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In recent years, aromatic substances have become the focus of environmental pollution-related concern due to their high stability and mutagenicity. In this regard, researchers have focused their attention on the development of photocatalytic processes to convert nitroaromatic compounds into aniline. In this work, the photocatalytic conversion of nitrobenzene (NB) to aniline (AN) was studied. The photocatalytic reaction was performed using commercial TiO2 (P25) and a photocatalytic aerogel, based on P25 embedded in syndiotactic polystyrene (sPS) aerogel (sPS/P25 aerogel) as photocatalysts. Different alcohols were used as hydrogen sources during the photocatalytic experiments. At the optimized operating conditions (photocatalysts dosage: 0.5 mg/L and 50% (v/v) EtOH%), an AN yield of over 99% was achieved. According to the results, this work could open avenues toward effective production of AN from NB using mild reaction conditions with sPS/P25 aerogel—in view of a possible scale-up of the photocatalytic process.

Fluorescent polypropylenes for the detection of PP-derived microplastic pollutants in organic tissues.

Monolithic aerogels of poly(2,6-dimethyl-1,4-phenylene)oxide (an industrially relevant polymer generally known as polyphenyleneoxide or PPO), have been prepared and characterized. These aerogels obtained by supercritical carbon dioxide extraction of thermoreversible gels of high molecular weight PPO present an apparent porosity typically in the range 70%–95% and high surface area values up to 750 m 2 /g. Moreover these aerogels are characterized by an excellent sorption uptake of organic pollutants at low vapor pressure and good recyclability by thermal regeneration. Good mechanical properties and dimensional stability of PPO monoliths is due to the polymer high molecular weight which ensures formation of a fibrillar morphology, already in the precursor gel. Sorption capacity and recyclability of PPO aerogels are mostly due to their crystalline phase: (i) its nanoporous-crystalline structure assures high specific surface areas; (ii) its high melting temperature assures a high thermal monolith stability as well as easy and cheap recovery of absorbed guest molecules by thermal treatments rather than by solvent extractions.

Summary Syndiotactic polystyrene films and aerogels with nanoporous crystalline phase and sulfonated amorphous phase have been prepared and characterized. The nanoporous nature of the crystalline phase confers to these materials a high solubility of volatile organic compounds (VOCs) from aqueous solutions even at low VOC concentrations while the sulfonation of the amorphous phase enhance the VOC sorption kinetics. These combined properties make these materials particularly interesting for a potential use as a sorption medium to remove traces of pollutants from water. Different aspects relative to the preparation, the structure and the transport properties of these innovative polymeric materials are described.

Stereoregular polymers like isotactic poly(N-butenyl-carbazole) (i-PBK), isotactic and syndiotactic poly(N-pentenyl-carbazole) (i-PPK and s-PPK), and poly(N-hexenyl-carbazole) (i-PHK and s-PHK) are synthesized using the stereospecific homogeneous “single site” Ziegler-Natta (Z-N) catalysts: rac-dimethylsilylbis(1-indenyl)zirconium dichloride (1)/methylaluminoxane (MAO) and diphenylmethylidene(cyclopentadienyl)-(9-fluorenyl)zirconium dichloride (2)/MAO. Catalytic activity is rationalized by density functional theory (DFT) calculations. All synthesized polymers are fully characterized by NMR, thermal, wide-angle X-ray diffraction, and fourier transform infrared spectroscopy analysis. Fluorescence measurements on isotactic and syndiotactic polymer films indicate that all polymers give rise to excimers, both “sandwich-like” and “partially overlapping.” Excimer formation is essentially driven by the polymer tacticity. Isotactic polymers generate both sandwich-like and partially overlapping excimers, while syndiotactic polymers give rise especially to partially overlapping ones. A theoretical combined molecular dynamics–time dependent DFT approach is also used to support the experimental results. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017

