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Two series of monomers, 4-[4-(allyloxy)benzoyloxy]phenyl-3-(perfluoro-nalkyl)ethyl propanoates and 2-(perfluoro-n-alkyl)ethyl-4-[4-(undec-10-en-1-oyloxy)-benzoyloxy] benzoates have been synthesised. These compounds contain a fluorinated chain obtained from 2-(perfluoro-n-alkyl)ethanol or from 2-(perfluoro-n-alkyl)ethyliodide, and an aliphatic chain containing a double bond attached to a mesogenic moiety. Their hydrosilylation with polymethylhydrogenosiloxane gives the corresponding polysiloxanes with a spacer with three or ten methylene units. These new fluorinated compounds are characterized by a combination of techniques consisting of differential scanning calorimetry (DSC) and thermal optical polarized microscopy. All the monomers exhibited smectogenic properties of type A but for the monomers with a long spacer between the unsaturated function and the mesogenic core (compounds B), the mesophase range decreases dramatically. All of the corresponding polysiloxanes exhibited a liquid crystalline behaviour over a wide temperature range. The influence of the fluorinated chain lengthening leads for the monomers and the polymers to an increasing of the transition temperatures.

In order to determine the in¯uence of`sulphur-containing' spacers on the formation of mesophases in low molecular mass compounds, we have examined the mesomorphic behaviour of molecules which consists of a 4-biphenyl unit linked to an unbranched¯uorinated chain via a short spacer including at least a sulphur atom. The synthesis of these compounds has been carried out from 2-F-butylethyl iodide or from the 2-F-alkylethyl mercaptans in the case of the F-hexyl and F-octyl tails. The mesomorphic properties have been characterized by polarized light microscopy and by di erential thermal analysis showing the peculiar contribution of each of the spacers. The in¯uence of the¯uorinated chain and the shape of the connector on the stability of the mesophases has been investigated. The compounds with a thioether or a hemithioacetal spacer showed no mesomorphic properties, whereas the structures with a thioester spacer showed a very interesting enantiotropic behaviour of the smectic A type over a wide temperature range. Furthermore from the series exhibiting liquid crystalline behaviour, increasing the number of¯uoromethylene units simultaneously increases both the melting and the clearing temperature. These mesomorphic properties within thē uorinated series are compared with those of their monocatenar hydrocarbon homologues.

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