Trans-planar Conformation Research Articles

Crystal Structure of Poly(7-heptalactone)

The crystal structure of poly(7-heptalactone) (PHL) is presented. PHL has been synthesized by a combination of ring-opening polymerization of η-heptalactone and reversible addition–fragmentation chain transfer polymerization using diphenyl phosphate as catalyst. The noncommercial available η-heptalactone monomer has been prepared by oxidation of cycloheptanone. The crystal structure of PHL has been determined by analysis of the X-ray powder diffraction profiles and of the electron diffraction patterns of single crystals and calculations of lattice energy. The structure has been refined with the Rietveld full-profile refinement. Chains in trans planar conformation are packed in an orthorhombic unit cell with axes a = 7.37 Å, b = 5.05 Å, and c = 10.07 Å according to the space group symmetry P21.

Mechanical Properties and Elastic Behavior of Copolymers of Syndiotactic Polypropylene with 1-Hexene and 1-Octene

The mechanical properties of copolymers of syndiotactic polypropylene (sPP) with 1-hexene and 1-octene have been studied and correlated with the structural transformations occurring during stretching and relaxation. These copolymers crystallize in disordered modifications of form I of sPP, with crystallinity, melting, and glass-transition temperatures that decrease with increasing comonomer content. All copolymers show remarkable mechanical properties and elastic behavior, with great improvement of flexibility and ductility with respect to the sPP homopolymer already at small comonomer concentrations. For low comonomer content up to 4–5 mol %, the elastic recovery is associated to a reversible conformational phase transition between the trans-planar form III or the trans-planar mesophase and the helical form II, which gives an enthalpic contribution to the elasticity. As the comonomer content increases, the trans-planar conformation is progressively destabilized and the observed elastic properties are not associated with any conformational transformation during stretching and relaxation. In these cases, copolymers behave as conventional thermoplastic elastomers where elasticity is purely entropic and crystals only act as topological constraints of the elastomeric network. These copolymers represent an exemplary case of the possibility to combine in the same materials desired and apparently incompatible physical properties of elasticity, crystallinity, and strength by controlling the crystallization behavior, achieved by tailoring the chemical structure in the synthetic process.

Investigation on the overlapping bands of syndiotactic polystyrene by using 2D-IR spectroscopy

In this work, WAXD and FTIR spectroscopy were utilized to investigate the phase transition of syndiotactic polystyrene (sPS) from amorphous phase to mesophase during the isothermal annealing process at 130 °C. Two dimensional (2D) correlation infrared spectroscopy was applied to reveal the sub-bands from the highly overlapping bands. The ∼900 cm−1 band is shown to be composed of two sub-bands. One band located around 906 cm−1 corresponds to the amorphous phase, another peak that occurs around 900 cm−1 is associated with mesophase. The trans-planar conformation band at 1223 cm−1 turns out to consist of two bands which might be related to trans-planar conformation with different sequence lengths.

Structure of Isotactic Ethylene/4-Methyl-1,3-pentadiene Alternating Copolymers Obtained from Postmetallocene Catalysts

The structure and thermal properties of a new class of isotactic copolymers of ethylene (E)/4-methyl-1,3-pentadiene (4MPD) with 4MPD content in the range 40–73 mol % and of isotactic poly-1,2-(4-methyl-1,3-pentadiene) (iP4MPD) homopolymer have been analyzed. The copolymers have been synthesized using 1,4-dithiabutanediyl-linked bis(phenolate) titanium complexes as catalyst precursors, activated by methylaluminoxane. Solution 13C NMR analysis reveals the tendency of the catalyst system to produce copolymers with alternating 4MPD/E constitution. All copolymers are crystalline and show melting temperatures in the range 90–100 °C. The samples with nearly equimolar concentration of units crystallize in a new crystalline form because of the alternating constitution. Structural analysis indicates that in this new form the alternating 4MPD-E sequences assume a nearly trans-planar conformation with periodicity c = 5.13 A and are arranged in a monoclinic unit cell with parameters a = 5.70 A, b (unique axis) = 14.95...