The synthesis and the microstructural characterization of isotactic poly (N-pentenyl-carbazole) (i-PPK) are reported. The achieved polymer is semicrystalline, the melting temperature is 138°C. i-PPK Fourier transform infrared spectroscopy (FTIR) spectrum and X-ray diffraction pattern are compared with those of polyvinylcarbazole. The observed differences are tentatively associated to higher flexibility of the i-PPK chains due to the alkylene group connecting the carbazole group to the main chain. The presence of intrachain and interchain interactions between carbazole units, which bring these groups to stack each other, is assumed. i-PPK optical properties are investigated and an electroluminescent behaviour of polymer is observed. Organic light emitting diodes based on i-PPK layers are founded to emit white light. POLYM. COMPOS., 36:1110–1117, 2015. © 2015 Society of Plastics Engineers

The controlled and pseudo-living ring-opening polymerization of a large ring size lactone, ω-6-hexadecenlactone (6HDL), was achieved for the first time by a dimethyl(salicylaldiminato) aluminum initiator. The obtained poly(ω-6-hexadecenlactone) was functionalized by thiol–ene coupling reaction. By epoxidation of the double bond a poly(6,7-epoxy-ω-hexadecalactone) was formed. The subsequent opening of the epoxide by NaCNBH3 produced a poly(6,7-epoxy-ω-hexadecalactone) with occasional inter and intra ether-type crosslinks. Modifications occurred without any change in the degree of polymerization. The obtained polymeric samples were characterized by NMR, GPC and DSC and X-ray diffraction analysis. The “polyethylene-like” orthorhombic crystal lattice of poly(ω-6-hexadecenlactone) is deformed in the poly(6,7-epoxy-ω-hexadecalactone) and is replaced by a hexagonal packing when the poly(ester) chains are hydroxylated. Furthermore, copolymerization of 6HDL with smaller e-caprolactone produced random copolymers, with average sequence block lengths of 2, while the sequential addition of 6HDL and e-caprolactone or rac-lactide allowed the preparation of block-copolymers. The block and random copolymers were also semicrystalline.

Fourier Transform Infrared spectroscopy (FTIR) and Wide-Angle X-Ray Diffraction (WAXD) measurements have clearly established the occurrence of a dual sorption ability of sulfonated syndiotactic polystyrene samples, which exhibit the nanoporous δ crystalline phase. In fact, large uptake (up to 20–30 wt%) of ionic liquid (IL; e.g. 1-ethyl-3-methylimidazolium dicyanamide) occurs only in the hydrophilic amorphous sulfonated phases and does not disturb the hydrophobic nanoporous crystalline δ phase. On the other hand, a large uptake of organic guests (e.g. naphthalene) occurs prevailingly in the nanoporous hydrophobic crystalline phase, independently of the presence of the IL in the amorphous phase, eventually leading to the formation of syndiotactic polystyrene co-crystalline phases. The thermal stability of IL can be largely increased by their inclusion in the amorphous phase of sulfonated syndiotactic polystyrene films. Copyright © 2012 John Wiley & Sons, Ltd.

Guest molecules, like cyclopentanone (CPO), N,N-dimethylformamide (DMF) tetrahydrofuran (THF) and 1,3-dioxolane (DOL), which are able to cocrystallize with poly(l-lactic acid) (PLLA), can also generate high degree of crystalline phase orientation, by simple sorption in amorphous unoriented PLLA films. This crystalline phase orientation is defined as a∥c∥ (a-parallel, c-parallel) because the ac plane of most crystallites is preferentially parallel to the film plane. This crystalline phase orientation, which is achieved without stretching even for high thickness films, is unprecedented for PLLA and can be maintained after suitable guest exchange procedures. The reported results confirm the recent hypothesis that cocrystallization with low-molecular-mass guest molecules could be a common route for getting uniplanar orientations of polymer crystalline phases, even in the absence of stretching. Wide angle X- ray diffraction analyses of unoriented and oriented PLLA samples have allowed proposing the structure of the cocrystalline form of PLLA with CPO. In particular, four PLLA chains exhibiting the 10/7 helical conformation and 16 CPO guest molecules are packed according the space group P212121 in an orthorhombic lattice with a = 1.61 nm, b = 1.26 nm, and c = 2.90 nm. The presence in PLLA cocrystalline structures of dense bc layers with close-packed polymer helices allows an easy rationalization of the observed uniplanar orientation.

