Vincenzo Venditto

Polymeric sensing films absorbing organic guests into a nanoporous host crystalline phase

Abstract Syndiotactic polystyrene (s-PS) semicrystalline films, including the nanoporous crystalline δ phase, have been tested as sensing elements of resonant sensors for vapors of volatile organic compounds (VOCs). In particular, the response to chloroform vapor of quartz crystal microbalance (QCM) sensors, coated with films of semicrystalline δ form of s-PS have been analyzed and compared to analogous systems coated with films of amorphous atactic polystyrene (a-PS). The sensitivity of sensors based on s-PS films was found markedly higher than those based on a-PS, particularly for low chloroform pressures. The higher sensitivity of the semicrystalline s-PS films is associated with a peculiar sorption mechanism: in fact, the organic compound, rather than being dissolved only into the amorphous phase, as it is generally the case for semicrystalline polymers, is mainly absorbed into the nanoporous crystalline phase, each molecule being confined into regularly spaced crystalline nanocavities, thus leading to a clathrate structure.

Regeneration of nanoporous crystalline syndiotactic polystyrene by supercritical CO2

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.

Molecular Sensing by Nanoporous Crystalline Polymers

Chemical sensors are generally based on the integration of suitable sensitive layers and transducing mechanisms. Although inorganic porous materials can be effective, there is significant interest in the use of polymeric materials because of their easy fabrication process, lower costs and mechanical flexibility. However, porous polymeric absorbents are generally amorphous and hence present poor molecular selectivity and undesired changes of mechanical properties as a consequence of large analyte uptake. In this contribution the structure, properties and some possible applications of sensing polymeric films based on nanoporous crystalline phases, which exhibit all identical nanopores, will be reviewed. The main advantages of crystalline nanoporous polymeric materials with respect to their amorphous counterparts are, besides a higher selectivity, the ability to maintain their physical state as well as geometry, even after large guest uptake (up to 10–15 wt%), and the possibility to control guest diffusivity by controlling the orientation of the host polymeric crystalline phase. The final section of the review also describes the ability of suitable polymeric films to act as chirality sensors, i.e., to sense and memorize the presence of non-racemic volatile organic compounds.

Possible model for chain end control of stereoregularity in the isospecific homogeneous Ziegler-Natta polymerization

Abstract 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.

Photoisomerization patterns based on molecular complex phases of syndiotactic polystyrene

Syndiotactic polystyrene (s-PS) films presenting molecular-complex crystalline phases with norbornadiene (N), possibly suitable for data storage systems with molecular size marks, have been thoroughly investigated. In particular, a procedure suitable for obtaining polymer films containing N molecules only in the crystalline phase and the long-term stability of this s-PS/N molecular-complex crystalline form have been investigated by thermogravimetric, infrared linear dichroism and X-ray diffraction experiments. The possible loss of guest molecules during photoisomerization processes from N to quadricyclane (Q) has also been studied. Moreover, the N and Q patterns obtained by photoisomerization and their long-term stability have been investigated by Fourier transform infrared microscopy.

Zirconium and hafnium Salalen complexes in isospecific polymerisation of propylene

The activity of dibenzylzirconium and dibenzylhafnium Salalen complexes in polymerisation of propylene with MAO as a cocatalyst is described. Three Salalen ligand precursors combining a bulky alkyl group (1-adamantyl) on the imine-side phenol and electron withdrawing halo groups of different sizes on the amine-side phenol were explored. All metal complexes were obtained as single diastereomers. An X-ray crystallographic structure of a hafnium complex of an additional ligand carrying the combination of tert-butyl and chloro substituted phenolates, 4-Hf, revealed a fac-mer wrapping of the Salalen ligand around the metal centre. All complexes led to active catalysts in propylene polymerisation and to isotactic polypropylene of high regioregularity. The zirconium complexes led to polypropylene having molecular weights of Mw = 132,000-200,000 and isotacticities of [mmmm] = 65.7-75.0%. The hafnium complexes led to polypropylene of higher molecular weights of Mw = 375,000-520,000 and higher stereoregularities of [mmmm] = 80.6-89.3%, the highest isotacticity obtained with 3-Hf.

C2‐Symmetric Zirconocenes for High Molecular Weight Amorphous Poly(propylene)

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.

Structural analogies between homogeneous and heterogeneous catalysts for the stereospecific polymerization of 1-alkenes

Abstract The stereospecificity of 1-alkene polymerization in the presence of Ziegler—Natta catalysts is discussed comparatively for heterogeneous and homogeneous systems. In particular, from molecular mechanics calculations on model catalytic sites, it is proposed that nonbonded interactions at the active center play a major role in determining the catalyst stereocontrol; in all cases, the chiral orientation assumed by the first C C bond of the growing polymer chain seems to be crucial for the onset of the enantioselectivity.

Crystal structure of syndiotactic poly (4-methyl-1-pentene)

Abstract 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 .

Ring-opening polymerization of ω-6-hexadecenlactone by a salicylaldiminato aluminum complex: a route to semicrystalline and functional poly(ester)s

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.