Molecular Engineering of Nonhalogenated Solution-Processable Bithiazole-Based Electron-Transport Polymeric Semiconductors

The electron deficiency and trans-planar conformation of bithiazole is potentially beneficial for the electron-transport performance of organic semiconductors. However, the incorporation of bithiazole into polymers through a facile synthetic strategy remains a challenge. Herein, 2,2′-bithiazole was synthesized in one step and copolymerized with dithienyldiketopyrrolopyrrole to afford poly(dithienyldiketopyrrolopyrrole-bithiazole), PDBTz. PDBTz exhibited electron mobility reaching 0.3 cm2 V–1 s–1 in organic field-effect transistor (OFET) configuration; this contrasts with a recently discussed isoelectronic conjugated polymer comprising an electron-rich bithiophene and dithienyldiketopyrrolopyrrole, which displays merely hole-transport characteristics. This inversion of charge-carrier transport characteristics confirms the significant potential for bithiazole in the development of electron-transport semiconducting materials. Branched 5-decylheptacyl side chains were incorporated into PDBTz to enhance polyme...

Highly Syndioselective 3,4-Trans Polymerization of (E)-1-(4-Methylphenyl)-1,3-butadiene by Fluorenyl N-Heterocyclic Carbene Ligated Lutetium Bis(alkyl) Precursor

(E)-1-(4-Methylphenyl)-1,3-butadiene (E-1-MPBD) synthesized via the “Wittig-type” reaction was polymerized with the ternary catalytic system (Flu-NHC)Lu(CH2SiMe3)2/AliBu3/[Ph3C][B(C6F5)4] (Flu-NHC = C13H8CH2CH2(NCHCHN(C6H2Me3-2,4,6)C) to afford a new product containing exclusively trans-3,4 (>99%) units with perfect syndiotacticity (rrrr > 99%). The regio-3,4 tacticity was proved by the IR and NMR (1H and 13C) spectroscopic analyses, while the 3,4-stereotacticity was confirmed by a model polymer with lower regularity (3,4 = 90.9%, rrrr = 49.3%) prepared by the binary catalytic system (Am-NHC)Lu(CH2SiMe3)2/[Ph3C][B(C6F5)4] (Am-NHC = 2,6-iPrC6H3N═C(C6H5)NCH2CH2(NCHCHN(C6H2Me3-2,4,6)C). The trans-planar conformation was uncovered through 2D-NOESY and 13C CPMAS NMR technologies. This represents the first stereo 3,4-syndioselective polymerization of 1,3-dienes achieved by a rare-earth metal based catalyst. Moreover, hydrogenating the resulting polymer gave the highly syndiotactic poly(4-methylphenyl-1-butene),...

The structure and phase transitions of crystalline polydimethylsilane [Me2Si] n revisited

The results of vibrational spectroscopy and X-ray diffraction obtained at the present level of the methods confirm the planar trans (all-A) conformation of the polydimethylsilane chain recently called in question. The differential scanning calorimetry data also verify the first order phase transitions at about –35 and 145 °C which proceed with retention of the all-A chain conformation and are of order-disorder type.

A revisitation of the polymorphism of poly(butylene-2,6-naphthalate) from periodic first-principles calculations

Periodic density functional theory calculations are here used to investigate polymorphism in poly(butylene-2,6-naphthalate) (PBN) and to understand the intra- and inter-molecular effects which are responsible for its behavior. Despite its similarity to poly(butylene terephthalate), the larger π-electron conjugation promoted by the presence of naphtyl rings generates peculiar intramolecular effects and stronger interchain packing interactions which cause some differences between the two polymers. This is particularly evident by comparing the structural and spectroscopic data predicted for the α and β crystals with respect to the respective one dimensional infinite chain models. Two different interpretations have been proposed in the previous literature to describe the structural transitions from the α to the β polymorph of PBN upon mechanical deformation or thermal treatments: from one hand, the setting on of a transplanar conformation on the methylene chains has been proposed while, on the other hand, a larger coplanarity of the ester groups and the naphtyl rings has been suggested. Our calculations reveal that both these effects are present in β-PBN and should be both taken into account to give an interpretation of the trends observed by IR spectroscopy and structural characterization.

