Fiorella Pradella

Crystal structure and spectroscopic analysis of melamine hydrobromide. Evidence for iso-melamine cations and charge-transfer complexes in the solid state

The crystal structure and vibrational analyses of melamine hydrobromide by FTIR and FT-Raman spectra, are presented. The crystallographic data show π-electron delocalization towards the amino substituents with ring nitrogen protonation in the solid state. Additionally, the presence of intermolecular hydrogen bonding interactions occurring between amino substituents and ring nitrogen lone-pairs gives rise to charge–transfer complexes, as demonstrated by UV–VIS reflectance in the solid state. FTIR and FT-Raman spectra allow the assignments of the vibrational modes in melamine hydrobromide by comparison with the corresponding deuteriated molecules, and show the existence of iso-melamine cations in the solid state.

A reappraisal of the photo-oxidation mechanism at short and long wavelengths for poly(2,6-dimethyl-1,4-phenylene oxide)

Abstract A detailed re-examination of the photo-oxidation mechanism of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) at both long and short irradiation wavelengths under accelerated conditions is reported. The formation rate of polydimethylphenoxy (PDMP) radicals is observed both in the presence and in the absence of ultra-violet (u.v.) light by using electron spin resonance spectroscopy. In order to explain the great stability of PDMP radicals, the bimolecular decay observed in the dark is discussed. In addition, spin-trapping experiments using nitrosodurene reveal the existence of benzylic-type radicals under u.v. irradiation conditions. Under mild oxidative conditions, i.e. thermo-oxidation at 80°C, the changes in the Fourier-transform infra-red spectra after 1215 h show the formation of two main absorption bands at 1694 and 1658 cm −1 , assigned to aromatic aldehydes and quinonic groups, respectively. The poor photo-stability of these two primary oxidation products generates, by fast photolytic conversion to benzoic acids and esters. a broad band centred at 1734 cm −1 under accelerated photo-oxidative conditions. Furthermore, the cyclic voltammetry of PPO solution in the dark indicates both the existence of superoxide radical anions in the absence of u.v. light and the electro-catalytic effect possibly induced by quinonic groups. In this work direct formation of radical cations and superoxide anions by a photo-induced electron-transfer reaction is suggested under photo-oxidative conditions. The catalytic role of quinonic groups on the further generation of superoxide anions and PDMP radicals at long irradiation wavelengths and in the dark is also considered on the overall PPO photodegradation.

Photodegradable polyolefins. Photo-oxidation mechanisms of innovative polyolefin copolymers containing double bonds

Abstract Photodegradable polyolefins based on ketone carbonyl and carbon monoxide copolymers are discussed. The proposed photo-oxidation mechanisms for these copolymers show efficient chain scission processes photoinduced by Norrish I and II reaction in polymer matrix. Other polyolefins copolymers containing unsaturation have been suggested as new innovative and photodegradable materials. In the photo-oxidative conditions, the formation of α,β-unsaturated ketone impart high yields of chains scissions leading to a rapid embrittlement and fragmentation of these copolymers.

Photochemical behavior of poly(organophosphazenes). XI. Photochemistry of poly[bis(4-benzylphenoxy) phosphazene]

In this paper we present results on the photolysis of poly[bis(4-benzylphenoxy)-phosphazene] in solution and in film, both in the presence and in the absence of molecular oxygen. Light irradiation of the polymer in oxygen-saturated CH2Cl2 solutions results in a remarkable degradation of the polyphosphazene, while in argon-purged solutions no appreciable variations of the polymer structure could be detected. The photolysis of poly[bis(4-benzylphenoxy)phosphazene] in films induces the cross-linking of the polymer regardless of the presence or the absence of molecular oxygen. The main process observed during the photochemistry both in solution and in the solid state of the polymer is the oxidation of the 4-benzylphenoxy group on the polyphosphazene, without involvement of the inorganic -P=N- backbone. The effect of temperature on the photolysis of the polyphosphazene substrate in film is also reported.

Grafting reactions onto poly(organophosphazenes). II: Photo-induced graft copolymerization of polymethylmethacrylate onto poly[bis(4-isopropylphenoxy)phosphazene]

Abstract This paper presents results on the photo-induced grafting of poly(methyl methacrylate) onto poly[bis(4-isopropylphenoxy)phosphazene], using benzophenone as photoinitiator. The formation of polyphosphazene-g-poly(methyl methacrylate) copolymers was followed by FTIR-ATR spectroscopy, and the products were characterized by contact angle and DSC measurements. Among the parameters affecting the photografting, irradiation time and the ratio methanol/unsaturated monomer used in the grafting experiments are of particular importance.

