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Half titanocenes (CpCH2CH2O)TiCl21 and (CpCH2CH2 OCH3)TiCl32, activated by methylaluminoxane are tested in styrene–1,3-butadiene copolymerization. The titanocene 1 is able to copolymerize styrene and 1,3-butadiene, with a facile procedure, to give products with high molecular weight. The analysis of microstructure by 13C-NMR reveals that the styrene homosequences in copolymers are in syndiotactic arrangement, while the butadiene homosequences are, prevailingly, in 1,4-cis configuration, according with behavior of 1 in the homopolymerizations of styrene and 1,3-butadiene, respectively. The reactivity ratios of copolymerization are estimated by diad composition analysis. All obtained copolymers have r1 × r2 values much larger than 1, indicating blocky nature of homosequences. The structural characterization by wide-angle X-ray powder diffraction and differential scanning calorimetry indicates that all copolymers are crystalline, with Tm varying from 171 to 239 °C, depending on the styrene content. The titanocene 2 did not succeed in styrene–1,3-butadiene copolymerization, giving rise to a blend of homopolymers. Compounds 1 and 2 were also tested in the polymerization of several conjugated dienes, and the obtained results were very useful to rationalize the behavior of both catalysts in the copolymerization of styrene and butadiene. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 815–822, 2010

Random styrene-ethylene copolymers have been synthesized by the hydrogenation of styrene-butadiene copolymers. The samples were characterized by 13 C NMR spectroscopy, X-ray powder diffraction, thermal analysis, and Fourier transform IR (FTIR) spectroscopy. In the 13 C NMR spectra, the resonance of the methylene carbon next to a methine bearing a phenyl group presents splitting due to its diastereotopic positions with respect to the phenyl substituents of the second and third inserted styrene units. A definitive assignment of the resonances of the SEES sequences (S = styrene, E = ethylene) was obtained by synthesis of a suitable model compound: 5,10-diphenyl-pentadecane. X-ray powder diffraction and thermal analysis show that these samples are amorphous. This is probably due to the shortness of the ethylene and styrene sequences in the copolymers. DSC thermograms show a single glass transition, at a intermediate temperature, between the Tg values of polystyrene and polyethylene, depending on the styrene content in the chain. FTIR spectra indicate the presence of styrene sequences in an isotactic arrangement.

The relationships between the structure and the viscoelastic properties of freeze/thaw PVA hydrogels obtained by repeatedly freezing and thawing dilute solutions of PVA in D 2 O(11% w/w PVA) in as-prepared and rehydrated states are investigated. Our results indicate that the PVA chains and solvent molecules are organized at different hierarchical length scales, which include the presence of micro-and macro-pores, into a network scaffolding. The porous network is ensured by the presence of crystallites, which act as knots interconnected by portions of PVA chains swollen by the solvent. X-ray diffraction and SANS techniques are used to obtain structural information at short (angstroms) and medium (nanometers) ranges of length scales, concerning the crystallinity, the size of small crystalline aggregates and the average distance between crystallites in PVA hydrogels. Indirect information concerning the structural organization on the large length scales (microns) are provided by viscoelastic measurements. The dynamic shear elastic moduli at low frequency and low strain amplitude, G', are determined and related to the degree of crystallinity. These data indicate that a minimum crystallinity of 1% is required for these PVA samples to exhibit gel behaviour and have allowed obtaining the order of magnitude of the average mesh size in these gels. Finally, it is shown that the negative effect of aging, inducing worse physical and mechanical properties in these systems, may be prevented using a drying/re-hydration protocol able to keep the physical properties of the as-prepared PVA hydrogels.

The structure of physical poly(vinyl alcohol) (PVA) hydrogels prepared by subjecting a PVA/D2O solution (11% w/w PVA) to freeze (−22 °C)/thaw (+25 °C) cycles has been systematically investigated by X-ray powder diffraction technique as a function of the number of cycles and aging time. The structural analysis has been extended to PVA “dried gels” and PVA hydrogels obtained by rehydrating the dried samples. The results of the present analysis confirm that highly stable PVA hydrogels, with a water uptake higher than 80%, may be obtained upon freeze/thaw cycles. The X-ray diffraction profiles of PVA hydrogels have been interpreted in terms of three components: “free water”, crystalline PVA aggregates, and swollen amorphous PVA. The degree of crystallinity and the size of the crystals increase with increasing the number of freeze/thaw cycles and the aging time. Our results support the hypothesis that PVA hydrogels have a porous structure, with pores mainly occupied by water. The porous walls consist of swoll...

The structure and rheological behavior of atactic poly(vinyl alcohol) (a-PVA) hydrogels prepared by freeze/thaw cycles were investigated as a function of polymer concentration and number of freeze/thaw cycles. The presence of phases with different mobilities was observed using 13 C CP/MAS and DP/MAS NMR experiments. The degree of crystallinity of the a-PVA-rich phase was determined by 1 H NMR free decay experiments. Measurements of the shear storage and loss modulus were performed at a fixed frequency of 1 Hz and a strain value of 0.1%, i.e. under conditions where the deformation imposed on the gel structure is entirely reversible. Results thus obtained showed that an increase in the number of freeze/thaw cycles induces an increase in the degree of crystallinity in the polymer-rich phase together with an increase in the storage modulus. The a-PVA hydrogels became more fragile as the number of freeze/thaw cycles was increased. Moreover, both the percentage of protons in a rigid environment measured by 1 H NMR and the storage modulus values tended to a limiting value after six freeze/thaw cycles. These results show that the first five or six freeze/thaw cycles play a very important role in determining the hydrogel structure and rheological properties. A more detailed comparison of NMR and rheological data led to the conclusion that the storage modulus is mainly controlled by the a-PVA crystallinity while the hydrogen bond interactions have a much smaller contribution.

Molecular dynamics simulations were employed in bulk polyisobutylene, to provide a detailed characterization of the methyl dynamics and to examine its relation to the β-relaxation as described by different experimental techniques. The simulation results were compared to new 13C NMR measurements probing methyl motion at temperatures above the glass transition, as well as to lower temperature literature NMR data. It was found that the 13C NMR results, and those for the β-relaxation reported by dielectric relaxation spectroscopy and neutron scattering experiments, can be interpreted on a common basis through a coupling mechanism between methyl rotation and a recently described fast relaxation motion associated with the chain backbone. In addition, a molecular mechanism for the description of the dielectric β-process observed in polyisobutylene is proposed, and investigated by simulations.