Diorganyl dichalcogenides with intramolecular coordination interactions: the synthesis and structure of bis(4,6-dimethylpyrimidin-2-yl) diselenide

Bis(4,6-dimethylpyrimidin-2-yl) diselenide, a new representative of dihetaryl diselenides, was obtained by successively treating selenourea with hydrochloric acid, acetylacetone, potassium hydroxide, and the system iodine—potassium iodide. The unusual planar trans-conformation of this compound (X-ray diffraction data) is stabilized by intramolecular attractive nonvalent Se…N interactions (Se…N, 2.926(1) A) and stacking of the π-systems of the aromatic heterocycles (the centers of the pyrimidine rings are spaced apart at 3.365(2) A). The energy parameters of the factors that make the molecular structure flattened were estimated by quantum chemical DFT calculations.

Structure of Self-Assembled Monolayers of Partially Fluorinated Alkanethiols with a Fluorocarbon Part of Variable Length on Gold Substrate

A series of self-assembled monolayers (SAMs) formed on Au(111) from partially fluorinated alkanethiols (PFAT) with variable length of the fluorocarbon chain, viz. F(CF2)n(CH2)11SH (FnH11SH, n = 6, 8, and 10), was studied by high resolution X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, and near edge X-ray absorption fine structure spectroscopy. The SAMs were found to be highly ordered and densely packed. The packing density was governed by the bulky fluorocarbon segments which adopted a helical conformation typical of this entity. With decreasing length of the fluorocarbon segment, progressive deterioration of the orientational order accompanied by a slight decrease in the packing density was observed in the fluorocarbon part of the FnH11SH SAMs. The conformation of the fluorocarbon segment was persistent through the series, with probably only a minor deterioration for n = 8 and 6. The hydrocarbon segments of the monolayers were, however, unaffected by the deterioration of the orientational order in the fluorocarbon part. They persistently exhibited all trans planar conformation, typical of densely packed assemblies of these moieties, while their orientation, given by the characteristic tilt and twist angles, mimicked that of the nonsubstituted alkanethiolate SAMs on Au(111). This behavior is explained by the effect of the bending potential which predefines the orientation of the hydrocarbon segments even if they are separated beyond the equilibrium spacing by the bulky fluorocarbon moieties.

Diorganyl ditellurides with intramolecular coordination bonds: synthesis and structure of bis(4,6-dimethylpyrimidin-2-yl) ditelluride

A reaction of 2-chloro-4,6-dimethylpyrimidine with sodium ditelluride in DMF afforded bis(4,6-dimethylpyrimidin-2-yl) ditelluride, a new representative of dihetaryl ditellurides. An X-ray diffraction study revealed an unusual planar trans-conformation of this compound, which is stabilized by both intramolecular attractive non-valent Te...N interactions (3.082(2) A) and π-stacking of the aromatic heterocycles (the centers of the pyrimidine rings are disposed at 3.421(3) ). The energy parameters of the factors that make the molecular structure flattened were estimated by quantum chemical DFT calculations.

Synthesis, spectroscopic, structural and conformational study of some tri-substituted ureas derived from N-methylpiperazine containing phenyl and N-heterocyclic substituents

A series of tri-substituted ureas containing an N-methylpiperazine moiety as well as phenyl and N-heterocyclic substituents were synthesized and studied by 1H, 13C NMR and IR spectroscopies. From 1H and 13C NMR data, in CDCl3 solution at room temperature, a fast inter-conversion of the piperazine ring with the N-CH3 group in equatorial position can be proposed. Amino–imino tautomerism is observed for both thiazole and benzothiazole derivatives. Moreover, with the exception of the imino form of the thiazole derivative, the aryl substituted N-carbamoyl group rotates freely. IR data show that the compounds adopt a planar trans conformation of the CONH moiety.

