A new microwave dielectric method for studying photoinitiated chain radical polymerization processes

Measurements of both real and imaginary parts of the dielectric constant at a fixed microwave frequency have been performed on n-butyl acrylate/poly(n-butyl acrylate) mixtures. The dielectric constant of the mixtures has been compared with that of neat n-butyl acrylate during UV photoinitiated polymerization. The values of the imaginary part of the dielectric constant thus obtained have allowed determination of the instantaneous monomer concentration and verified the kinetic equation for the photoinitiated polymerization process. The limits of validity of the method as well as its capability of providing detailed kinetic information are also discussed.     

Ethylene polymerization by novel Ziegler–Natta‐type catalysts obtained in situ by the oxidative addition of 8‐hydroxyquinoline‐based ligands to bis(1,5‐cyclooctadiene)nickel(0) and methylaluminoxane

Novel catalytic systems, prepared in situ by the oxidative addition of 8‐hydroxyquinoline ligands to bis(1,5‐cyclooctadiene)nickel(0) and activated by methylaluminoxane, were studied in ethylene polymerization. When 8‐hydroxyquinoline was employed, only oligomeric products were obtained. On the contrary, 5,7‐dinitro‐8‐hydroxyquinoline gave linear polyethylene (PE), but with a modest activity. For the catalyst based on 5‐nitro‐8‐hydroxyquinoline, the productivity was largely dependent on the content of free trimethylaluminum (TMA) present in the commercial aluminoxane. The progressive optimization of the TMA/oligomeric methylaluminoxane ratio increased the productivity, which reached 700 kg of PE/(mol of Ni × h), by an order of magnitude. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 200–206, 2006     

Long-wavelength fluorescent indicators for the determination of oxygen partial pressures

A series of stable heterocyclic indicators with λ_ex = 469–566 nm and λ_em = 511–652 nm allows the fluorimetric determination of ca. 1–100% partial pressures of oxygen by quenching of fluorescnce, especially in toluene solution.     

Photophysical study on a new class of benzophenone containing photoinitiators of polymerization: Properties of poly(4-acryloxybenzophenone) and a model compound

The photophysical characteristics of poly 4(2-acryloxybenzophenone), poly-ABP, and its model compound 4(2-methylpropionyloxy)benzophenone, IBP, have been determined by steady state and pulse methods. In IBP the observed properties are mainly due to the n,π1 triplet of the benzophenonic moiety. In poly-ABP, the formation of singlet excimers competes with the intersystem crossing leading to benzophenonic triplet. The triplet lifetime in polymer is reduced, compared to the model compound. This fact is explained in terms of intramolecular hydrogen abstraction of the triplet.     

Model reactions for stereochemistry of Ziegler-Natta polymerization: Asymmetric hydroisomerization of α-olefins by chiral titanium tetra-alkoxides/tri-isobutyl aluminum catalytic systems

Homogeneous catalytic systems based on optically active low-molecular-weight and macromolecular titanium alcoholates and tri-isobutyl aluminum were used in the isomerization and hydrogenation of chiral and prochiral olefins. The reactions performed in a nitrogen and hydrogen atmosphere show a different behavior in regard to reaction rate, composition of reaction mixture, and stereochemical control. The catalyst based on macromolecular titanium alcoholate displays lower activity and a varying influence on the stereochemical pathway with respect to the catalysts based on low- molecular-weight alcoholates. The data obtained in both cases strongly support the absence of a stereocontrol by the growing chain in the stereoelective polymerization of racemic α-olefins by Ziegler-Natta catalysts based on optically active precursors. Finally evidence is provided in favor of the intermediate step responsible for asymmetric isomerization which does not occur via η-allyl complex.     

A two-step conversion of alpha,beta-unsaturated ketones to their alpha-carbalkoxy or alpha-carbamoyl derivatives

Several enones are converted into their alpha-iodo derivatives in excellent yields and carbonylated with palladium catalysis in the presence of alcohol or amines to the alpha-carbonyl enones in satisfactory yields.     

Optically active hydrocarbon polymers with aromatic side chains. II. Poly-(S)-p-sec-butylstyrene

The synthesis of optically active p-sec-butylstyrene (I) has been carried out starting with (S)-2-phenylbutane (II) having optical purity 88–91%. The optical purity of I thus obtained was found to be 73–75%. The polymerization of I with stereospecific coordinated anionic catalysts gave amorphous polymers, as in the case of many other p-substituted styrene derivatives. The fractions obtained from these polymers have very similar rotatory power at 589 nm which is practically equal to that of polymer of I obtained by nonstereospecific radical initiator and of low molecular weight structural models. Accordingly the ¹L_b electronic transition of the aromatic chromophore shows a very low rotatory strength in all samples examined. This result is related to the lack in solution of conformations with a predominant single chirality of the main chain of the macromolecules derived from I.     

Optically active polymers containing side-chain photochromic moieties: Synthesis and chiroptical properties of copolymers of trans-4-(phenylazo)-1-naphthyl acrylate and trans-4-(1-naphthylazo)-phenyl acrylate with (?)-menthyl acrylate

The synthesis of photochromic optically active copolymers from trans-4-(phenylazo)-1-naphthyl acrylate (PANA), or trans-4-(1-naphthylazo)-phenyl acrylate (NAPA), and (?)-menthyl acrylate (MtA) is described. The copolymers prepared, having different contents of trans-phenylazonaphthalene moieties, have been characterized by IR, ¹H-NMR, UV, and GPC techniques. The chiroptical properties have been investigated by circular dichroism (CD) and the induced optical activity on the side-chain trans-phenylazonaphthalene chromophores discussed in terms of different conformational situations of the macromolecules in both the copolymer series. © 1994 John Wiley & Sons, Inc.     

Selective synthesis of isobutanol by means of the Guerbet reaction: Part 2. Reaction of methanol/ethanol and methanol/ethanol/n-propanol mixtures over copper based/MeONa catalytic systems

The synthesis of isobutanol via the Guerbet condensation between methanol and ethanol was studied by using sodium methoxide (MeONa) as soluble basic component and copper-based catalysts as heterogeneous dehydrogenating/hydrogenating metal species. The effect of the nature of the catalyst and the relative amount of its individual components with respect to the reacting alcohols as well as of temperature on productivity and selectivity of the process was investigated. The collected data indicated that the copper chromite/MeONa was more active than Cu-Raney/MeONa system. The reaction was shown to proceed with the formation only of n-propanol and isobutanol. Ethanol conversion up to 61% with selectivity to isobutanol up to 98.4% was obtained. The same catalytic systems were also employed in the reaction of the methanol/ethanol/n-propanol ternary mixture. Again copper chromite/MeONa resulted more active than the Cu-Raney/MeONa system. Finally, experiments were carried out on methanol/n-propanol mixtures in the presence of the copper chromite/MeONa catalytic system by recycling both the recovered solid copper component and the liquid reaction mixture for evidencing eventual copper leaching by MeONa. On the basis of the obtained results it was concluded that in the Guerbet reaction copper chromite works as heterogeneous catalyst.