Syndiotactic‐specific radical polymerization of N,N‐dimethylacrylamide in the presence of tartrates: A proposed mechanism for the polymerization

Radical polymerization of N,N‐dimethylacrylamide (DMAAm) was investigated in the presence of tartrates, such as diethyl L ‐tartrate, diisopropyl L ‐tartrate, and di‐n‐butyl L ‐tartrate, in toluene at low temperatures. Syndiotactic polymers were obtained in the presence of tartrates, whereas isotactic polymers were obtained in the absence of tartrates. The syndiotactic‐specificity increased with increasing amount of tartrates and with decreasing polymerization temperature. NMR analysis suggested that DMAAm and tartrates formed a 1:1 complex through double hydrogen bonding. A mechanism for the syndiotactic‐specific radical polymerization of DMAAm is proposed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1192–1203, 2009     

The effect of the N‐substituent s‐trans to the carbonyl group of N‐methylacrylamide derivatives on the stereospecificity of radical polymerizations

Radical polymerization of N‐methylacrylamide (NMAAm), N,N‐dimethylacrylamide (DMAAm), and N‐methyl‐N‐phenylacrylamide (MPhAAm) was investigated in toluene at low temperatures. Atactic, isotactic, and syndiotactic polymers were obtained by the polymerization of NMAAm, DMAAm, and MPhAAm, respectively, indicating that the stereospecificity of the radical polymerization of acrylamide derivatives depended on the N‐substituents of the monomer used. From the viewpoint of monomer structure, the origin of the stereospecificity of radical polymerization of NMAAm derivatives is discussed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6534–6539, 2009     

Sensitivity of the NTB phase formation to the molecular structure of imino-linked dimers

ABSTRACT

Here we report on the synthesis and mesomorphic properties of a series of imino-linked dimeric molecules. In order to improve our understanding of the structure–N_TB phase correlations, we have studied the impact of geometric and electronic factors arising from varying mesogenic units, different spacer lengths and from the ratio (n/m) between the lengths of terminal chains (n) and spacer (m). From the perspective of the molecular geometry, the results show that the stability of the N_TB phase results from increasing effective molecular bending and with the broadening of the mesogenic unit, in particular near the spacer, and that the n/m ratio plays a substantial role in conjunction with the specific mesogenic unit. A computational study of the electronic properties shows that a broadening of the mesogenic core in the vicinity of the spacer is associated with an increased anisotropy of the electrostatic potential distribution. Within a given series of materials our study suggests that the incidence of the N_TB phase and its thermal stability are governed by the synergy of specific geometrical factors and the anisotropy of the electrostatic potential distribution of the mesogenic core.     

Non-adiabatic dynamics of pyrrole: Dependence of deactivation mechanisms on the excitation energy

Non-adiabatic dynamics simulations were performed for pyrrole at time-dependent density functional theory level using the trajectory surface hopping approach. Initial conditions were prepared based on the UV-absorption spectrum so as to simulate monochromatic absorption in three distinct spectral regions. The results showed predominance of the NH-stretch mechanism for excited-state relaxation. With increasing initial energy, however, other mechanisms are activated as well, even though they still occurred for a minor fraction of the trajectories. Dynamics starting at the origin of the absorption spectrum exhibited internal conversion to the ground state with a time constant of 20 fs. In contrast, dynamics starting at higher energies gave rise to much longer time constants for internal conversion near 200 fs.     

Evidence for a biogenic, microorganismal origin of rock varnish from the Gangdese Belt of Tibet

In the present study we examined material from the Ashikule Basin of Tibet. Chemical analyses were performed by use of energy dispersive X-ray spectroscopy and electron probe microanalysis to clarify whether the varnish layers that had developed on the surface of the rhyolite are indeed composed of varnish bodies and silica glaze. Electron microscopic analyses revealed that the surface of the varnish is covered both by filamentous hyphae bacterial and cocci-shaped forms. Within the varnish mineral layer in those samples two forms of bacteria-like microorganisms exist; cocci as tightly packed bacterial aggregates [within varnish bodies], and bacillus-like microorganisms [within the varnish matrix, that surrounds the varnish bodies]. The bacillus-like forms are embedded in a network of filaments that have diameters between 35 and 45 nm. These bacilli with the surrounding filaments are assumed to have been involved in biofilm formation (synthesis of extracellular polymeric substances) prior to their live burial. We concluded that the formation of the varnish layers was for the most part biogenically driven by microorganisms.     