The nanoporous crystalline phase of a cheap commercial polymer shows, already at room temperature, large uptake of gas molecules (e.g., butadiene and carbon dioxide) leading to polymer/gas clathrate phases. These clathrate phases have been characterized by X-ray diffraction and infrared linear dichroism measurements. Gas release kinetics are strongly reduced as a consequence of clathrate formation and can be further reduced by (010) orientation of the host phase, i.e., by placing the ac layers of close-packed enantiomorphous polymer helices nearly parallel to the film plane. The reported results indicate that this robust and easy-to-process polymeric crystalline framework is suitable for gas storage and controlled release.

The crystal structures of two molecular complex phases of syndiotactic polystyrene (s-PS) with 1,3,5-trimethyl-benzene (TMB) and 1,4-dimethyl-naphthalene (DMN) have been described. These structures present a monoclinic unit cell in which the s(2/1)2 polymer helices and guest molecules are packed according the space group P21/a and unit cell constants: a=17.3 A, b=15.4 A, c=7.8 A and γ=95.7° for s-PS/TMB and a=17.4 A, b=17.2 A, c=7.8 A and γ=116.4° for s-PS/DMN. Both structures can be described as intercalates, since they present ac layers of polymer helices alternated to layers of contiguous guest molecules and a guest/monomeric-unit molar ratio of 1/2, as recently observed only for s-PS/norbornadiene molecular complex. On the basis of a comparison between crystalline structures and X-ray diffraction data of several s-PS molecular complexes, a simple criterion to anticipate their clathrate or intercalate nature has been suggested.

An efficient method to align low-molecular-mass molecules by enclosing them as isolated guests of the nanoporous host δ phase of syndiotactic polystyrene oriented films, is presented. This new method is compared with the classic method consisting in the absorption of low- molecular-mass molecules as a solute of amorphous phases of highly oriented semicrystalline polymeric films. It is shown that this new molecular alignment method provides a powerful tool to study in detail several features (spectroscopic properties, molecular motions) of relevant molecules, by separating with unprecedented selectivity their anisotropic spectral responses. The reported results refer to infrared and solid-state- 2 H-NMR spectra.

In this contribution we analyse sorption and transport of several gases in semicrystalline syndiotactic polystyrene with nanoporous crystalline δ form. Investigation was performed on amorphous samples and on samples characterized by different degrees of crystallinity. Sorption isotherms of carbon dioxide, nitrogen and oxygen in the crystalline phase have been determined starting from experimental results obtained for semicrystalline and amorphous samples. Corresponding isosteric heats of sorption were evaluated for the crystalline and amorphous phase. Permeation tests were also performed to gather information on mass transport properties of semicrystalline samples, evaluating average diffusivities of carbon dioxide and oxygen, in the limit of small concentrations as function of degree of crystallinity.

Cosyndiotactic random copolymers of styrene/p-chlorostyrene (S/p-CS) have been prepared for the entire composition range. Their polymorphic behavior has been studied by means of wide-angle X-ray diffraction, differential scanning calorimetry, Fourier transform infrared spectroscopy, and ultraviolet spectroscopy. For all samples, the possible formation of clathrate structures has been investigated. A new clathrate phase, being colored and comprising trans planar chains, has been obtained for copolymers with high chlorinated comonomer unit content in the presence of the p-chlorostyrene guest. Both colored and trans-planar clathrates are unprecedented for styrene-based syndiotactic homopolymers and copolymers. A phase diagram of this copolymer system in the presence of the p-chlorostyrene guest has been reported.

The different kinds of crystalline orientations which can occur in industrially relevant isotactic polypropylene (i-PP) films are compared with those generally observed for other semicrystalline polymers. The peculiar bimodality typical of the uniplanar-axial orientation which is present in two-step biaxially stretched i-PP films is rationalized in terms of the two alternative stackings on the (0k0) face of bilayers of macromolecules of the monoclinic unit cell.

