Homopolymerization of methyl methacrylate by novel salicylaldiminate‐nickel/methylaluminoxane catalysts obtained by oxidative addition of the chelate ligand to a nickel(0) precursor

Salicylaldimine ligands, such as 5‐nitro‐N(2,6‐diisopropylphenyl)salicylaldimine, 3,5‐dinitro‐N(2,6‐diisopropylphenyl)salicylaldimine, and 3‐phenyl‐N(2,6‐diisopropylphenyl) salicylaldimine were checked in the oxidative addition to bis(1,5‐cyclooctadiene)nickel(0) to prepare, after activation by methylaluminoxane (MAO), novel nickel‐based catalytic systems active in the polymerization of methyl methacrylate. The catalytic behavior of the aforementioned systems, in terms of activity, molecular weight, and polydispersity of the resulting poly(methyl methacrylate) as well as its stereoregularity degree, was investigated as a function of the Al/Ni molar ratio, reaction temperature, and nature of the salicylaldimine ligand. The effect of ethylene atmosphere present during the preparation of the catalyst precursors was also investigated. The results are discussed and compared with those previously obtained by bis(salicylaldiminate)nickel(II)/MAO catalytic systems. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1716–1724, 2003     

Synthesis and characterization of the intermetallic compound NiSbS

The results recently obtained by our group to get new thermoelectric materials belonging to the M–Pn–Ch ternary systems (M=Co, Fe, Ni; Pn=P, As, Sb, Ch=S, Se, Te) are here reported. We have considered the Ni–Sb–S ternary system, and some homogeneous samples of composition near to 1:1:1 were prepared employing a new and simple synthetic route, starting from pure elements. Scanning electronic microscopy, electron-probe microanalysis and X-ray powder diffraction were used to investigate the microstructure. The NiSbS lattice parameters were determined and the crystal structure was refined by Rietveld method. The crystal cell of NiSbS belongs to the P213 space group with a = 0.5931 nm. The thermal stability of the ternary compounds Ni–Sb–S was investigated by DSC technique. Electrical resistivity and thermoelectric power measurements at room temperature and at 77 K were performed on platelets obtained by cleavage of the bulk.     

Photochromism of 4-acryloxyazobenzene/(−)-menthyl acrylate copolymers

Copolymers of trans-4-acryloxyazobenzene ( AAB ) with (?)-menthyl acrylate ( MtA ) have been prepared by free radical initiation; comonomer reactivity ratios have been found to be r _AAB = 0.89 and r _MtA = 0.53. Dependence of chiroptical properties on copolymer composition has been investigated and contribution to circular dichroism by azobenzene chromophores of isolated and sequence AAB units, as well as the influence of the trans to cis photoisomerization, have been determined. Kinetics of photoinduced trans → cis and thermally induced cis → trans isomerization of azobenzene side chains have also been examined and the kinetic parameters evaluated with relation to copolymer structure. The results have been discussed in terms of light-induced secondary structure reversible modifications.     

Induzierte optische Aktivit?t in Copolymeren von (?)-Menthylacrylat mit Styrol

Ohne Zusammenfassung     

Optically active copolymers from p-vinyltrifluoroacetophenone

Copolymers of p-vinyltrifluoroacetophenone (VTFA) with optically active (?)-menthyl acrylate (MtA) have been prepared by free radical initiation. The values of reactivity ratios (r_VTFA = 4.28 and r_MtA = 0.26) indicate that counits are inserted substantially at random into the copolymer macromolecules. CD spectra show a couplet typical of exciton splitting in the absorption region of trifluoromethylphenyl (TFA) chromophore. Dependence of ellipticity on composition and sequence distribution suggests that TFA chromophores are in a chiral environment provided by the dissymmetric conformation of the copolymer macromolecules.     

Guerbet condensation of methanol with n-propanol to isobutyl alcohol over heterogeneous copper chromite/Mg–Al mixed oxides catalysts

The synthesis of isobutyl alcohol (ⁱBuOH) from methanol (MeOH) and n-propanol (PrOH) through the Guerbet condensation has been studied at 200 °C and under inert atmosphere (3.0 MPa of N₂), using a two-component heterogeneous catalytic system based on pre-activated copper chromite and Mg–Al mixed oxides deriving from hydrotalcite-type (HT) precursors with different Mg/Al ratios. All the investigated catalysts displayed a significant activity, with an almost complete selectivity to ⁱBuOH. Unlike the copper chromite/soluble sodium methoxide system, the catalysts were tolerant of the co-produced water and did not display any appreciable deactivation during the course of the reaction. The catalyst productivity was found to increase by reducing the Mg/Al ratio in the heterogeneous base, according to the increase of the fraction of medium–strong and strong basic sites which favour the aldol condensation between the aldehydes derived from MeOH and PrOH.     

Semiconducting poly(monocyanoacetylenes)

Poly(monocyanoacetylenes) (PMCA) were synthesized by anionic, Ziegler–Natta, metathesis, and photo initiations. The Ziegler–Natta-catalyzed polymers probably have highly stereoregular cis-transoid structure that contains very few defects and the nitrile groups are difficultly cyclized. It has M?_n = 1100. PMCA obtained by anionic polymerization at ?78°C has M?_n ～ 4800; it is rich in trans-transoid structures but probably contains other isomeric units as well. The unpaired spin concentrations in these polymers are very high, comparable to that in trans-polyacetylene (PA) isomerized above 150°C. UV irradiation initiated rapid polymerization of cyanoacetylene in solid state at low temperature but the products were bleached in color after long irradiation. The unpaired spins in PMCA are immobile; nitrile cyclization causes some decrease in EPR linewidth and increase in room-temperature conductivity (σ_RT). There was also a large increase in unpaired spin concentrations to about 200 monomer units/spin. Iodine doping increases σ_RT to about 10^?3 (ω cm)^?1 but the dopant is readily removed by evacuation and the polymer returns to its original insulating state. The properties of pristine and doped PMCA, such as EPR g-value, ΔH_pp, T₁, T₂, and σ_RT are very similar. The similarities persist after cyclization and doping for this pair of polymers. These properties are also compared with those of poly(methylacetylene), poly(phenylacetylene), poly(dicyanoacetylene) and PA, and the significance is discussed.     

Real meromorphic functions and linear differential polynomials

We determine all real meromorphic functions f in the plane such that f has finitely many zeros, the poles of f have bounded multiplicities, and f and F have finitely many non-real zeros, where F is a linear differential polynomial given by F = f (k) +Σk-1j=0ajf(j) , in which k≥2 and the coefficients aj are real numbers with a0≠0.