Arlete Queirós

Studies on thermal and thermo-oxidative degradation of poly(ethylene terephthalate) and poly(butylene terephthalate)

Abstract A comparative study was conducted into the thermo-oxidative degradation of poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT). Degradation of the polymer films and model compounds, ethylene dibenzoate (EDB) and butylene dibenzoate (BDB), was carried out in an oxygen atmosphere at 160°C. On the basis of the compounds identified by GC–MS a mechanism is proposed for the degradation of the model compounds that involves the oxidation at the α-methylene carbon with formation of unstable peroxides and carboxylic acids. From the studies performed under N 2 at 160 °C it could be concluded that benzoic acid and esters are products of the thermal degradation, while benzoic and aliphatic acids, anhydride and alcohols are due to thermo-oxidative degradation. In contrast to the thermo-oxidative degradation of other polymers, for PET and PBT, especially at the beginning, thermal degradation plays an important role. The results clearly showed that PET is more stable towards degradation than PBT.

Thermooxidative studies of poly(ether-esters) 1. Copolymer of poly(butylene terephthalate) and poly(ethylene oxide)

To determine the thermo-oxidative degradation mechanism of a copolymer of poly(butylene terephthalate) and polybutylene oxide, degradation experiments were carried out on this polymer and a model compound (dibutyleneglycol dibenzoate) at 140°C in an oxygen atmosphere. The changes were analysed using various analytical techniques, such as oxygen uptake measurements, measurement of CO/CO2 formation, 1H and 13C NMR, FT-IR and GC–MS. On the basis of the results, an oxidation mechanism for the model compound was proposed and it was concluded that oxidation is initiated at the methylene carbon adjacent to the ether oxygen. The thermal oxidation mainly results in the formation of a stable ester together with a formate, aliphatic and aromatic acids and an alcohol. Based on the similar degradation products found for the model compound and the polymer, it was concluded that the thermo-oxidation mechanisms in these two compounds are similar.

On carboxylate as a leaving group at the active site of Mo nitrogenase: electrochemical reactions of some MO and W carboxylates, formation of mono-, di- and tri-hydrides and the detection of an MoH2(N2) intermediate

Abstract A carboxylate ligand can function as a leaving group at a reduced molybdenum dihydride centre and this exposes a site at which dinitrogen or other small molecules can bind ; dihydrogen loss occurs after substrate binding ; protic attack intercepts carboxylate loss affording a trihydride intermediate which can partake in a hydrogen evolution cycle : it is suggested that carboxylate loss from the reduced Mo centre of nitrogenase might similarly lead to substrate binding and thence obligatory evolution of H 2 .

A comparative study of the mechanism of the thermo-oxidative degradation of poly(ethylene 2,6-naphthalate) and poly(butylene 2,6-naphthalate)

Abstract A comparison is made between the thermo-oxidative degradation of poly(ethylene 2,6-naphthalate) (PEN) and that of poly(butylene 2,6-naphthalate) (PBN) using oxygen uptake, FTIR (before and after chemical treatments) and color measurements. The model compounds ethylene dinaphthoate (EDN) and butylene dinaphthoate (BDN) were used to identify possible oxidation products. The results showed that thermal oxidation of PEN and PBN yields mainly naphthoic acid end groups and anhydrides, which indicates that the oxidation is initiated at the methylene group in the α position to the ester group. The results clearly showed that PEN is more stable towards oxidation than PBN.

Influence of diene content on the photodegradation of ethylene-propylene-diene (EPDM) elastomers

The photodegradation of EPDM based on dicyclopentadiene was followed by FTIR spectroscopy and the main photo-oxidation products were identified by derivatization reactions. It could be found that the photodegradation is initiated at the diene with formation of α,β–unsaturated carbonyl compounds and is then propagated to the ethylene-propylene segments. The propagation reactions are accelerated and, hence, the rate of photodegradation increases with the diene content.

Ligand rotamers and redox isomerism: metallo-pseudo-prolines

A ligand rotation which orientates the oxygen atom of an amide group towards or away from a neighbouring metal centre provides rotamers A and B with E° potentials which differ by 150 mV; oxidation of the metal centre of B switches the system from a state in which there is a small difference in the relative populations of the rotamers to one in which the cis-amide conformation greatly predominates; this new metallocycle system can be viewed as a pseudo-proline with redox sensitive rotameric properties.