dimethylsulfoxid-Komplexe von Antimon- und Bismuttrichlorid

Three Stereoisomeric Square-Pyramidal Complexes [Ma₃b₂]: Investigations of the Existence and the Structures of Crystalline Dimethyl Sulfoxide Complexes of Antimony and Bismuth Trichlorides The formation of solid complexes MCl₃ · n DMSO (M = Sb, Bi; n = 1–4) was reinvestigated. In each system, only two of four presumptive complexes could be isolated as crystalline solids: SbCl₃ · DMSO ( 1 a ) was prepared for the first time, SbCl₃ · 2 DMSO ( 1 b ), BiCl₃ · 2 DMSO ( 2 b ) and BiCl · 3 DMSO ( 2 c ) were reproduced according to literature data. Evidence is presented as to the non-existence of BiCl₃ · 4 DMSO, contrary to previous claims in the literature. A unit cell determination showed 2 c to be structurally identical with the monomeric fac-octahedral complex BiCl₃(DMSO)₃ obtained fortuitously and described elsewhere [Z. anorg. allg. Chem. 620 (1994) 1037]. The compounds 1 a (monoclinic, space group P2₁/c), 1 b (monoclinic, space group P2₁/n) and 2 b (monoclinic, space group C2) represent examples of each of the three possible geometric isomers of a square-pyramidal complex [Ma₃b₂]. In the formally 1/1 adduct 1 a , which is in fact [Sb(1)Cl₃(DMSO)₂ · Sb(2)Cl₃], the Sb(1) atom of the complex unit displays square-pyramidal geometry with the DMSO ligands situated in the apical and one of the basal positions. These units are linked into chains by SbCl₃ molecules acting as Cl-acceptors. Sb(2) forms two chelating chloro-bridges with cis-Cl atoms of one neighbouring complex and a third chloro-bridge with the remaining Cl of the symmetry-related second neighbour. The resulting Cl₆-geometry around Sb(2) is distorted octahedral. 1 b consists of square-pyramidal molecules, in which the DMSO ligands occupy two basal cis-positions; the monomeric units are loosely linked through the apical Cl atoms to form a chain of octahedra sharing trans-vertices. The asymmetric unit of 2b is a square pyramid with trans-basal DMSO ligands. The pyramids are connected by symmetry-equivalent basal chlorine atoms into chains of octahedra sharing cis-vertices. 1a displays remarkably short Sb? O dative bonds (204.7/212.9 pm); the M? O bond distances of 1b and 2b are 223.0/234.6 and 234.5/238.7 pm, respectively.     

Mechanism and functional role of antibody catalysis

The light (L) chain of a model antibody (Ab) was deduced to contain a serine protease-like catalytic site capable of cleaving peptide bonds. The catalytic site is encoded by a germline V_L gene. The catalytic activity can potentially be improved by somatic sequence diversification and pairing of the L chain with the appropriate heavy chain. Autoimmune disease is associated with increased synthesis of antigen (Ag)-specific Abs, but the reasons for this phenomenon are not known. Only recently has attention turned to the functional role of the catalytic function. Preliminary studies confirm that the catalytic cleavage of peptide bonds is a more potent means to achieve Ag neutralization, compared to reversible Ag binding. Administration of a monoclonal Ab to VIP in experimental animals induces an inflammatory response in the airways, suggesting that catalytic autoantibodies to this peptide found in airway disease and lupus are capable of causing airway dysfunction. The phenomenon of autoantibody catalysis can potentially be applied to isolate efficient catalysts directed against tumor or microbial Ags by exposing the autoimmune repertoire to such Ags or their analogs capable of recruiting the germline V_L gene encoding the catalytic site.     

Catalytic self-assembled monolayers on Au nanoparticles: the source of catalysis of a transphosphorylation reaction

The catalytic activity of a series of Au monolayer protected colloids (Au MPCs) containing different ratios of the catalytic unit triazacyclononane?Zn^II (TACN?Zn^II) and an inert triethyleneglycol (TEG) unit was measured. The catalytic self‐assembled monolayers (SAMs) are highly efficient in the transphosphorylation of 2‐hydroxy propyl 4‐nitrophenyl phosphate (HPNPP), an RNA model substrate, exhibiting maximum values for the Michaelis–Menten parameters k_cat and K_M of 6.7×10^?3 s^?1 and 3.1×10^?4 M , respectively, normalized per catalytic unit. Despite the structural simplicity of the catalytic units, this renders these nanoparticles among the most active catalysts known for this substrate. Both k_cat and K_M parameters were determined as a function of the mole fraction of catalytic unit (x ₁ ) in the SAM. Within this nanoparticle (NP) series, k_cat increases up till x ₁ ≈0.4, after which it remains constant and K_M decreases exponentially over the range studied. A theoretical analysis demonstrated that these trends are an intrinsic property of catalytic SAMs, in which catalysis originates from the cooperative effect between two neighboring catalytic units. The multivalency of the system causes an increase of the number of potential dimeric catalytic sites composed of two catalytic units as a function of the x ₁ , which causes an apparent increase in binding affinity (decrease in K_M). Simultaneously, the k_cat value is determined by the number of substrate molecules bound at saturation. For values of x ₁ > 0.4, isolated catalytic units are no longer present and all catalytic units are involved in catalysis at saturation. Importantly, the observed trends are indicative of a random distribution of the thiols in the SAM. As indicated by the theoretical analysis, and confirmed by a control experiment, in case of clustering both k_cat and K_M values remain constant over the entire range of x ₁ .     

