The synthesis of a number of 3-alkyl and 3-carboxy substituted pyrroles; their chemical polymerisation onto poly(vinylidene fluoride) membranes, and their use as gas sensitive resistors

Abstract

A number of synthetic pathways to 1-phenylsulphonyl and 1-tosylsulphonyl pyrrole have been reviewed and repeated. Two novel methods of producing this pre-cursor to 3-substituted poly(pyrroles) have been developed utilising crown ether phase transfer reagents or the aprotic solvent dimethylsulphoxide (DMSO). The most effective method of producing 1-phenylsulphonyl pyrrole was via the reaction of 1-phenyl or p-toluene sulphonamide and 2,5 diethoxytetrahydrofuran (the Clauson–Kaas method). The method was facile and yielded 81% of a highly pure product. 1-Phenylsulphonyl and 1-tosylsulphonyl pyrrole were utilised as pre-cursors, in the synthesis of a range of 3-substituted pyrroles via Friedel–Crafts acylation reactions. Subsequent de-protection followed by reduction yielded the 3-alkyl substituted pyrrole monomers, whilst, a thallium transposition via the Willgerdot–Kindler reaction yielded the 3-carboxy derivatives which were then deprotected to give the 3-carboxy pyrrole monomers or esterified to yield the 3-substituted ester analogues. The 3-substituted pyrrole monomers were then polymerised chemically onto poly(vinylidene fluoride) membranes to yield conducting polymer films. The resulting 3-substituted polymer films were tested for their gas sensitivity towards a number of classes of volatile organic compounds (VOCs). The 3-substituted polymers were found to exhibit enhanced sensitivity and differing selectivities to the test vapours when compared to poly(pyrrole) alone. The selectivity of the resulting sensors to certain organic vapours could be controlled by altering the substituents on the pyrrole ring, for instance where alkyl chains were added the responses of the resulting sensors to non-polar vapours such as hexane were enhanced. The addition of longer alkyl chains to the pyrrole ring resulted in a progressive enhancement in the sensitivity to non-polar vapours whilst the sensitivity to polar compounds remained constant or was reduced.