Stefania Pragliola

C2-symmetric zirconocenes in the polymerization of conjugated diolefins.

C 2 -symmetric zirconocenes activated by methylaluminoxane were utilized as catalysts in the polymerization of 1.3-diolefins. The results indicate that the most crowded catalytic precursor rac[CH 2 (3-tert-butyl-1-indenyl) 2 ]ZrCl 2 (1) is also the most active one, giving 1,4-polymerization of 1,3-butadiene and (Z)-1,3 pentadiene and 1,2-polymerization of (E)-1,3-pentadiene and 4-methyl-1,3-pentadiene. Probably, the different behavior of 1 with respect to other C 2 -symmetric zirconocenes utilized is due to the different stability of the bond between the last inserted monomer unit and the metal, as well as to the coordination of incoming monomer.

Polymerization of Phenyl-1,3-butadienes in the Presence of Ziegler-Natta Catalysts

2-Phenyl-1,3-butadiene and (E)-1-phenyl-1,3-butadiene were polymerized in the presence of homogeneous Ziegler-Natta catalytic systems. The products were analyzed by means of 13C NMR spectroscopy. Monomer reactivities and polymer structures were compared with those obtained by using the same catalytic systems in the polymerizations of isoprene, (E)-1,3-pentadiene and 1,3-butadiene. The results were tentatively rationalized by considering the stability of the allyl groups resulting after any insertion of the monomer into the growing chain.

Crosslinking of a fibrous sample to a macro-oriented mesomorphic network

Preliminary results on the synthesis and characterization of anisotropic networks, oriented on a macroscopic scale, are reported. Fiber samples of segmented thermotropic liquid-crystalline polymers bearing the oxypentenyl lateral substituent have been crosslinked via thermally activated radical reaction. This was made possible by immersion of fiber samples in dichloromethane containing t-butylperoxybenzoate as activating agent, thus allowing its diffusion in the samples. Subsequent annealing at 145°C brings us to an anisotropic network with no loss of the original orientation. A mesophase is stabilized and no structural modification is observed by heating samples from room temperature up to 400°C, where thermal decomposition takes place. Crosslinked fibers exhibit good tensile properties, at both room temperature and at 150°C.

Highly Isotactic Poly(N-butenyl-carbazole): Synthesis, Characterization, and Optical Properties

The synthesis of isotactic poly(N-butenyl-carbazole) (-PBK) by using homogeneous isospecific Ziegler-Natta catalytic system is reported. The achieved polymer is crystalline and shows, to DSC and X-ray analysis, two distinct crystalline phases. -PBK FTIR spectrum and X-ray diffraction pattern are compared with those of poly(N-vinylcarbazole) (PVK). The observed differences are tentatively associated with higher flexibility of the -PBK chains due to the alkylene group connecting the carbazole group to the main chain. -PBK optical properties are also compared with those of PVK and isotactic poly(N-pentenyl-carbazole) (-PPK), a higher homologue of -PBK recently used as emitting layer in organic light emitting diodes (OLEDs) showing white light emission. The close similarity of the fluorescence spectra of -PBK and -PPK is a promising basis for optical applications of this polymer.

Highly isotactic poly(N-pentenyl-carbazole): A challenging polymer for optoelectronic applications

The synthesis and the microstructural, morphological, optical characterization of poly (N-pentenyl-carbazole) are reported. The obtained polymer present a stereoregular isotactic microstructure and is cristalline with a Tm of 138°C. Photoluminescence analysis of chlorobenzene polymer films, having high homogeneity and low roughness, show that PPK emits in a broad spectrum range 350-600 nm.

Liquid Crystalline Behaviour of Polymer Networks Based on Segmented Chain Mesogenic Polymers

Abstract The synthesis of some crosslinked polymers based on a segmented-chain polymeric mesogen containing variable amounts of a side chain substituent is reported. The unswollen networks show thermotropic mesomorphism of nematic character whose stability decreases with increasing substitution and crosslink density. Depending on the same parameters, the swollen networks may show optical anisotropy at room temperature whose origin is attributed to phase separation

Blue and white OLEDs with highly stereoregular polymers

Organic light emitting diodes (OLEDs) are fabricated using two novel polymers, isotactic and syndiotactic poly(N-pentenyl-carbazole) (i-PPK and s-PPK), as emitting layers. The electroluminescence spectra of the devices are analyzed in comparison with the photoluminescence spectra of polymer films and are related to their microstructure properties. Blue light emitting diodes are obtained, except for the i-PPK based multilayer OLED, which emits white light due to three contributions (electroplex fluorescence, excimer phosphorescence and electromer emission). The results show that the polymer tacticity strongly affects the photoconductive properties of the devices.

Influence of molecular weight on the structure and ageing behavior of quenched syndiotactic poly(propylene)

Four samples of syndiotactic poly(propylene) were synthesized by changing either the polymerization temperature or the catalytic system. Samples with different molecular weights were obtained, ranging between 27 000 and 826 000, whereas the fraction of syndiotactic pentads was found very near in the different samples, ranging between 88 and 91%. The polymer powders were quenched from the melt in an ice-water bath, extracted at room temperature and aged for 1 month. Changes in the structural organization during the ageing time at room temperature were followed by X-rays, transport properties and infrared analysis. The X-ray diffrac- tograms showed that all the fresh samples crystallized in the disordered helical form I, containing also a fraction of chains in trans-planar conformation. The helical crystallinity was found very similar for all the samples, and a small increase of this parameter, not higher than 4-6%, was observed during the ageing time. At variance, the study of sorption of a vapor at low activity indicated the presence of an intermediate phase not crystalline, yet impermeable to the vapors, increasing during the ageing at room temperature. The fraction of the intermediate phase was very high in the lowest molecular weight sample, and decreased on increasing the molecular weight, reaching a constant value for molecular weights higher than 80 000. The analysis of the infrared spectra showed that the helical bands increase during the ageing, indicating that the intermediate phase is composed by very small and disordered helical crystals. The elastic modulus of each sample was measured soon after quenching and during the ageing. It was found a consistent increase of this parameter, not ascribable only to the small increase of crystallinity. The increase of the elastic modulus of all the samples was correlated to the increase of the intermediate phase derived by sorption, and a good dependence on this parameter was found.