Search results for "RPE"

Dipolar Relaxation in Functionalized Poly-p-phenylenes Bearing Ultrastrong Dipoles Perpendicular to the Backbone

2018

Local polymer dynamics are studied in polymers bearing dipoles rigidly attached to the backbone. The compounds are based on cyano-substituted dihydrobenzimidazoles bearing ultrastrong dipole moments (∼12 D per repeat unit) incorporated in a poly-p-phenylene backbone, giving rise to polymers with rigid dipoles perpendicular to the chain. They belong to type B polymers according to the Stockmayer classification. They are ideal model systems for studying rotational isomers in the gas phase and the self-assembly and local dynamics in the solid state. Gas phase calculations (DFT) provided the dipole moments, the energetic barriers, and the backbone conformation as a function of the dipole streng…

Structure and dynamics of grafted polymer layers: A Monte Carlo simulation

1991

The bond fluctuation model of polymer chains on lattices is used to study layers of polymers anchored with one end at a hard wall, assuming good solvent conditions and repulsive interactions between the monomers and the wall. Chain lengths from N=10 to N=80 and grafting densities σ from 0.025 to 0.20 are considered, both for the ‘‘quenched’’ case, where the anchor points are kept fixed at randomly chosen surface sites, and the ‘‘annealed’’ case, where lateral diffusion of the anchored ends at the wall is considered. Profiles of monomer density and free end density, chain linear dimensions parallel and perpendicular to the wall, as well as corresponding mean square displacements of inner and…

Anomalous Structure and Scaling of Ring Polymer Brushes

2011

A comparative simulation study of polymer brushes formed by grafting at a planar surface either flexible linear polymers (chain length $N_L$) or (non-catenated) ring polymers (chain length $N_R=2 N_L$) is presented. Two distinct off-lattice models are studied, one by Monte Carlo methods, the other by Molecular Dynamics, using a fast implementation on graphics processing units (GPUs). It is shown that the monomer density profiles $\rho(z)$ in the $z$-direction perpendicular to the surface for rings and linear chains are practically identical, $\rho_R(2 N_L, z)=\rho_L(N_L, z)$. The same applies to the pressure, exerted on a piston at hight z, as well. While the gyration radii components of ri…

1984

The fiber patterns of a series of liquid crystalline polymers were analysed. For polymers with smectic phases and for polymers with long spacers and nematic phases, the mesogenic groups orient perpendicularly to the polymer main chains which are parallel to the fiber axis. The fiber pattern indicate a short range order of the mesogenic groups analogous to that in low-molecular weight liquid crystals. For polymers with short spacer groups and nematic phases qualitatively different fiber patterns are found. These fiber patterns are indicative for a special nematic phase formed by ribbon-like molecules.

Lateral versus perpendicular segregation in mixed polymer brushes.

2002

Grafting of incompatible polymers on a substrate prevents macrophase separation and the chains self-assemble laterally. Mixed brushes are exposed to different solvents and the morphology is observed via atomic force and x-ray photoemission microscopy. In a nonselective solvent the different species segregate into parallel cylinders ("ripple structure"). Upon exposure to a selective solvent, we encounter a transition to a "dimple" structure, in which the unfavored component forms clusters. Simultaneously, we observe an enhanced perpendicular segregation. The experimental observations are compared to self-consistent field calculations, where qualitative agreement is found.

Anisotropic Effect after Stretching of the Spin-Crossover Compound [Fe(II)(2,6-bis-(benzimidazol-2′-yl)pyridine)2] (C1O4)2 in Polyvinylalcohol Polyme…

1999

Abstract [Fe(II)(2,6-bis-(benzimidazol-2′-yl)-pyridine)2](C1O4)2 diffused into a poly(vinyl)alcohol polymer matrix, exhibits an thermally induced spin-crossover. After stretching the matrix, the compound orients itself in the matrix. Polarised UV/VIS spectra measured parallel and perpendicular to the stretching direction allowed to calculate the distortion of the molecule in the matrix by means of semi-empirical PPP SCF-CI calculations.

30-Norfriedelane triterpenes from the stem bark ofCaloncoba glauca

1992

Abstract The stem bark of Caloncoba glauca has yielded three friedelane triterpenes of which two are characterized by the loss of C-30 and the presence of a C-27, C-15 lactone. They were identified, with considerable use of long-range C H coupling studies, as the known compound trichadonic acid (3-oxofriedelan-27-oic acid) and two new 30-norfriedelane derivatives, 3,27-dioxo-30-norfriedelan-20(29)-en-27,15α-lactone (caloncobalactone) and 3,27-dioxo-21β-hydroxy-30-norfriedelan-20(29)-en-17,15α-lactone (21β-hydroxycaloncoba lactone).

Friedelane triterpenes from the stem bark of Caloncoba glauca

1993

Further investigation of the stem bark of Caloncoba glauca has yielded three minor (D:A)-friedo-oleanane derivatives. These have been identified, on the basis of NMR studies, as the known compound kokoonol [3-oxo-(D:A)-friedo-olean-27-ol] and the novel natural products kokoonal [3-oxo-(D:A)-friedo-olean-27- al] and 3β,21β-dihydroxy-30-nor-(D:A)-friedo-olean-27-oic acid.

Polyacanthoside A, a new oleanane-type triterpenoid saponin with cytotoxic effects from the leaves of Acacia polyacantha (Fabaceae)

2018

The chemical investigation of the leaves and stem bark of Acacia polyacantha (Fabaceae) led to the isolation of a new oleanane-type triterpenoid saponin named polyacanthoside A 1 together with fifteen known compounds. Their structures were established from spectral , mainly HRESIMS, 1D NMR and 2D NMR and by comparison with literature data. The cytotoxicity of compound 1 and the analogues 8 as well as doxorubicin was determined in a panel of 9 cancer cell lines including sensitive and drug resistant phenotypes. Unlike the analogue 8, compound 1 as well as doxorubicin displayed cytotoxic effects in all the 9 tested cancer cell lines with IC50 values ranged from 8.90 μM (towards CCRF-CEM leuke…

Stereoselective synthesis of 7,11-guaien-8,12-olides from santonin. Synthesis of podoandin and (+)-zedolactone A.

2000

Photochemical rearrangement of hydroxy ester 2, easily obtained from santonin (1), afforded butenolide 4, a good starting material for the synthesis of 7,11-guaien-8,12-olides. Compound 4 has been transformed into compound 10, which has been used for the synthesis of podoandin (5) and (+)-zedolactone A (ent-6). Regioselective elimination of the acetyl group on C10 afforded directly podoandin (5). For the synthesis of ent-6, a hydroxyl group has been regio- and stereoselectively introduced at the 4alpha-position through the 3alpha,4alpha-epoxide 15. The basic hydrolysis of the 10-acetyl group in compound 18 took place with concomitant intramolecular conjugated addition of the alkoxide to the…