Molecular engineering of liquid crystal polymers by living polymerization. II. Living cationic polymerization of 11-[(4-cyano-4′-biphenyl) oxy] undecanyl vinyl ether and the mesomorphic behavior of the resulting polymers

The synthesis and living cationic polymerization of 11-[(4-cyano-4′-biphenyl)oxy]-undecanyl vinyl ether ( 6 – 11 ) are described. The mesomorphic phase behavior of poly( 6 – 11 ) with different degrees of polymerization was compared to that of 6 – 11 and of 11-[(4-cyano-4′-biphenyl) oxy] undecanyl ethyl ether ( 8 – 11 ) which is the model compound of the monomeric structural unit of poly( 6 – 11 ). 6 – 11 displays a monotropic S_A and a monotropic nematic mesophase while 8 – 11 an enantiotropic S_A mesophase. Poly( 6 – 11 ) with low degrees of polymerization exhibits an enantiotropic S_A mesophase. Poly( 6 – 8 ) with high degrees of polymerization displays an enantiotropic S_X (i. e., an unidentified smectic phase) and an enantiotropic S_C mesophase. These results demonstrate that the transformation of the nematic mesophase of the monomer into a smectic mesophase after polymerization, occurs at the level of monomeric structural unit.     

Quantitative evaluation of photodegradation and racemization of poly(l-lactic acid) under UV-C irradiation

To obtain details of poly(l-lactic acid) (PLLA) photodegradation behavior, PLLA films were irradiated by UV-C light (λ = 253.7 nm) to directly excite carbonyl groups, resulting in a rapid decrease in the molecular weight accompanying a gradual decrease in the optical purity of monomeric units in the chains. The racemization during the photodegradation was first detected as a result of the chain scission by irradiation. From quantitative analyses of the molecular weight and the monomeric unit composition, it was found that the chain scission ratio and the d-lactate unit ratio increased in parallel during the irradiation, suggesting that approximately one d-lactate unit formed for every chain scission. From a mechanistic consideration, the racemization equilibrium was proposed to occur at both carboxyl and hydroxyl chain ends.     

Coenzyme models. 32. Catalytic activity of polymer-bound thiazolium ions as estimated by flavin trapping technique

Three thiazolium-containing polymers, Th-7, Th-33, and Th-18-Py [where Th-x and Py mean x mol % thiazolium unit and pyridinium unit (54 mol %), respectively], were synthesized from partially p-chloromethylated polystyrene. The catalytic activities of these polymer catalysts in acyloin condensation of aldehydes and decarboxylation of α-keto acids were estimated kinetically by oxidative trapping of the key intermediate by flavin (flavin-trapping technique). In aqueous solution at 30°C, the catalytic activity of Th-18-Py and Th-33 in condensation of p-chlorobenzaldehyde was comparable with that of the cationic-micelle-bound thiazolium ion, whereas Th-7 and a monomeric thiazolium compound (N-benzylthiazolium bromide) scarcely exhibited any catalytic activity. The catalysis of the polymer-bound thiazolium ions was sensitively suppressed by increased ionic strength. These results suggest that the pendent thiazolium ion is activated by the relatively high charge density along the polymer chain: the cationic environment is able to facilitate dissociation of the thiazolium ion to the ylid form and deprotonation of the thiazolium-aldehyde adduct to the key intermediate.     

Bis(2‐but­yl‐N,N′‐diisopropyl­amidinato)dichloro­hafnium(IV)

The title compound, [Hf(C₁₁H₂₃N₂)₂Cl₂], is a monomeric hafnium(IV) complex containing two bidentate amidinate ligands and two cis Cl atoms. The crystals are triclinic (space group ) and there is one independent six‐coordinate monomer with a highly distorted octa­hedral geometry in the asymmetric unit. The reported structure is the first hafnium–amidinate complex to be characterized successfully by single‐crystal X‐ray diffraction.     

Crystal structure of polyisobutylene

The crystal structure of polyisobutylene was determined by x-ray analysis. The orthorhombic cell, with a = 6.88 Å, b = 11.91 Å, c (fiber axis) = 18.60 Å (space group: P2₁2₁2₁ ? D), contains two molecular chains each consisting of eight monomeric units in the fiber identity period. The chain conformation is essentially an (8/3) helix, but deviates appreciably from the exact (8/3) helix symmetry. The symmetry of the molecular chain is only a twofold screw axis in exact sense, and a crystallographic asymmetric unit consists of four monomeric units. The torsional angles are where M denotes the methyl group. The averaged skeletal C? CH₂? C and C? CM₂? C bond angles are 128° and 110°, respectively. The large C? CH₂? C bond angles may be due to steric respulsion between the adjacent methyl groups, giving intramolecular distances larger than 3.09 Å.     