Ultrafast Structural Evolution of Photoactive Yellow Protein Chromophore Revealed by Ultraviolet Resonance Femtosecond Stimulated Raman Spectroscopy

We studied ultrafast structural dynamics of the chromophore of photoactive yellow protein, trans-p-coumaric acid (pCA), using newly developed ultraviolet resonance femtosecond stimulated Raman spectroscopy (UV-FSRS). The UV-FSRS data of the anionic form (pCA–) in a buffer solution showed clear spectral changes within 1 ps, followed by a spectrally uniform decay with a time constant of 2.4 ps. The observed spectral change indicates that the structural change occurs in excited pCA– from the Franck–Condon state to the S1 potential minimum in the femtosecond time region. The S1 Raman spectra exhibit spectral patterns that are similar to the ground-state spectrum, suggesting that pCA– yet retains a planar-trans conformation throughout the S1 lifetime. We concluded that S1 pCA– undergoes a femtosecond in-plane deformation, rather than a substantial Cet═Cet twist. With these femtosecond vibrational data, we discuss possible roles of the initial structural evolution of pCA in triggering the photoreceptive functio...

Nucleator-Enhanced Molecular Orientation and Crystallization in Melt-Spun Monofilaments of Syndiotactic Polypropylene

Different from other semi-crystalline polymers, the crystallization of syndiotactic polypropylene (sPP) into helical sequences under flow is largely inhibited due to favorable formation of trans-planar conformations. The results presented in this study, using micro-FTIR, 2D-WAXS, and 2D-SAXS characterization, indicate that molecular orientation and crystallization in melt-spun sPP monofilaments can be dramatically enhanced by a sorbitol-based nucleator. It is suggested that, similar to the oriented crystallization in the nucleated isotactic polypropylene (iPP), the phase-separated nanofibrils, aligned under flow prior to sPP crystallization, act as shish structure (primary nuclei) to mediate the molecular orientation and crystallization of sPP.

Syndiotactic Polystyrene ε Cocrystals With n-Alkanes: Guest Uptake and Crystal Structure

s-PS ε-clathrates with different n-alkanes, ranging from n-hexane to n-dodecane, have been obtained for the first time and their structure has been investigated by means of X-ray diffraction, FT-IR spectroscopy, and molecular mechanics calculations. The diffraction data of all the s-PS/n-alkane ε-clathrates are in agreement with the orthorhombic unit cell with axes a = 1.62 nm, b = 2.20 nm, and c = 0.79 nm and the space group Pbcn, proposed for the nanoporous ε form, indicating that the crystal packing of the chains in the clathrate forms remains substantially unaltered independently of the length of the n-alkane. Molecular mechanic calculations provided conclusions that the n-alkanes' guest, independent of their length, are arranged in the channels in fully trans-planar conformation with their main molecular axis almost parallel to the axis of the cavity. The possibility of having n-alkane guest with a bent conformation in the channels was also ruled out. A detailed structural study was carried out for the cocrystalline form containing n-octane. Supplemental materials are available for this article. Go to the publisher's online edition of Soft Materials to view the free supplemental file.