Copolymerizations of ethylene with 1,3-butadiene in the presence of catalytic systems based on C 2 -symmetric zirconocenes rac -(CH 3 ) 2 Si(2-R-4-R′-1-indenyl) 2 ZrCl 2 (where R = CH 3 – or H and R′ = C 6 H 5 – or H) are compared. The chemical nature and the relative amount of constitutional comonomer units from butadiene (1,4- trans , methylene-1,2-cyclopentane and methylene-1,2-cyclopropane) are strongly affected by the bulkiness of the substituent on the indenyl ligands. The unsubstituted indenyl zirconocene system rac -(CH 3 ) 2 Si(indenyl) 2 ZrCl 2 /methylalumoxane (MAO) inserts 1,3-butadiene leading to both cyclopentane and 1,4- trans units, whereas exclusively cyclopentane constitutional units are obtained from rac -(CH 3 ) 2 Si(2-methyl-1-indenyl) 2 ZrCl 2 /MAO. The catalytic system rac -(CH 3 ) 2 Si(2-methyl-4-phenyl-1-indenyl) 2 ZrCl 2 /(MAO) is able to incorporate about 30% of butadiene units into polymer chains and to form up to 10% of cyclopropane units. An unusual insertion mechanism for conjugated diolefins, that involves a butadiene η 2 primary coordination and insertion leading to formation of cycloalkane units, is largely predominant for all used catalytic systems. Mechanistic studies and DFT calculations indicate that the chemoselectivity of the reaction depends on the bulkiness of the substituents on the indenyl ligands of catalyst.

Details of the preparation and polymerization behavior of two chiral zirconocene catalysts are presented. The results of experimental and molecular-mechanics evaluation of their stereoselectivity with respect to liquid-propene polymerization are compared with those of a related catalyst system. The elastic properties of the obtained amorphous poly(propylene) due to the presence of small γ-form crystallites, are compared with those of a high-molecular weight atactic poly(propylene) obtained in the presence of a C 2v catalyst.

Clathrates with tetrahydrofuran of styrene-p-methyl styrene co-syndiotactic copolymers have been characterized by wide-angle X-ray scattering and differential scanning calorimetry. Stable clathrate structures with THF molecules have been observed in whole range of composition. In particular, syndiotactic polystyrene like and syndiotactic poly-p-methylstyrene like clathrate phases are obtained for p-methylstyrene contents lower and higher than 35% by mol, respectively.

A comparative X-ray diffraction study on unoriented and oriented samples of ethene based random copolymers with styrene and propene, obtained by single site metallocene catalysts is presented. For ethene-styrene copolymers, as usual for ethene copolymers with comonomers bulkier than propene, an orthorhombic crystal phase is generally observed. However, an oriented pseudo-hexagonal crystalline phase has been obtained in stretched samples, and compared with that one known for ethene-propene copolymers.

The crystalline structure of isotactic poly(vinylcyclobutane), claimed to be tetragonal, has been revised on the basis of fiber diffraction spectra. The chain symmetry is 41, as already assessed, but the conformation is different (the identity period is 6.91 A and not 6.60 A). The space group is the orthorhombic P212121 with lattice constants a = 17.14 A, b = 8.99 A, and c = 6.91 A. The excellent quality of the oriented samples allowed a full structure analysis and refinement. Integrated diffraction intensities, from a stretched fiber, measured with a Fuji image plate detector, have been utilized in a constrained refinement (the helix was assumed strictly of 41 symmetry and the C−C bond lengths were set to 1.54 A) obtaining a final R index of 15%. The refined chain bond angles are 103 and 107°, and the chain torsion angles are 165 and 273°.

Guest desorption procedures for s-PS clathrate samples, leading to formation and regeneration of the nanoporous δ phase, are compared. An extraction procedure, based on supercritical carbon dioxide, allows an easy and fast recovery of the guest molecules operating under relatively mild conditions (90–200 bar, 40°C) and generates a completely empty δ form, also starting from the most stable s-PS clathrate forms. In agreement with a previously proposed crystal structure of the nanoporous δ form, X-ray diffraction patterns of δ form powders obtained by this procedure do not show the peak, which is reduced but still apparent in samples extracted with previous procedures based on boiling solvents. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2077–2082, 1999

