Angelina Altomare

Synthesis, chiroptical properties and photoresponsive behaviour of optically active poly[(S)-4-(2-methacryloyloxypropanoyloxy)azobenzene]

Abstract A novel optically active monomer containing a photochromic chromophore, such as trans-(S)-4-(2-methacryloyloxypropanoyloxy)azobenzene and its low molecular weight model compound trans-(S)-4-(2-pivaloyloxypropanoyloxy)azobenzene were prepared. Free radically initiated homopolymerization of the methacrylic monomer afforded in fairly high yield a rather high molecular weight polymer which was molecularly and structurally characterized. The molecular mobility of the polymer in all trans configuration, as well as chiroptical properties, photoisomerization kinetics and photochromic behaviour were investigated and compared with those of the low and high molecular weight related compounds. The polymer photoresponsive properties were evaluated by circular dichroism measurements at various extents of photoisomerization. All the results are discussed in terms of structural requirements of the macromolecules.

From homogeneous to supported metallocene catalysts

Abstract The mechanism of the monoalkene homogeneous polymerization with metallocene/methylalumoxane (MAO) system is reviewed with reference to structural and stereochemical control of the polymers as derived by the type of ligands around the transition metal (Zr and Ti). Major disadvantages of these homogeneous catalysts are the lack of morphology control and reactor fouling. Supported catalysts can allow to overcome these drawbacks and be utilized in cheaper bulk or gas-phase processes. Also they may permit the use of less MAO, while still offering facile control of polymer properties by ligand variation. Binding of well-defined transition metal complexes onto carefully prepared inorganic surfaces is certainly a very attractive approach to create new heterogeneous catalysts and to characterize highly reactive species. In this paper attention is on the use of alumina, silica and zeolites for supporting metallocenes of Ti and Zr. The amount of metallocene which can be attached to the surface of these supports is much lower than expected on the basis of simple physical absorption. This seems to indicate that attachment takes place at specific sites of the surface. In any case the amount of hydroxyl groups seems to play a determining role as shown by structural analysis and catalytic activity of olefin polymerization of the resulting heterogenized complexes. The formation of cationic species on the inorganic surface was proposed in a very analogous way as proposed in solution. Results concerning homo- and copolymerization of monoalkenes with different supported metallocene complexes are presented to discuss the effect of support nature and surface features.

Light-induced conformational changes in chiral polymers with photochromic side chains

Preparation de polymeres optiquement actifs avec un squelette hydrocarbure et peptide portant des groupes photoisomerisables azobenzene et stilbene dans les chaines laterales par copolymerisation et par modification chimique de polymeres preformes. Etude du comportement photochrome et de la conformation en solution par absorption UV, fluorescence et dichroisme circulaire

Zeolite supported polymerization catalysts: Copolymerization of ethylene and α-olefins with metallocenes supported on HY zeolite

Abstract Copolymerization of ethylene with higher α -olefins, such as 4-methyl-1-pentene, 1-hexene, 1-octene and 1-dodecene, was carried out with both aspecific and isospecific bridged metallocene complexes, comparing the catalytic behaviour following metallocene supportation on methylalumoxane-pretreated HY zeolite (SiO 2 /Al 2 O 3 =5.7). A decrease in specific activity was generally observed after adsorption on zeolite of both non bridged and ansa -metallocene complexes, though molecular weights of the copolymers were found to be higher especially when employing α -olefins as polymerization solvents. On the other side, the incorporation of the α -olefin comonomer was reduced in the case of supported biscyclopentadienylzirconium dichloride, whereas similar contents of olefin units were incorporated in the copolymers prepared with soluble or supported ansa -zircono- or hafnocene. NMR analysis and DSC determined melting points of the produced copolymers are connected with a shape selectivity effect promoted by the support.

Methacrylic polymers containing permanent dipole azobenzene chromophores spaced from the main chain: synthesis and characterization

Three photochromic monomers containing a permanent dipole photochromic azobenzene group separated from the methacryloyl moiety by a polymethylene spacer, namely 4-(4-oxy-4′-cyanoazobenzene)but-1-yl methacrylate, 6-(4-oxy-4′-cyanoazobenzene)hex-1-yl methacrylate, and 8-(4-oxy-4′-cyanoazobenzene)oct-1-yl methacrylate, were synthesized in three steps starting from 4-cyanoaniline. These monomers were homopolymerized and copolymerized with an optically active monomer, (−)-menthyl methacrylate, in the presence of AIBN as a radical initiator. The polymeric materials, having a molar content of photochromic units comprised between 5 and 100% and molecular weights of about 15000, were characterized by an almost random distribution of monomeric units and a glass transition temperature ranging from 35 to 131°C. Polymer samples having a content of aromatic units larger than 75% were characterized by a thermotropic liquid crystalline structure, as evidenced by optical microscopy, X-ray diffraction and DSC measurements. Two-dimensional correlations spectroscopy (COSY) allowed an appreciable contribution by intramolecular charge transfer mesomers to the electronic distribution of the azobenzene chromophore to be highlighted.

