Structural-Chemical Conception of Stabilization of Aromatic Polymers as Exemplified by Arylene Carboranes

Abstract

The paper discusses the specific features of thermal behaviour of carborane-containing polymers and the stabilizing effect of the carborane groups as exemplified by arylene carboranes. The results obtained explain as a whole the stabilizing effect of aromatic polymers by carborane-containing additives.At the present time the requirements for a prolonged thermal stability of polymers, the working temperatures of the corresponding composites, considerably exceed those of the stability of chemical groups and chemical bonds which form even the most thermally stable polymers (cyclo-chain polyheteroarylenes, polyorganosiloxanes). The stabilizers of thermal and thermal-oxidative degradation acting by the known classical mechanisms cannot be effective at 350-40O0C, at least for a prolonged time. At the same time it has sufficiently long been established in practice that the polymers with carborane-12 groups in the macromolecule make it possible to achieve higher working temperatures of polymeric materials.'A sufficient number of papers dealing with investigations and explanations of the specific features of thermal behaviour of carborane-containing polymers and the stabilizing effect of the carborane groups was published. But most often the conceptions developed in them are either of purely phenomenological or of insufficiently well-founded, hypothetical character due to complex investigations of chemical transformations especially in three-dimensional polymeric matrix. The present lecture reports the results of investigations on the mechanisms of carborane-12 transformations by the action of high temperatures and oxygen that can explain this unusual thermal behaviour of carborane-containing polymers and formulate a general approach to stabilization of polymeric materials intended for operation at elevated temperatures.In the last two decades almost all known classes of polymers containing carborane-12 groups in the backbone or as side groups have been prepared. Investigations of their properties showed that the introduction of carborane groups into the polymer chain increases polymer thermal stability and their composites, this showing itself most effectively in case of aromatic polymers. A similar result can be achieved by introduction of carborane-containing additives into polymers primarily such as aromatic derivatives. Thus it can be said about the effect of carborane-12 groups stabilizing thermal and thermal-oxidative degradation of aromatic polymers.In the behaviour of carborane-containing polymers under the action of elevated temperatures and oxygen a number of specific features can be marked which differentiate them from that of their ordinary analogs. For instance, under conditions of thermal-oxidative degradation the chemical transformations in the matrix of carborane-containing polymers are observed in most cases at lower temperatures (200-350°C) that allows for classifying them as thermostable polymers in the adopted meaning of this word.' Unusual small weight losses at high temperatures is the second most e...

Syntheses and properties of carboranylpyrroles

“…The inductively electronwithdrawing carboranes typically increase the stability of pyrrole and polypyrroles by decreasing their reactivity towards electrophiles and oxidizing agents. To date, several carborane-containing polymers have been synthesized and characterized [11][12][13]. These macromolecular systems show high resistance to combustion and unique two-dimensional structures.…”

Synthesis and Electrochemistry of Carboranylpyrroles. Toward the Preparation of Electrochemically and Thermally Resistant Conjugated Polymers

“…Indeed, such functional groups could confer on the host matrix novel properties, such as a thermal and/or chemical stability, optoelectronic characteristics, and an ordered structure. For instance, numerous polymers in which the icosahedral carboranes were linked by aromatic units (generally benzene) have been synthesized and characterized. − These macromolecular systems showed either an extreme resistance to combustion 3,6 or a two-dimensional grid-shaped structure using the boron cage as a molecular connector . Other poly(carborane−siloxane) elastomers incorporating acetylenic units showed also remarkable thermal and oxidative stabilities …”

Excited State Characterization of Carborane-Containing Poly(dihexyl fluorene)s

“…The icosahedral boron cluster, o -carborane (C 2 B 10 H 12 ), displays unique electronic, steric, and chemical properties, including high hydrophobicity, high thermal and chemical stabilities, and three-dimensionally delocalized electrons. − Carboranes have been combined with conjugated systems in an attempt to provide thermal stability to semiconducting polymers. − Incorporating carborane into conjugated backbones also drastically influences the emission properties of these systems, which are influenced by the dihedral angle between the carbon–carbon bond in carborane and the π-conjugated unit . For this reason, carborane has been widely studied in many conjugated monomer and polymer systems with aromatic building blocks including p -phenyleneethynylene, ,, naphthalene, carbazole, , and triphenylamine, to name a few. ,,,, These studies have shown that introducing carborane into a conjugated system can cause the system to experience excimer emissions, AIE, crystallization-induced emission, ICT, and twisted ICT, in addition to the local emission of the conjugated building block.…”