Annealing induced variation of crystalline structure and mechanical properties of syndiotactic polypropylene fibers

Syndiotactic polypropylene (sPP) as-spun fiber (sPP1) and drawn fiber (sPP2) were prepared by melt-spinning and melt-spinning/hot-drawing, respectively. The structure transition of the two fibers induced by annealing at different temperatures and the corresponding mechanical properties were subsequently investigated by the combination of Fourier transform infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD) and tensile testing. The results indicate that the chain conformation and crystal forms of the two sPP fibers are not obviously changed at low annealing temperature (40°C). With increasing the annealing temperature, the trans-planar conformation and mesophase in sPP1 and sPP2 fibers can be completely transformed to helical conformation and crystal form I under tension. Upon removing the tension, a small amount of mesophase and trans-planar conformation will be regained. The mechanical properties of the annealed fibers are manifestly dependent on their initial structure and the annealing temperature.

Molecular Origin of Enhanced Strength in the Monofilament of Syndiotactic Polypropylene Because of Annealing: A Micro-FTIR Study

The conformational and polymorphic transformations in the melt-spun monofilament of syndiotactic polypropylene upon annealing and subsequent stretching have been first explored by micro-FTIR studies. The results indicate that annealing of as-spun monofilament gives rise to an unusual increase in the molecular orientation of helical conformations in both amorphous phase and ordered crystals as a result of structural transitions of transplanar conformations to helical ones as well as mesophase to ordered crystals. The increased molecular orientation in the annealed monofilament is mainly responsible for the enhanced strength during stretching. Furthermore, high stress exerted by the molecular chains of annealed monofilament is confirmed by the significant structural transitions opposite to that induced by annealing.

“Too Small, Too Big, and Just Right” − Optical Sensing of Molecular Conformations in Self-Assembled Capsules

Irradiation of dimethylbenzil within a cylindrical capsule gives bright green phosphorescence, while irradiation of benzil and dimethoxybenzil in the same capsule results in high energy blue fluorescence. This difference is likely due to the geometric restrictions imposed by the capsule's space on its excited guests, forcing a trans-planar conformation in some cases and cis-skewed in others.

Electrospinning of Syndiotactic Polypropylene from a Polymer Solution at Ambient Temperatures

A syndiotactic polypropylene (sPP) fibrous membrane was successfully prepared from a multicomponent solvent system at room temperature. As demonstrated by SEM, the morphology of sPP was strongly dependent upon the environment, especially the ambient temperature during electrospinning. The crystal structure of electrospun sPP fibers exhibits a mixture of forms I and III, as determined by WAXD. It is hypothesized that the less stable form III crystals with trans-planar chains are induced in the electrospun sPP fibers due to rapid, stress-induced crystallization during the electrospinning process. To confirm the existence of a trans-planar conformation, FT-IR and Raman were employed. However, it was found that form I is the dominant crystal structure in electrospun sPP fibers.

Dependence of electrical properties of polypropylene isomers on morphology and chain conformation

The electrical properties of polypropylene isomers were correlated with the morphology and chain conformation of differently obtained isotactic (iPP) and syndiotactic (sPP) samples. In the case of iPP, a crystallized and a smectic sample were prepared, whereas for sPP two crystalline helical samples and a mesophase with the chains in trans-planar conformation were considered. The phase composition was obtained for all the samples comparing x-ray diffractograms and transport properties of vapours, which give the crystallinity and the amorphous fraction, respectively. The fraction of mesophase was obtained by the difference of the previous values. The study of the morphology evidenced similarities and differences among the samples, which were discussed and correlated with the phase composition. The electrical conductivity was measured for all the samples, and the syndiotactic isomer showed the lowest value as well as a dependence on the structure. In contrast, the isotactic isomer showed the same behaviour for either polymorph. Based on the structural and electrical results, a phenomenological explanation of the conduction mechanisms taking place in the different forms has been proposed. In particular, the current in the iPP seems to be controlled by Schottky emission, i.e. by field-assisted thermo-ionic injection of carriers from the electrode into the polymer, whereas for the sPP more than one mechanism is likely to be effective, although the ionic transport appears as the predominant one. The experimental data confirm a different behaviour of the ionic conduction properties for the different polymorphs, highlighting the greater insulating characteristics of the mesomorphic structure of the syndiotactic isomer.