Antonino Recca

A topological approach to the interaction of lanthanide shift reagents with the carbonyl group : Its relevance to conformational analysis

Abstract A topological approach to the LIS simulation process of compounds bearing the carbonyl functional group is reported. The Lanthanide was always found to complex in the less hindered position, along the sp 2 lone-pair orbital of the O atom. The relevance to chemistry of such an approach is that only chemical significant positions are considered as feasible sites of complexation of the Lanthanide with the substrate, disregarding all other positions. More important, the results obtained have thrown some light on the factors that may influence the equilibria of conformationally mobile systems, in the presence of LSR. The reliability of the method employed has been tested in several disparate carbonyl derivatives.

Conformational preferences and electronic effects in selenophene and tellurophene carbonyl derivatives investigated by lanthanide induced shifts

Computer simulation of the lanthanide induced shifts has been applied to study of the conformational preferences in the 2-formyl and 2-acetyl derivatives of furan, thiophene, selenophene and tellurophene. The results assign a nearly equipopulated mixture of s-cis and s-trans conformers to the furan, and a preponderance of the s-trans form to the thiophene, selenophene and tellurophene derivatives. This difference is interpreted as due to the interaction between the heteroatom and carbonyl oxygen lone pairs. The 2-N,N-dimethylcarboxyamide derivatives of furan, thiophene and selenophene are found to exist mainly in a quasi-planar s-cis form. The barriers to the rotation about the amide bond in these amides have been measured and related to the electronegativity of the heteroatom.

Synthesis, characterization and study of the thermal properties of new polyarylene ethers

Abstract In order to investigate the contrasting propensities for crystallization between aryl sulphone and aryl ketone polymers, new aromatic monomers containing sulphone, ketone and ehter linkages were synthesized and reacted to prepare new polyarylene ethers. The polymers obtained were characterized by solution viscosity, thermal gravimetric analysis and differential scanning calorimetry (d.s.c.). The effect of the presence of ketone and/or sulphone linkages in the polyarylene ethers during synthesis and on the final properties is discussed.

Evaluation of the influence of various (ether, ketone and sulfone) groups on the apparent activation energy values of polymer degradation

Abstract The kinetics of degradation of four model polymers containing ether, carbonyl and sulfone groups was studied in both N 2 flow and static air atmosphere, with the aim of evaluating the influence of various groups on the apparent activation energy values ( E a ) of degradation. The initial temperatures of decomposition ( T i ) and the temperatures of half decomposition ( T 1/2 ), were also determined. The degradation of all polymers was carried out in the scanning mode and the Kissinger equation used to evaluate the E a values of the first degradation stage. The results allowed us to make a kinetic stability classification among various groups (sulfone>ketone>ether) which could be explained on the basis of the double-bond character of the polymer chain links. The E a values showed a trend analogous to the glass transition temperatures ( T g ), in agreement with literature reports. By contrast, the characteristic temperatures of decomposition showed a different trend which could not be interpreted on the basis of the experiments until now performed.

Natural Ageing of Automotive Polymer Components: Characterisation of New and Used Poly(propylene) based Car Bumpers

The chemical and morphological structure of new and post-consumer ten years old poly(propylene) based car bumpers has been investigated and their mechanical properties have been correlated to the chemical and physical degradation of the material. Poly(propylene) matrix and a rubbery ethylene-propylene phase were the constituents of the bumpers. A strong impairment of the mechanical properties has been found in old samples which showed also a distinctive phase separation. The modification of the chemical structure was much less evident: oxidation on the exposed surface involving the poly(propylene) phase has been highlighted in old bumpers, and possible crosslinking. These relatively small changes, however, are responsible for the pronounced physical ageing. A tentative explanation is a poor interfacial adhesion of rubber particles from which the crazes can propagate further to form cracks.

Dynamic stereochemistry of bis(N-isopropylsalicylaldiminato) beryllium(II): enantiomerization process

Bis(N-isopropylsalicylaldiminato)beryllium has been prepared. Its variable temperature 1H NMR spectrum has been investigated in 1,2-dichlorobenzene, and the free energy of activation (22.1 kcal/mole at 139°C) thus determined for enantiomerization (inversion at the beryllium tetrahedral atom) without prior separation of the optical antipodes.

Dynamic Stereochemistry of cis-Octahedral bis(β-Diketonato) complexes

Abstract The dynamic aspects of the stereoisomerizations of some bis(β-diketonato) complexes have been investigated with the aid of permutational analysis and topological representations. The analysis indicates that the lowest energy rearrangement mode (M 4 ) occuring in such complexes, and involving a reversal of the helecity of the structure, is permutationally equivalent to the “one-ring flip” mechanism of diaryl derivatives of the type Ar 2 ZX, which are shown to be stereochemically correspondent to the bis-chelate molecules. The activation energies associated with such arrangements have been determined by variable temperature NMR studies. Only for bis(acetylacetonato) bis-(triphenylphosphine)ruthenium(II) is the activation barrier high enough to allow resolution of the two enantiomers at ambient temperatures.

Surface characterisation of collagen-based bioartificial polymeric materials.

An electron spectroscopy for Chemical Analysis (ESCA) characterization of films based on collagen-poly(vinylalcohol) blends, cross-linked by dehydrothermal treatment is reported. The semiquantitative analysis indicates a remarkable enrichment of collagen in the surface with respect to its initial percentage utilized to prepare the blends.

Static sterochemistry of the β-diketonate complexes of group II metals

Abstract The static sterochemistry in solution of the β-diketonate complexes of Be, Mg, Ca, Ba and Zn was investigated using 1H-NMR techniques. It was ascertained that the bis-acetylacetonate complexes of Mg, Ca and Ba are polymeric in solution, in contrast to the dipivaloylmethanate complexes of Mg and Zn which are monomeric in solution. The benzoylacetonate chelates of all the metals studied were found also to be monomeric in solution. Chemical shift values and other arguments led us to favor a pseudo-tetrahedral D2d idealized structure for all the complexes investigated.

A preliminary investigation of the surface photo-oxidation of copolymers of styrene and 2-(2-hydroxy-3-vinyl-5-methylphenyl)-benzotriazole

ESCA has been used to monitor the changes in the surface composition of polystyrene and 2-(2-hydroxy-3-vinyl-5-methylphenyl)-benzotriazole/styrene (HVB/sty) copolymers during uv (λ > 300 nm) irradiation in air. Oxygen uptake in the surface is more rapid for the copolymers and is dependent on the benzotriazole content of the surface. On extended exposure the nitrogen content in the surface increases. The behaviour of the HVB in the surface is indicative of photo-sensitisation and not of photo-stabilisation.