Comparison of head-to-head and head-to-tail polystyrenes by thermally stimulated discharge

Thermally stimulated depolarization (TSD) experiments were carried out on several polystyrene samples. They included normal head-to-tail polystyrene (atactic) obtained by anionic polymerization of styrene, amorphous and substantially crystalline isotactic polystyrene, and the newly available head-to-head polystyrene. By TSD, six maxima of current intensity occurred at specific temperatures. Their features are compared for the various samples. Only three peaks could be identified with transitions which had been found by other techniques. Peak 5, located near T_g, is the primary relaxation. Maximum 6 could be the transition found above the T_g by torsional braid analysis and called T₁₁ for polystyrene samples. Maximum 1 seems to correspond to what is sometimes referred to as the γ transition.     

Enantioselective synthesis of DIANANE, a novel C2-symmetric chiral diamine for asymmetric catalysis

DIANANE (endo,endo-2,5-diaminonorbornane) is a novel chiral C(2)-symmetric diamine, based on the rigid bicyclo[2.2.1]heptane scaffold. Schiff-base ligands derived from DIANANE have already found use in asymmetric catalysis, e.g., in the highly enantioselective Nozaki-Hiyama-Kishi reaction. We herein describe a practical synthesis of enantiomerically pure DIANANE, starting from norbornadiene in four steps: (i) Pd-MOP catalyzed Hayashi-hydrosilylation/Tamao-Fleming oxidation, (ii) oxidation to norbornane-2,5-dione, (iii) endo-selective reductive amination with benzylamine, and (iv) hydrogenolytic debenzylation. None of the steps involves chromatographic purification. For the Tamao-Fleming oxidation, the use of hydrogen peroxide in the form of its urea clathrate instead of aqueous solution proved beneficial. By the above sequence, enantiomerically pure (ee >or=99%) DIANANE was obtained from norbornadiene in 40-50% overall yield. The relative and absolute configuration of DIANANE was confirmed by X-ray crystallography of the DIANANE bis-tosylamide, and of its bis-camphorsulfonamide. Furthermore, the synthesis and X-ray crystal structure of the Schiff-base ligand derived from DIANANE and 3,5-di-tert-butyl salicylic aldehyde are reported.     

High order harmonic generation optimization with an apertured laser beam

We report a systematic study of high order harmonic generation with an infrared laser apertured by an iris, as a function of the aperture size. Measurements were made of harmonic generation efficiency for different gas species, laser energies and focal geometries. Harmonic efficiencies as a function of aperture show a characteristic peaked shape, which is independent of gas species and harmonic order. A one dimensional, time dependent simulation of harmonic generation in a gas cell, taking into account experimentally measured transverse coherence of the laser, closely reproduces the observed behaviours. We show that the aperture diameter which maximizes harmonic yield is the result of a compromise between considerations of focal geometry and ionization (which favour small apertures); and harmonic dipole amplitude and phase (which favour large apertures). Received 31 May 2002 Published online 24 September 2002 RID="a" ID="a"e-mail: kazamias@ensta.fr RID="b" ID="b"UMR 7639 du CNRS     

Time-domain interferometry using synchrotron radiation applied to diffusion in ordered alloys

Time-domain interferometry of synchrotron radiation (TDI) has recently been used as a tool for investigating diffusion in glasses. This work deals with an extension of this technique to ordered structures. In a TDI experiment performed on the B2 alloy CoGa at the APS the intensity scattered into Bragg directions showed no detectable quasielastic signal. Experimental lower limits of the elastic contribution are given. They are in accordance with the coherent scattering function derived in this paper. This result indicates that TDI can be applied to diffusion in crystalline solids, e.g. intermetallic alloys, by using diffuse scattering. Requirements and limitations of diffuse scattering experiments are discussed. Received 21 September 2000 and Received in final form 13 December 2000     

Primary resonance excitation of electrically actuated clamped circular plates

We investigate the response of an electrically actuated clamped circular plate to a primary resonance excitation of its first axisymmetric mode using an analytical reduced-order model (macromodel). We discuss the inf luence of the number of modes retained in the discretization on the predicted solutions. The reduced-order model, which is a system of coupled nonlinear ordinary-dif ferential equations, accounts for general residual stress and strain hardening and allows for general material and geometric design variables. Our reduced-order model is robust up to the pull-in instability and is general enough to be an ef fective design tool for capacitive micromachined ultrasonic transducers.     

