Macromol. Chem. Phys. 2002, 203, 975–984 975 Synthesis of New Side-Chain Liquid-Crystalline Polyoxetanes with Two Mesogenic Groups Connected by a Flexible Spacer in the Side Chain Arµnzazu del Campo,* a Antonio Bello, Ernesto PØrez Instituto de Ciencia y Tecnología de Polímeros (C.S.I.C.), Juan de la Cierva, 3, 28006 Madrid, Spain Fax: +34-91-5644853 Keywords: asymmetric dimers; liquid-crystalline polymers (LCP); phase behavior; ring-opening polymerisation; spacer effects; Introduction In the last few decades a great number of liquid-crystal- line molecules have been synthesised and analysed. [1] Most of them consist of a calamitic rigid core, containing at least two ring systems, which is linked at both ends to flexible chains or small polar groups. Other non-conven- tional geometries have been published where several mesogens are connected in such a way that they are still able to build phases with orientational order. Among them, dendrimers, metallomesogens, piramidal mole- cules, V-shaped molecules, star-like compounds, dimers, trimers or longer linear or cyclic oligomers constitute sig- nificant examples. Dimers or twin molecules consist of two mesogenic units connected in their longitudinal direction through a flexible spacer. They are especially interesting due to the resemblance of their properties to those of semiflexible main-chain liquid-crystalline poly- mers. [2] This fact makes them useful as model compounds for the prediction of the properties of more complicated macromolecular systems. The properties of a mesophase are known to be strongly dependent on the molecular structure. [3] The size, nature and conformation of each molecular segment, as well as the interactions between them, are important parameters to modify when looking for predefined sym- metry or properties. For liquid-crystalline dimers it is known that the chemical nature of the central spacer, its length and its parity play an important roll in the final mesomorphic behaviour. [2] The most general expression of this peculiar spacer-effect is the alternation in the clearing temperatures and in the enthalpy and entropy changes of the phase transitions [4] on varying the number of carbons in the spacer, n. In some cases this influence goes even further and the type of mesophase formed on Full Paper: The synthesis of a different geometry of side- chain liquid-crystalline polymers is reported where two mesogenic units connected by a flexible spacer are attached as pendent groups of a polymeric chain. The resulting structure is the following, where R 1 ,R 2 and R 3 are methylenic or oxymethylenic segments: The monomers were obtained by linking the dimer struc- ture to an oxetane ring. These substituted oxetanes were polymerised by a boron trifluoride-initiated cationic ring- opening reaction to obtain the corresponding polymers. The polymerisation proceeds with high yields, in spite of the steric hindrance derived from the bulkiness of the side chain, and the presence of some functional groups in the molecule that compete against the cyclic ether for the nucleophilic attack at the propagating centre. A fraction of cyclic oligomer is obtained together with high molecu- lar-weight polyoxetane, as a result of back-biting reac- tions during the polymerisation. Several systems with spacers of different length and parity were synthesised in order to analyse the influence of the nature of the spacer on the phase behaviour. The formation of mesophases was proved by means of differential scanning calorimetry, X- ray diffraction and microscopic analysis. Most of the monomers form smectic structures at subambient tem- peratures. The polymers display a similar phase behav- iour, although shifted to higher temperatures. Macromol. Chem. Phys. 2002, 203, No. 7 i WILEY-VCH Verlag GmbH, 69469 Weinheim 2002 1022-1352/2002/0704–0975$17.50+.50/0 a Present address: Max Planck Institut für Polymerforschung, Ackermannweg, 10, 55118 Mainz, Germany Fax: +49-(0)6131–379100; E-mail: delcampo@mpip-mainz. mpg.de