Random copolymers of tetrafluoroethylene with minor amounts of fluorinated comonomers (hexafluoropropene, perfluoromethylvinylether and perfluoropropylvinylether) of different compositions are characterized mainly by X-ray diffraction but also by differential scanning calorimetry and density measurements in an attempt to establish the possible inclusion of the respective side groups into the crystallites. Structural characterizations are conducted at room temperature (where all the considered copolymer samples are in the less ordered crystalline form I) as well as at -40 degrees C (where all the considered copolymer samples are in the more ordered crystalline form II). Increases of the lateral dimension of the unit cell and decreases of the size and amount of crystalline domains, with comonomeric unit content, are observed which are similar for different comonomer units (of largely different bulkiness) as well as for the two crystalline forms. The reported experimental results suggest a substantial exclusion from the crystallites not only of the bulkier comonomeric units (from perfluoropropylvinylether), but also, at least for high comonomer compositions, of the smaller comonomeric units (from perfluoromethylvinylether and hexafluoropropene). (C) 1998 Elsevier Science Ltd. All rights reserved.

Crystalline methylene-1,2-cyclopentane (E-1,2CP) random copolymers, containing up to 15% of cyclic units, are obtained in high yields by copolymerization of ethylene and 1,3-butadiene in the presence of rac-[CH2(3-methyl-1-indenyl)2]ZrCl2/MAO catalyst. The highly selective formation of trans methylene-1,2-cyclopentane units occurs by an intramolecular insertion of the double bond pendant from polymeril chain after an unusual primary vinyl insertion of 1,3-butadiene. Thermal properties and structure of E-1,2CP copolymers, as function of 1,2-cyclopentane molar content, are analyzed and compared with those of other ethylene copolymers. The variation in melting temperature and crystallinity observed in E-1,2CP copolymers is consistent with that observed for ethylene copolymers with comonomers bulkier than propylene. Moreover, the 1,2-cyclopentane units, as well as comonomers bulkier than propylene, cause only slight deformations of polyethylene orthorhombic unit cell parameters. These data are rationalized by assuming that 1,2-cyclopentane units, are confined in amorphous phase, while the polyethylene crystalline phase remains pure.

A preliminary model for the crystal structure of form III of isotactic poly(4-methylpentene-1) is suggested on the basis of structure factor calculations. Chains in 4(1) helical conformations are packed in a tetragonal unit cell with axes a = 19.38 angstrom and c = 6.98 angstrom; the space group is I4(1). The polymorphic behavior of poly(4-methylpentene-1) is discussed on the basis of conformational energy calculations on the isolated chain. The presence of different modifications is in accordance with energy minima present in the conformational energy map.

Examples of the application of simple molecular mechanics techniques to the study of the polymerization mechanism of various types of Ziegler-Natta catalysts are reported. Details are given on the new mechanism of enantioselectivity suggested by molecular mechanics studies. The validity of the latter, involving a “chiral orientation of the growing chain” which had never been suspected before, has been experimentally proved both for homogeneous and heterogeneous catalysis.

X-ray diffraction patterns of unoriented samples of a recently synthesized highly crystalline poly(3-methyl-1,3-pentadiene) show the presence of two polymorphic forms. For one of these forms a fibre pattern is presented and the identity period of the chain is determined. To obtain information on the type of tacticity of the polymer, geometric and energy analyses and Fourier transform calculations on isolated chain models have been performed. It is suggested that the experimentally observed chain axis repeat value is more easily accounted for by a 1,4- cis -syndiotactic structure.

The model previously proposed for the heterogeneous isospecific Ziegler-Natta polymerization, which agrees with a large number of experimental facts, has been examined in more detail. In particular a semiquantitative evaluation of the discriminating effects between the two faces of the co-ordinated prochiral l-propene molecule at the insertion step has been attempted. This study is presented together with a re-examination of the model at the co-ordination step, which considers in more detail the influence of the assumed values for the metal-carbon bond lengths on the stereospecificity. In agreement with previous suggestions, the analysis indicates the presence of enantioselectivity ( si olefin favoured on a Λ site independently of the configuration of the growing chain) at the insertion step also in the absence of enantioselectivity at the co-ordination step.