Aldehyde Polymerization,Helical Polymers,Macromolecular Asymmetry,and Oligomers

Abstract

A historical development of the polymerization of higher aldehydes is presented. It shows the development of the polymerization of the simplest example, formaldehyde, the very ancient and to some extent incorrect history of other aldehyde polymerizations, and actually starts with the full understanding of this field as of the early 1950s. Much of the early work is incoherent and inconsistent, not surprisingly because of the limited knowledge of polymer chemistry that was available at that time. This contribution tries to correlate the polymerization of formaldehyde, the higher aliphatic aldehydes, and the trihaloacetaldehydes within the framework of a comprehensive understanding of the field of polymer science. The extensive progress in the field of trihaloacetaldehyde polymerization has led to the development of cryotachensic polymerization, the concept of optical activity based on macromolecular asymmetry, ultimately the recognition of the importance of the study of oligomers of stereoregular polymers, and the very essence of the embryonic state of addition polymerization.     

Preparation and Characterization of Head-to-Head Polymers. II. Head-to-Head Poly(methyl Acrylate)

Head-to-head poly(methyl acrylate) was prepared by esterification of the known alternating copolymer of ethylene and maleic anhydride. Some of the chemical,physical, and mechanical properties and the thermal degradation behavior of head-to-head poly(methyl acrylate) were studied and compared with those of head-to-tail poly(methyl acrylate). The T_g of the head-to-head polymer was higher than that of the head-to-tail polymer, but the solubilities of both types of polymers of comparable molecular weight were similar. Head-to-head poly(methyl acrylate) degraded thermally at approximately the same temperature and with a rate similar to head-to-tail poly(methyl acrylate). Unlike poly(methyl cinnamates) which cleanly degraded to monomers, poly(methyl acrylates), head-to-head and head-to-tail, degrade to very small molecules, such as CO₂, methanol, but also larger polymer fragments and char. Trace amounts of monomers (methyl acrylate) were also observed.     

Functional Polymers. LVII. Electronic Absorption Spectra of Benzotriazole Derivative/Electron Acceptor Systems

The electronic absorption spectra of charge-transfer complexes (CTC) of benzotriazole derivatives including 2(2H-hydroxyphenyl)2H-benzotriazole-containing copolymers with various electron acceptors were investigated. While no charge-transfer interaction was observed with weak acceptors, strong acceptors such as trinitrofluorenone and pyromellitic anhydride exhibited an absorption of the contact charge-transfer type with these donors. When the very strong acceptor tetracyanoethylene was used as acceptor, new peaks of a CTC type appeared at longer wavelengths. From the wavelengths of the absorption maxima and the equilibrium constants of the CTC, the electron-donating ability of several related (2(2-hydroxyphenyl)2H-benzotriazole derivatives was estimated as follows: 2(2-Hydroxy-5-methylphenyl)2H-benzotriazole > 2H-benzotriazole > 2(4-hydroxyphenyl)2H-benzotriazole > 2(2-acetoxy-5-methylphenyl)2H-benzotriazole > copolymers containing 2(2-hydroxyphenyl)2H-benzotriazole groups.     

Bisorthoesters as Polymer Intermediates. III. Oligomers Containing Purine Rings

Hexa-n-propyl orthoisophthalate, hexa-n-propyl orthoterephthal-ate, and in one case hexaethyl orthooxalate were condensed with aminomalondiamine dihydrochloride or aminomalonamideamidine dihydrochloride in DMSO or DMF between 100°C and the boiling point of DMF to oligomers containing adenine and hypoxanthine rings. For example, hexa-n-propyl orthoisophthalate and amino-malonamidamidine dihydrochloride in dimethyl sulfoxide at 100°C gave an oligomer of poly (2,8-hypoxanthinediyl-1,3-phenylene). The condensation was limited to oligomers of relatively low molecular weight, partly because of the low solubility of the oligomers under reaction conditions and partly also because the four reactive amino groups of the aminomalonamideamidine and the aminomalondiamidine did not react simply as difunc-tional tetramines to form linear aromatic polymers; this unequal reactivity of the amine functions caused side reactions to occur. Such side reactions have not been observed in the reactions of bisorthoesters with tetrafunctional aromatic orthodiamines, such as 3,3′,4,4′-tetraaminobiphenyl.     

Head-to-Head Polymers. XXVIII. Attempted Syntheses of Linear Head-to-Head Polyisobutylene

2,2,3,3-Tetramethyl-1,4-dibromobutane, when used as monomer for polymerization by Wurtz-type polycondensation, gave head-to-head polyisobutylene which is branched. Under similar conditions, 2, 5-dimethyl-2, 5-dibromohexane gave no polymer. Copolymerization of ethylene with tetramethylethylene under various conditions gave polyethylene of modest molecular weight with about 5% tetramethylene units in the polymer. 1,1,4, 4-Tetramethyl-1,3-butadiene (2,5-dimethylhexadiene-2,4) polymerized with BF₃ initiator to high molecular weight trans- 1,4-poly-(1,1,4,4-tetrarnethylbutadiene-1,3). The polymer could not be hydrogenated with soluble hydrogenation catalysts and only partially by chemical reduction with diimide. Under forcing conditions, incorporation of portions of the decomposition products of the precursor of the diimide was observed.