Photochemistry and photophysics of copolymers of (−)menthyl acrylate with 4-hydroxystilbene acrylate

Copolymers of trans-4-hydroxystilbene with (—)menthyl acrylate exhibit upon irradiation transacis isomerization of the side-chain stilbene chromophores analogously to trans-4-hydroxystilbene-2-methylpropanoate, chosen as low molecular weight model compound. U.v. measurements on polymer samples indicate a deviation from first-order kinetics. In the fluorescence spectra of all-trans polymer samples both excimer and monomer emissions are observed. On irradiation the excimeric component is quenched much faster than the monomeric one. Fluorescence polarization experiments indicate a higher conformational rigidity of the stilbene chromophore when inserted in polymeric systems. Comparison of chiroptical properties of both unirradiated and irradiated copolymers suggests that on irradiation a local conformational rearrangement of macromolecules takes place.

Optically active polymethacrylates with side-chain L-lactic acid residues connected to push-pull azobenzene chromophores

Abstract Novel optically active methacrylic homopolymers with side-chain l -lactic acid moieties interposed through ester bonds between the main chain and trans -4-azobenzene chromophores bearing in the 4′-position electron withdrawing groups, such as the cyano and nitro residues, have been synthesized and structurally characterized. Investigation by circular dichroism of the chiroptical and conformational properties of the polymers in solution indicates a much higher optical activity of these latter with respect to that of the corresponding low molecular weight model compounds, purposely prepared, due to the presence of dissymmetric conformations of one prevailing handedness. The trans to cis photoisomerization kinetics of the azoaromatic double bond has been measured in solution for polymers and models and the resulting data compared with those of previously reported similar photochromic derivatives. The influence of the nature of the electron withdrawing group in the 4′-position of the azobenzene chromophore on the chiroptical properties of the polymers is also discussed.

Effect of the zeolite HY-support on the monoalkene polymerization by group IV metallocenes

Various metallocene complexes of the IV group transition metals supported on HY zeolites were investigated in the polymerization of ethylene and propylene. A comparative study with the same catalysts operating in solution was also performed. Generally, a decrease in the catalytic activity after support was observed, whose extent depends on the preliminary treatment of the support with aluminum alkyl. In general, better stability with the time and higher molecular weight of the polymers were obtained. The high isotactic stereospecificity of chiral metallocene catalysts for propylene polymerization was maintained after support. In the case of the aspecific bis(indenyl) complex, a modest but significant effect on the stereospecific control was observed due to the steric influence of the support.

Supported transition metal complexes for ethylene polymerization

Abstract This paper describes the preparation of some active organometallic species derived from nickel, titanium and zirconium fixed on the surface of different supports and their behaviour for the catalytic polymerization of ethylene. In the case of nickel, the transition metal species were attached to crosslinked polystyrene resins through a carbon atom of the P O chelate ligand. The formation of high molecular weight linear polyethylene is discussed in comparison to homogeneous analogs giving oligomeric products under the same conditions. Titanium containing species on the MgCl2 surface are obtained by coprecipitating the metals from solution of their alcoholates or carboxylates. These species are shown to be sensitive to the presence of carboxylate ligands which affect the catalytic process. Finally biscyclopendienyl dichloro or dimethyl zirconium have been supported on modified HY zeolite to give catalysts showing the same activity as the homogeneous analogs but higher stability during the polymerization reactions. General aspects deriving from these results are discussed with reference to the possibility of improving the design of well defined physically heterogeneous catalysts.

Structure–activity relationship of new NLO organic materials based on push–pull azodyes: 4. Side chain polymers

Abstract Several new polymeric materials containing push–pull azobenzene derivatives in the side chains were synthesized by polymer-analogous reactions involving the coupling of the diazonium salts of 4-substituted anilines with a preformed polymer, poly( N -methacryloyl- N ′-phenylpiperazine). The second harmonic generation properties of these glassy macromolecular materials were investigated in order to understand the influence of the chromophore chemical structure on dipole orientability and stability of orientation. The results thus obtained were compared with those of analogous guest–host systems and a lack of chromophore mobility was highlighted for the side chain polymers. The use of a short flexible spacer should probably improve the NLO performance.