The transport of chloroform in films of atactic polystyrene and of semi- crystalline syndiotactic polystyrene in its nanoporous form (d-form) has been investi- gated by gravimetric analysis. Experimental tests have been performed at 35, 49, and 56 8C and at several vapor pressures ranging from 0.5 to 100 Torr. Nonequili- brium lattice fluid prediction of the amorphous sorption behavior was used to enucle- ate the sorption contribution of the crystalline nanoporous phase from semicrystalline samples. Sorption behavior of the crystalline phase has been interpreted on the basis of Langmuir equation. Moreover, the chloroform sorption at low activities in the crystalline nanoporous phase has been predicted by using Grand Canonical Monte Carlo molecular simulations. Isosteric heats of sorption were also experimentally evaluated for the crystalline phase, and compared with the corresponding prediction of molecular simulation. V C 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46

Syndiotactic polystyrene (sPS) films with uniplanar-axial orientation of their co-crystalline phase with a fluorescent guest (naphthalene, NP) have been prepared and the orientation of their crystalline phase has been fully characterized by X-ray diffraction measurements. The prepared films have been examined in detail using Nishijima's method, where angular distribution of NP polarized fluorescence intensities was measured at each setting film angle by the rotation of films around the excitation light beam. This polarized fluorescence method turns out to be quite effective for monitoring the orientation of fluorescent guest molecules in the films. In particular, in semicrystalline films presenting uniplanar-axial orientation of their sPS/NP clathrate phase, NP molecules were found to exhibit a high three-dimensional orientational order all over the films. The experimental data have also allowed to determine the orientation of the NP guest molecule with respect the axes of the co-crystal unit-cell: in satisfactory agreement with molecular modeling results, angles formed by the short and the long axes of NP with the c axis are not far from 80° while the angle between the short axis of NP and the a axis is nearly 105°.

A conformational study of a possible model for chain end control in the homogeneous isotactic specific Ziegler-Natta polymerization is presented. The model site consists of a complex with two ligands (olefin and growing chain) besides two cyclopentadienyl ligands. The geometries corresponding to the located energy minima are very similar to those of the chiral model previously proposed by us for the site control in the homogeneous isospecific Ziegler-Natta polymerization. In this case the diastereoisomeric intermediates for isotactic and syndiotactic propagation are energetically similar but, according to our assumptions, the insertion path for the propene would be easier in the case of isotactic propagation.

A comparison between 13C NMR spectra of polymer solutions and Fourier transform infrared spectra of polymer films for several ethene–butadiene copolymers, including most butadiene as 1,2-cyclopropane and 1,2-cyclopentane units, is presented. This spectral comparison, also with the help of a low-molecular-mass model compound specifically synthesized for this purpose, has allowed to locate infrared absorbances associated with the cyclic comonomer units. In particular, a satisfactory calibration curve for evaluation of the amount of the reactive cyclopropane units, from the absorbance of the 1020 cm−1 band, has been achieved. An analysis of 1,2-cyclopropane unit reactivity in annealed films, has also pointed out that the presence of remnants of the polymerization catalyst can produce large increase in cyclopropane reactivity.

Syndiotactic polystyrene (s-PS) films including a fluorescent guest (naphthalene, NP) in their nanoporous δ crystalline phase have been prepared and characterized by X-ray diffraction, infrared linear dichroism, molecular modeling, and fluorescence depolarization techniques. A nearly perpendicular orientation of the fused rings of NP guest molecules with respect to the chain axis of the δ crystalline structure has been established by molecular mechanics and by evaluation of directions of transition moment vectors of infrared vibrational modes. The fluorescence depolarization is more efficient for NP guest molecules into the host phase than for NP molecules simply absorbed in the amorphous phase. This could be due to a more efficient resonance energy transfer between guest molecules, possibly associated with their ordered positioning and orientation into the host nanoporous polymeric crystallites.

