Thermally induced structural transformations of linear coordination polymers based on aluminum tris(diorganophosphates)

Abstract

The thermal transitions of inorganic–organic hybrid polymers composed of linear aluminum tris(diorganophosphate) chains with a general formula of catena-Al[O₂P(OR)₂]₃ (where R = C₁–C₈ alkyl group or phenyl moiety) have been studied by means of DSC, powder XRD, TGA and TG-QMS, as well as optical spectroscopy. DSC and XRD reveal that most of them undergo reversible structural transformations in the solid state between −100 and 200 °C caused by the changes in conformation of their organic substituents; however, a translational displacement of the rigid polymeric chains occurs only in the case of the derivative bearing long 2-ethylhexyl groups, which becomes liquid at about 140 °C. The thermal decomposition of the studied polymers begins between 200 and 265 °C depending on the type of organic substituent R decorating their aluminophospate core. TGA combined with mass spectrometry of the evolved gaseous products shows that the pyrolytic decomposition of Al[O₂P(OR)₂]₃ proceeds either through β-elimination of olefin (for compounds with C₂–C₈ aliphatic ligands), or a homolytic cleavage of the P–OR bond (for methyl and phenyl derivatives); both processes are accompanied by condensation of the newly formed POH groups and liberation of water. Powder XRD, FTIR and SEM analyses of the solid residues indicate that thermolysis of Al[O₂P(OR)₂]₃ accompanied by olefin elimination leads to the formation of condensed aluminum phosphates, mainly aluminum cyclohexaphosphate, exhibiting porous morphology. On the other hand, thermal degradation of methyl or phenyl derivatives results in amorphous aluminophosphate residues, and the latter contains conducting carbonaceous phases.