Silicon-containing polyphilic bent-core molecules: the importance of nanosegregation for the development of chirality and polar order in liquid crystalline phases formed by achiral molecules

Polyphilic molecules composed of a bent aromatic core, oligo(siloxane) units, and alkyl segments were synthesized, and the self-organization of these molecules was investigated. Most materials organize into polar smectic liquid crystalline phases. The switching process of these mesophases changes from antiferroelectric for the nonsilylated compounds via superparaelectric to surface-stabilized ferroelectric with increasing segregation of the silylated segments. It is proposed that the siloxane sublayers stabilize a polar synclinic ferroelectric (SmC(s)P(F)) structure, and the escape from a macroscopic polar order as well as steric effects leads to a deformation of the layers with formation of disordered microdomains, giving rise to optical isotropy. Another striking feature is the spontaneous formation of chiral domains with opposite handedness. For two compounds, a temperature-dependent inversion of the optical rotation of these domains was found, and this is associated with an increase of the tilt angle of the molecules from < 45 degrees to > 50 degrees. This observation confirms the recently proposed concept of layer optical chirality (Hough, L. E.; Clark, N. A. Phys. Rev. Lett. 2005, 95, 107802), which is a new source of optical activity in supramolecular systems. With increasing length of the alkyl chains, segregation is lost and a transition from smectic to a columnar phase is found. In the columnar phase, the switching process is antiferroelectric and takes place by rotation of the molecules around the long axes, which reverses the layer chirality; that is, the racemic ground-state structure is switched into a homogeneous chiral structure upon application of an electric field.     

Synthesis and structure of the scandium‐rich indides Sc5Ni2In4 and Sc5Rh2In4

The ternary indides Sc₅ Ni₂ In₄ and Sc₅ Rh₂ In₄ were synthesized by arc‐melting of the elements and subsequent annealing. A structural investigation by X‐ray powder and single crystal diffraction revealed: Lu₅ Ni₂ In₄ type, Pbam, a = 1716.3(2), b = 755.1(1), c = 335.22(5) pm, wR2 = 0.0721, 844 F² values for Sc₅ Ni₂ In₄, and a = 1754.3(3), b = 765.0(1), c = 332.97(6) pm, wR2 = 0.0363, 1107 F² values for Sc₅ Rh₂ In₄ with 36 variables per refinement. Both structures can be described as intergrowths of distorted AlB₂‐ and CsCl‐related slabs, where the transition metal (T) atoms have a trigonal prismatic and the indium atoms a distorted square prismatic coordination. The shortest interatomic distances occur for Sc T and T In. The crystal chemistry and chemical bonding in these intermetallics are briefly discussed. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:364–368, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20106     

Heterogeneous freezing on pyroelectric poly(vinylidene fluoride-co-trifluoroethylene) thin films

Active deicing of technical surfaces, such as for wind turbines and heat exchangers, currently requires the usage of heat or chemicals. Passive coating strategies that postpone the freezing of covering water would be beneficial in order to save costs and energy. One hypothesis is that pyroelectric active materials can achieve this because of the surface charges generated on these materials when they are subject to a temperature change. High-quality poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) thin films with a high crystallinity, prefererd edge-on orientation, low surface roughness, and comprised of the β-analogous ferroelectric phase were deposited by spin-coating. Freezing experiments with a cooling rate of 1 K min⁻¹ were made on P(VDF-TrFE) coatings in order to separate the effect of different parameters such as the poling direction, film thickness, used solvent, deposition process, underlying substrate, and annealing temperature on the achievable supercooling. The topography and the underlying substrate significantly changed the distribution of freezing temperatures of water droplets in contact with these thin films. In contrast, no significant effect of the thickness, morphology, or pyroelectric effect of the as-prepared domain-state on the freezing temperatures was found.