Grafting reactions onto poly(organophosphazenes)-III. Light-induced graft copolymerization of poly-N,N′-dimethyl-acrylamide onto mobile-hydrogen-containing phosphazene polymers

Abstract In this paper we report the light-induced grafting copolymerization of N , N ′-dimethylacrylamide onto four poly(organophosphazene) films i.e. poly[bis(4-isopropylphenoxy)phosphazene], poly[bis(4-s-butylphenoxy)phosphazene], poly[bis(4-benzylphenoxy)phosphazene] and poly[bis(2,2′,2″-trifluoroethoxy)phosphazene]. The process was carried out in N , N ′-dimethylacrylamide/methanol mixtures in presence of benzophenone as sensitizer. The yield of the grafting process was evaluated as a function of the phosphazene structure, of the relative amounts of N , N ′-dimethylacrylamide and methanol in the reaction mixtures, and of the swelling capability of the polymer films. The obtained polyphosphazene-g-poly- N , N ′-dimethylacrylamide copolymers were characterized by i.r. spectroscopy, both ATR and transmission, and by contact angle measurements.

Matrix and solution photochemistry of [C5(CH3)5Rh(CO)2]

Abstract The photochemical behaviour of [C 5 (CH 3 ) 5 Rh(CO) 2 ] has been examined in a polystyrene matrix and in a hexane solution. Photodissociation of CO yields the coordinatively unsaturated [C 5 (CH 3 ) 5 RhCO] fragment as the primary photoproduct. Photodissociation of the C 5 (CH 3 ) 5 Rh bond is observed in solution upon long wavelength irradiation of the [C 5 (CH 3 ) 5 Rh(CO) 2 ] giving ·Rh(CO)2 and C 5 (CH 3 ) 5 · radicals as primary photoproducts. The identity of the radicals has been demonstrated by spin trapping ESR spectroscopy experiments. After prolonged photolysis in a polystyrene matrix, the CH activation product [C 5 (CH 3 ) 5 Rh(CO)(H)(polystyrene)l is formed via [C 5 (CH 3 ) 5 RhCO]. The role of the [C 5 (CH 3 ) 5 RhCO] intermediate in solution reactions is discussed.

Electron transfer reactions of bis[dicarbonyl(pentamethylcyclopentadienyl)iron] with 2,3-dichloro-5,6-dicyanobenzoquinone

Abstract The thermal and photochemical reactions of [(C 5 Me 5 )Fe(CO) 2 ] 2 with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) yield the binuclear complex {[(C 5 Me 5 )Fe(CO) 2 ] 2 ·(DDQ) 2 }. EPR studies using nitroso spin traps support the conclusion that the primary products are [(C 5 Me 5 Fe(CO) 2 ] + radicals. The metal-centered radical transfers an electron to DDQ acceptor to yield {[(C 5 Me 5 )Fe(CO) 2 ] 2 ·(DDQ) 2 } as the only Fe-containing product. Spectroscopic evidence for dimerization of DDQ − to form diamagnetic [DDQ] 2− 2 in the solid is discussed.

Photochemical behaviour of poly(organophosphazenes). 14. Photooxidation of poly[bis(4-isopropylphenoxy) phosphazene] under accelerated conditions

The photooxidation of poly[bis(4-isopropylphenoxy)phosphazene] under accelerated conditions has been followed by FTIR and UV visible spectroscopic techniques. The main photooxidation products are acetophenone and phenol groups. In addition, acetone vapors have been detected by GC MS combined techniques concomitant with the IR spectral changes in the CH stretching region, suggesting a significant decrease in the isopropyl moieties. The presence of polymeric sequences having phenol groups under our conditions gives origin to further oxidation reactions due to electron transfer or radical recombination or to hydrogen abstraction reactions promoted by unhindered phenoxyl radicals. In addition, the absence of UV visible light, i.e., under thermooxidation reaction at 60° C has demonstrated that phenol groups are the main responsible of secondary oxidation products. The complexity of the photooxidation mechanism in the solid state for this polymer makes it difficult to determine a definitive degradation mechanism under both thermo- and photooxidative conditions.