Armand Soldera

Energetic analysis of the two PMMA chain tacticities and PMA through molecular dynamics simulations

Abstract Simulated dilatometry techniques have been applied to compute the glass transition temperatures, T g s, of the two poly(methyl methacrylate) (PMMA) chain tacticities, and poly(methyl acrylate) (PMA). Since the difference in T g s between the two configurations was accurately simulated, further analysis could be carried out. This article more particularly deals with energetic and structural analysis of the difference. Thus this analysis showed that the non-bond energy and the bending angle energy associated with the intradiad backbone angle, principally contribute to the energetic difference between the two PMMA configurations. Following the free volume theory, these two energetic variations allow an increase in T g in comparison to PMA, and an enlargement of the difference in the T g s between the two PMMA configurations. Actually, these two energetic contributions stem from the substitution of the hydrogen atom attached to the chiral carbon atom in the PMA repeat unit by a methyl group. The same behavior is encountered with the two poly(ethyl methacrylate) (PEMA) chain tacticities.

A strategic approach to the synthesis of functionalized spirooxindole pyrrolidine derivatives: in vitro antibacterial, antifungal, antimalarial and antitubercular studies

A series of spiro[pyrrolidin-2,3′-oxindoles] has been synthesized by exo-selective 1,3-dipolar cycloaddition reaction of a stabilized azomethine ylide, generated in situ by thermal [1,5]-prototropy, across various (E)-3-arylidene-1-phenyl-pyrrolidine-2,5-diones. The stereochemistry of these N-heterocycles has been confirmed using an X-ray diffraction study. To rationalize the observed regio- and stereoselectivity, DFT calculations at the B3LYP/6-31G(d,p) level were employed. It was found that this reaction preferentially affords the kinetic product. The compounds have been screened for their in vitro antibacterial, antifungal, antimalarial and antitubercular activities. Several compounds exhibited good activities comparable to those of established standard drugs.

Regio- and Stereoselective Synthesis of Spiropyrrolizidines and Piperazines through Azomethine Ylide Cycloaddition Reaction

A series of original spiropyrrolizidine derivatives has been prepared by a one-pot three-component [3 + 2] cycloaddition reaction of (E)-3-arylidene-1-phenyl-pyrrolidine-2,5-diones, l-proline, and the cyclic ketones 1H-indole-2,3-dione (isatin), indenoquinoxaline-11-one and acenaphthenequinone. We disclose an unprecedented isomerization of some spiroadducts leading to a new family of spirooxindolepyrrolizidines. Furthermore, these cycloadducts underwent retro-1,3-dipolar cycloaddition yielding unexpected regioisomers. Upon treatment of the dipolarophiles with in situ generated azomethine ylides from l-proline or acenaphthenequinone, formation of spiroadducts and unusual polycyclic fused piperazines through a stepwise [3 + 3] cycloaddition pathway is observed. The stereochemistry of these N-heterocycles has been confirmed by several X-ray diffraction studies. Some of these compounds exhibit extensive hydrogen bonding in the crystalline state. To enlighten the observed regio- and stereoselectivity of the [3 + 2] cycloaddition, calculations using the DFT approach at the B3LYP/6-31G(d,p) level were carried out. It was found that this reaction is under kinetic control.

Computation of densities, bulk moduli and glass transition temperatures of vinylic polymers from atomistic simulation

The aim of molecular modeling is to mimic reality by considering approximations appropriate to the scale at which the simulation is carried out. At the atomic level, forcefields that represent average atomic interactions are used. However, the phase space has to be adequately explored in order to compare successfully computed and experimental properties. The procedure exposed in this article considers an initial selection of relevant configurations on which a simulated annealing process is applied using the first generation forcefield OPLS, followed by a uniform hydrostatic compression using the second generation forcefield COMPASS©. The resulting data are fitted by an equation of state, from which density and bulk modulus are determined. The glass transition is then simulated and T gs are computed. Our approach is tested using a series of vinylic polymers, which differ from each other by small variations in atomic interaction combinations. The excellent agreement with experimental data shows the validity of the procedure exposed. Moreover, a clear linear relationship between simulated and experimental T gs is revealed.

Functionalization of molecular glasses: effect on the glass transition temperature

Molecular glasses constitute an appealing class of materials combining the advantages associated with small molecules with the potential to form glassy phases. However, the current design of functional molecular glasses is highly dependent on a trial and error approach. By achieving a better understanding of the microscopic behaviors that govern the propensity of a system to generate a glassy state, the glassy behavior of new molecules can be identified before their synthesis. Full-atomistic simulation is genuinely invaluable for achieving this goal. A series of functionalized triazine-based molecular glasses previously synthesized and characterized were thus used as models to carry out simulations. To estimate the link between microscopic calculations and macroscopic properties, mechanical equilibrium and isotropy have first to be achieved. The reproducibility of the simulation results and their linear correlations with experimental data are two essential criteria for corroborating the validity of our method. Mobility of the core and the functional “headgroups” can then be interpreted, rationalizing the effect of molecular structure on the formation of glasses, and on the important differences in Tg observed for this series of compounds.