Synthesis of pillar[5]arene dimers and their cooperative binding toward some neutral guests

Three pillar[5]arene dimers, bridged by a flexible aliphatic chain (H1) or a relatively rigid phenylene unit (H2 and H3), were synthesized, with the possible synthetic strategies being discussed. The dimers could significantly enhance the binding affinities toward neutral model substrates in comparison with monomeric 1,4-dimethoxypillar[5]arene (H4) through the cooperative binding of two pillar[5]arene moieties. The molecular binding ability and selectivity are discussed from the viewpoints of the size/shape-fit concept and multiple recognition mechanism.     

Reversible solvent and temperature induced “monomeric-like” —“Polymeric-like” transitions in alkyl bonded silica

Summary The study of behaviour of homologous series (and more precisely, the plots of methylene selectivity versus the carbon number, n, of the alkyl chain of the homologue) on monomeric and polymeric phases in liquid chromatography enabled us to characterize two kinds of retention mechanism. These mechanisms are not independent; changes in the temperature or mobile phase composition allow passage from one to the other. The first, for which the methylene selectivity versus n decreases with a discontinuity, defines a retention mechanism where solutes are inserted between the grafted chains. It occurs for monomeric phases at ambient temperature, for polymeric phases, at high temperature or with a strong solvent like THF, CH₂Cl₂, AcMe or CHCl₃. The second gives a plot of methylene selectivity versus n with a maximum. It is consistent with a partition mechanism with total immersion of the solute in the bed of the stationary phase. It occurs for a polymeric phase at ambient temperature or for a monomeric one at low temperature.     

Crystal structure of poly(ω‐pentadecalactone)

The crystal structure of poly(ω‐pentadecalactone) (PPDL) synthesized by enzyme‐catalyzed polymerization was determined by full‐profile refinement. A pseudo‐orthorombic monoclinic unit cell with dimensions a = 7.49(1), b = 5.034(9), and c = 20.00(4)Å (fiber axis), and α = 90.06(4)° hosts two monomeric units belonging to polymer chains with opposite orientation, according to the P2₁ space‐group symmetry. A close similarity to the crystal structure of poly(?‐caprolactone) was evident. However, the even number of backbone atoms in the monomer unit of PPDL leads to a lower crystal symmetry. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1009–1013, 2003     

Copper(II) complexes containing pyridine-based and phenolate-based systems: Synthesis,characterization, DFT study,biomimetic catalytic activity of catechol oxidase and phenoxazinone synthase

A template Schiff condensation of 2,6-pyridine dicarbaldehyde or 2,6-diformyl-4- bromophenol and 1,3–diamino-2-hydroxy propane or 3,4-diaminotoluene in the presence of copper(II) salts (CuX₂) (X = Cl, Br, CH₃COO, or ClO₄) affords different types of copper(II) complexes. Depending on the employed molar ratio of the dicarbonyl compounds and diamines, different types of copper(II) complexes formed during the template condensation reaction. Structural formulation of the complexes was confirmed by elemental analysis (C, H, N, and M), physical measurements such as thermal analysis (TAG & DTG), molar conductivity, and magnetic moments in addition to spectral studies (UV–Vis, IR, and ESR). Homobinuclear in a four-coordinate square planar and five-coordinate square pyramidal and trigonal bipyramidal in monomeric structures are proposed. A mononuclear hexa-coordinate in an octahedral geometry is suggested as well. Oxidase biomimetic catalytic activity of these newly synthesized copper(II) complexes was examined toward the aerobic oxidation of 4-tert-butylcatechol (4-TBCH₂) and o-aminophenol under catalytic conditions. Both catalytic and kinetic investigations demonstrate promising oxidase catalytic activity and based on the kinetic results, probable mechanistic catalytic implications are discussed. Geometrical structures of representative copper(II) complexes were determined by optimizing their bond lengths, bond angles, dihedral angles, and the structural index (τ).     

Modelling of θ-conditions for bisphenol—A polycarbonate single chains

Assessing conformational dimensions of macromolecules is a topic of long-standing interest. Because of its simplicity, it is attractive to investigate the chain properties in θ-conditions. Under these special conditions, the effects of excluded volume of the segments of the polymer chain vanish. The molecular chain is only subject to local constraints resulting from the bond structure and the hindrance to rotations about bonds. To model θ-conditions a contour length dependent cutoff is introduced ensuring that only nonbonded interactions of atoms of neighbouring monomeric units are taken into account for energy calculations. Using this energy model we will show that it is possible to model θ-conditions of a single bisphenol-A polycarbonate (BPA-PC) chain in vacuum by two different methods: (i) (Pseudo-) Langevin dynamics simulations and (ii) regular reassignment of randomly generated atom velocities during a molecular dynamics simulation. Both methods can be used to avoid oscillative dynamic behaviour of the chain. Calculations of the end-to-end vector and the radius of gyration of the equilibrium ensembles derived from simulations at different temperatures show good agreement with experimental data. Thus our model techniques are well suited to simulate θ-conditions with small computational expense.