The crystal structure of the molecular complex of syndiotactic polystyrene (s-PS) with bicyclo-[2.2.1]hepta-2,5-diene (norbornadiene, NB) is presented. In analogy to the already described s-PS molecular complexes, this structure presents a monoclinic unit cell (cell constants a = 17.5 Å, b = 14.5 Å, c= 7.8 Å, and gamma= 107.8°) in which the s(2/1)2 polymer helices and guest molecules are packed according to the space group P21/a. However, the structure of the s-PS/NB complex presents layers of contiguous guest molecules alternated with layers of enantiomorphous polymer helices and hence can be described as an intercalate phase. On the other hand, all the known structures of other s-PS molecular complexes present isolated molecules (or a couple of small molecules) into the cavities typical of the nanoporous s-PS ä form and hence can be described as clathrate phases. The molar ratio between the monomeric unit and the guest is 2/1 for the s-PS/NB intercalate phase, while it is generally 4/1 for the s-PS clathrate phases.

The possibility to obtain crosslinked polyethene (XLPE) from new ethene copolymers containing cyclopropane rings (C3), synthesized by ethene/1,3-butadiene copolymerizations with a hindered metallocene catalyst, has been explored. In particular, the thermal crosslinking of these copolymers, for C3 molar content in the range 0.3–2.7%, has been studied for annealing temperatures up to 200 °C and compared with the thermal behavior of other ethene–butadiene copolymers. Only for the ethene–C3 copolymers, the annealing procedures in the temperature range 160–200 °C lead to high gel fractions. These crosslinking processes occur in reasonable times and leave nearly unaltered the polymer crystallinity. Moreover, annealing procedures up to 140 °C do not produce appreciable crosslinking. All these features suggest that these new copolymers could be suitable for possible industrial fabrication processes of XLPE.

The possible formation of clathrate phases with several different guest molecules for co-syndiotactic random copolymers of styrene and p-methylstyrene has been investigated. In the presence of suitable guests, like carbon disulfide and tetrahydrofuran, clathrate phases are stable for the entire copolymer composition range. However, discontinuous changes between clathrates structurally similar to those observed for syndiotactic polystyrene and syndiotactic poly(p-methylstyrene) have been observed by wide-angle X-ray diffraction and differential scanning calorimetry. Wide copolymer composition ranges of stability have been observed for carbon disulfide/syndiotactic polystyrene and for tetrahydrofuran/syndiotactic poly(p-methylstyrene) clathrates. A phase diagram inclduing clathrate and crystalline forms of this copolymer system has been presented.

The transport of chloroform in films of semicrystalline syndiotactic polystyrene (s-PS) in its nanoporous form (δ-form) has been investigated by gravimetric analysis and time-resolved FTIR spectroscopy. Experimental tests have been performed at 56 °C and at several vapor pressures ranging from 5 to 100 Torr. Sorption and desorption kinetics have been monitored with both techniques, and the dependence of diffusion coefficients with penetrant concentration was investigated, evidencing Fickian features typical of systems characterized by diffusivity increasing with concentration. Analysis of the vibrational spectrum of chloroform sorbed in the crystalline phase showed significant perturbations when compared to the spectrum of the isolated molecule or the molecule absorbed in amorphous s-PS. This perturbation has been attributed to host−guest molecular interactions whose strength was found to be relatively small. Conformational rearrangements of macromolecular chains in the amorphous phase were detected as a ...

A stereoregular alternating ethylene−styrene copolymer, also stretchable in fiber form, has been obtained with rac-isopropylidenebis(1-indenyl)zirconium dimethyldiamide activated by the methylalumoxane catalytic system. From X-ray diffraction fiber spectra, an isotactic structure has been assigned to this copolymer as well as the previously described stereoregular ethylene−styrene copolymers. The polymer has zigzag planar chain conformation with phenyl groups oriented perpendicularly to the chain axis. The crystallographic symmetry in B2/m. Lattice constants (monoclinic, unique axis c): a = 10.23, b = 15.53, c = 5.12 A; γ = 98.6°. The disagreement index for measured reflections is R1 = 0.10.

Chemical separations based on absorption phenomena into syndiotactic polystyrene samples including a nanoporous crystalline phase are described. These separation phenomena are due to the presence into the crystalline phase of cavities whose size and shape have been studied through sorption experiments relative to linear hydrocarbons and through calculation procedures relative to the available crystalline structures of different polymorphic and clathrate forms of syndiotactic polystyrene.

