Activated Monomers and Nucleophilic Reagents in Addition and Polymerization Reactions

A number of examples of addition and polymerization reactions is presented with special emphasis on the chemical behaviors of activated monomers and/or activated nucleophilic reagents. Lithium alkoxyethanolate forms a complex with lithium alkyl. Spectroscopic studies showed this complex to possess agent-separated ion pairs. The nature of the complex is characterized by the enhanced reactivity of styrene in the copolymerization reaction with butadiene initiated by the complex. Magnesium alkyl can be sufficiently activated by magnesium alkoxyethanolate to polymerize styrene and diene. Aluminum alkyl and zinc alkyl are able to induce the anionic polymerization of vinyl ketones, but not of unsaturated esters or nitriles. Aluminum or zinc alkoxyethanolates fail to activate their corresponding metal alkyls. Bipyridyl, sparteine, triphenylphosphine, HMPT, and related Lewis bases, however, activate aluminum alkyl enough to react with carbon-carbon double bonds of the unsaturated esters and nitriles. Crotononitrile can be polymerized by the AIR₃-HMPT system to form a colorless polymer, where possible side reactions between CN and AIR₃ are prevented by HMPT. Mutual activation through complex formation is confirmed by a model system of a vinyl ketone with organozinc compounds. AIR₃-HMPT does not polymerize vinyl ketones because of a lack of complex formation. N-Carboxy-α-alanine anhydride (NCA) can be polymerized with zinc alkyl as initiator. The formation of activated NCA by proton abstraction from the NH group is shown to be the essential stage for polymerization. Zinc alkyl is also activated by conventional acid anhydrides. The propylene oxide ring can be cleaved with the ZnR₂-phthalic anhydride system, which is the initiation step in the alternate copolymerization between propylene oxide and the acid anhydride. The propagation mechanism of the CO₂-epoxide copolymerization is also discussed.     

Modal interaction in chaotic vibrations of a shallow double-curved shell-panel

Experimental results and analytical results are presented on chaotic vibrations of a shallow double-curved shell-panel subjected to gravity and periodic excitation. Modal interactions in the chaotic responses are discussed. The shell-panel with square boundary is simply supported for deflection. In-plane displacement at the boundary is elastically constrained. In the experiment, time histories of the chaotic responses at the spatial multiple positions of the shell-panel are measured for the inspection of modal interaction. In the analysis, the shallow shell-panel is assumed to have constant curvatures along to orthogonal directions and geometric initial imperfection. The Donnell-Mushtari-Vlasov type equation is used as governing equation with lateral inertia force. Assuming deflection with multiple modes of vibration, the governing equation is reduced to a set of nonlinear ordinary differential equations by the Bubnov-Galerkin procedure. Chaotic responses are integrated numerically. The chaotic responses, which are obtained by the experiment and the analysis, are inspected with the Fourier spectra, the Poincaré projections, the maximum Lyapunov exponents and the Lyapunov dimension. Contribution of modes of vibration to the chaotic responses is analyzed by the principal component analysis, i.e., Karhunen-Loève transformation.     

Magnetic-field control of quantum critical points of valence transition

We study the mechanism of how critical end points of first-order valence transitions are controlled by a magnetic field. We show that the critical temperature is suppressed to be a quantum critical point (QCP) by a magnetic field, and unexpectedly, the QCP exhibits nonmonotonic field dependence in the ground-state phase diagram, giving rise to the emergence of metamagnetism even in the intermediate valence-crossover regime. The driving force of the field-induced QCP is clarified to be cooperative phenomena of the Zeeman and Kondo effects, which create a distinct energy scale from the Kondo temperature. This mechanism explains the peculiar magnetic response in CeIrIn(5) and the metamagnetic transition in YbXCu(4) for X=In as well as the sharp contrast between X=Ag and Cd.     

Molecular level approach to the origin of the steric control in organozinc catalyst for oxirane polymerization

A new type of single crystals was formed by a reaction between diethylzinc and DL-1-methoxy-2-propanol. The constituent unit of this single crystal was shown to be a two to two complex of zinc dialkoxide and ethylzinc alkoxide, [MeOCH² CH(Me)OZnOCH(Me)CH² OMe]² · [EtZnOCH(Me)CH² OMe]², which has a chair type structure. The two to two complex was found to exhibit higher activity and stereospecificity in the PO polymerization compared with those of the one to six complex which were reported previously. Molecular level elucidation was made on the basis of NMR studies for polymerization system with a partly deuterated two to two complex as initiator. The excellent stereospecificity in isotactic propagation was ascribed to the highly chiral hole around the active site of the chair type complex. Another series of study revealed chemical behavior of cyclohexene oxide (CHO), which is different from that of PO, toward several organozinc compounds. The two to two complex was the only catalyst which showed high activity for both MO and CHO polymerizations. Three samples (I), (II) and (III) of poly-(CHO) were prepared with Et²Zn, EtZnOR and the two to two complex, respectively. After careful examinations of each of the three samples, it was concluded that most parts of the main chain of polymer (III) molecule were composed of syndiotactic triad, SS-RR-SS(or RR-SS-RR), while polymer (I) and (II) molecules contained syndiotactic and heterotactic triads in comparative amounts. In the CHO polymerization, the chiral hole of the two to two complex cannot recognize the mode of orientation of the achiral monomer, and the complex seems to serve as a simple bulky group which facilitates syndiotactic addition of CHO monomer molecule to the active site, strong steric effects being exerted from the penultimate unit of the growing chain during the propagation stage.