Tacticity induced molecular microstructure dependence of the configurational properties of metallocene-synthesized polypropylenes

Stereoblock polypropylene (PP) produced using metallocene catalysts consists of alternating stereoregular and stereoirregular sequences. Through a control of the isotactic and atactic sequence lengths, the Bernoullian replication probability and the rate of isomerization of the metallocene catalysts, polymeric chains of various molecular microstructures could be generated. The dependence of the microstructure on the various polymerization parameters is examined in terms of the isotactic pentad content, [mmmm]. The dependence of the physical properties of PP on the molecular microstructure induced by the chain tacticity was also investigated both experimentally and theoretically using Metropolis Monte Carlo techniques. It was shown that the intrinsic viscosity and the characteristic ratio of the polymer are greatly influenced by the stereoregularity of the polymeric chains, which was attributed to a possible steric hindrance between the neighbouring methyl side groups. Other configuration-dependent physical properties such as the radius of gyration, the persistence length and the molar stiffness function were also investigated for PP of different stereoregular microstructures. It was shown by examining the distribution of the dihedral angles that the pentane effect resulting from the steric hindrance of the side groups is the underlying principle for the microstructure dependence of the configurational properties of stereoblock PP.

Pyridinium salt liquid crystals Effect of mesogen extension and alkyl chain length

Abstract The thermotropism of 1-n-alkyl-(4-methyl and 4-tolyl)pyridinium bromides were compared for alkyl chain lengths ranging from n = 12 to 22 carbons. A smectic A mesophase is present in both series for the longer chain compounds, n ≥ 16, with the clearing temperature being similar for both series but increasing rapidly with chain length. The series with the elongated mesogen also possesses an ordered mesophase identified as smectic G. The transition between this mesophase and the SA or isotropic phase in the 4-tolyl series, and the transition to and from the crystalline phase in both series, are affected relatively little by the alkyl chain length. It seems that the SA mesophase is governed primarily by the amphiphilic character of the substances, whereas elongation of the ionic head group is responsible for the appearance of a more ordered mesophase at intermediate temperatures.

Mid-infrared optical properties of a polymer film: comparison between classical molecular simulations, spectrometry, and ellipsometry techniques

Abstract Infrared optical properties of amorphous polymers, polymethylmethacrylate and polychlorotrifluoroethylene, are predicted following a procedure based on molecular simulation and Kramers–Kronig relations. The amorphous phase of polymer is first simulated with periodic boundary conditions. Normal mode vibrations and integrated intensities are then extracted from simulation data. These parameters allow the computation of the imaginary part, k , of the complex refractive index, N . Using Kramers–Kronig relations, the real part, n , of N , can be determined. The resulting spectrum is compared to spectra coming from two experimental techniques, spectrometry and ellipsometry. Three observations follow from such a comparison. Firstly, the ellipsometry is the most appropriate technique for this specific measurements. Secondly, some problems still exist in the intensity determination in the low frequency region. Thirdly, the mid-IR region is accurately depicted.

Luminescent P-Chirogenic Copper Clusters

P-chirogenic clusters of the cubanes [Cu4I4L4] (L = chiral phosphine) were prepared from (+)- and (-)-ephedrine with L = (S)- or (R)-(R)(Ph)(i-Pr)P (with R = CH3 (seven steps) or C17H35 (10 steps)) with e.e. up to 96%. The X-ray structure of [Cu4I4((R)-(CH3)(Ph)(i-Pr)P)4] confirmed the cubane structure with average Cu···Cu and Cu···I distances of 2.954 and 2.696 Å, respectively. The cubane structure of the corresponding [Cu4I4((S)-(CH3)(Ph)(i-Pr)P)4] was established by the comparison of the X-ray powder diffraction patterns, and the opposite optical activity of the (S)- and (R)-ligand-containing clusters was confirmed by circular dichroism spectroscopy. Small-angle X-ray scattering patterns of one cluster bearing a C17H35 chain exhibit a weak signal at 2θ ~ 2.8° (d ~ 31.6 Å), indicating some molecular ordering in the liquid state. The emission spectra exhibit two emission bands, both associated with triplet excited states. These two bands are assigned as follows: the high energy emission is due to a halide-to-ligand charge transfer, XLCT, state mixed with LXCT (ligand-to-halide-charge-transfer). The low energy band is assigned to a cluster-centered excited state. Both emissions are found to be thermochromic with the relative intensity changing between 77 and 298 K for the clusters in methylcyclohexane solution. Several differences are observed in the photophysical parameters, emission quantum yields and lifetimes for R = CH3 and C17H35. The measurements of the polarization along the emission indicate that the emission is depolarized, consistent with an approximate tetrahedral geometry of the chromophores.

Study of the influence of ester orientation on the thermal stability of the smectic C phase: Simulation investigation

Two series of mesogenic molecules have been simulated using molecular dynamics with a full atomistic representation. The two series differ from each other only with the orientation of an ester linker inside the rigid core. Experimentally, this small difference leads to drastically different polymorphisms: one displays the smectic A phase while the other exhibits the smectic C and the smectic A phase. From molecular dynamics trajectories, intermolecular potentials are extracted and analyzed. It is then found that the minor difference inside the rigid core can be related to these various interactions. Finally, experimental and simulation results are integrated in a coherent analysis. The correlation between both types of data offers interesting insights in the role of the Coulomb and van der Waals interactions in the molecular origin of the smectic C phase.