Structural (X-ray diffraction), melting (differential scanning calorimetry), as well as mechanical (tensile tests) characterizations on uncrosslinked ethene-propene copolymer samples, obtained using a metallocene-based catalytic system and having an ethene content in the range 80-50% by mol, are reported. Samples with an ethene content in the range 80-60% by mol present a disordered pseudohexagonal crystalline phase, whose melting moves from 40°C down to -20°C as the ethene content is reduced. The dramatic influence of the crystalline phase on tensile properties of uncrosslinked ethene-propene copolymers is shown. In particular, highest elongation at break values are obtained for samples being essentially amorphous in the unstretched state and partially crystallizing under stretching. On the other hand, lowest tension set values (most elastic behavior) are observed for samples presenting, already in the unstretched state, microcrystalline domains acting as physical crosslinks in a prevailing amorphous phase.

A model for the crystal structure of syndiotactic poly(4-methyl-1-pentene) is presented. According to the present analysis, chains having helical conformations with unit twist t = 210° (1.714 conformational repeat units, each composed of two monomeric units per helix turn) and unit height h = 3.91 A , corresponding essentially to s( 12 7 )2 symmetry, are packed in a tetragonal unit cell with a = 18.03 A . The density is 0.878 g cm−3 with four chains in the unit cell, space group P 4 .

The synthesis of isotactic, syndiotactic and atactic poly(allyltrimethylsilane) (PATMS) is described, along with the characterization of these polymers using thermal methods, X-ray diffraction, solution and solid-state nuclear magnetic resonance (n.m.r.). Isotactic PATMS was produced from rac- Me 2 SiInd 2 ZrCl 2 MAO (methylalumoxane) and displayed chains with an s ( 3 1 ) symmetry in the crystalline domains. Syndiotactic PATMS, produced from Me 2 C(Cp)(Flu)ZrCl 2 MAO , was a semicrystalline polymer with chains in the crystalline phase exhibiting s ( 2 1 )2 helical symmetry. A combined X-ray diffraction and solid-state n.m.r. analysis on poorly crystallized samples of syndiotactic PATMS indicates the occurrence in the crystalline phase, for given crystallization conditions, of disorder in the packing of chains, which preserve the helical conformation. Atactic PATMS, an amorphous polymer, was produced using an Ind 2 ZrCl 2 MAO catalyst.

A preliminary characterization of the crystalline structure of syndiotactic poly(4-methy-1-1-pentene) is presented. X-ray diffraction patterns of powder and fiber samples are reported. On the basis of X-ray diffraction data as well as conformational energy calculations, a helical chain conformation with a sequence of dihedral angles of the kind almost-equal-to (TTGG)n having symmetry s(12/7)2 and chain repetition c = 46.91 angstrom is suggested. A tetragonal unit cell with axis a = 18.03 angstrom is proposed.

The thermal properties and the crystalline structure of poly(methylenecyclopentane) samples with different microstructures are investigated. Similar disordered crystalline structures are formed, independently of the microstructure of the sample. These structures present hexagonal packing of the chain axes and extended chains with configurational and conformational disorder, which generate intermolecular rotational and translational disorder. The distance between adjacent parallel chains decreases with the increase of rings with the cis configuration ; correspondingly, the melting temperature strongly increases. Both interchain distances and melting temperatures seem poorly dependent on the relative stereochemistry between rings. The degree of crystallinity and the correlation length of the crystalline domains tend to increase with the stereoregularity in the configuration of the rings (both cis and trans). The melting entropy increases with all the examined kinds of stereoregularity (cis, trans, and isotactic).

The orientation of the crystalline δ nanoporous phase in syndiotactic polystyrene films, obtained by different procedures, have been characterized. For both solution cast and biaxially stretched films a high degree of uniplanar orientation, corresponding to the tendency of the ac crystallographic planes, to be parallel to the film plane has been observed and rationalized. According to molecular dynamics simulations of diffusion of small molecules into the δ nanoporous phase, this uniplanar orientation would minimize the molecular diffusivity through the nanoporous crystalline phase.