New Bilaterally Linked Mesogens in Main-Chain Polymers with Exhibition of Biaxial Fluctuation in Nematic Phase Tat-Hung Tong, † Andrew Primak, ‡ Satyen Kumar, ‡ and Liang-Chy Chien* ,† Liquid Crystal Institute, Department of Chemical Physics, and Department of Physics and NSF ALCOM Center, Kent State University, Kent, Ohio 44242 Received December 5, 1997; Revised Manuscript Received February 19, 1998 ABSTRACT: The synthesis of a new series of liquid crystalline polyesters with bilaterally linked mesogens is described. The investigation of their mesomorphic behaviors by differential scanning calorimetry, X-ray diffraction, and polarization microscopy is presented. These new polymers exhibit only the nematic phase. X-ray diffraction studies reveal the existence of biaxial fluctuation in the nematic phase in two homologues of the series. Introduction Most commonly encountered polymeric liquid crystals (PLCs) have mesogens linked at their ends to the polymer backbone by flexible spacers. PLCs with me- sogens linked bilaterally by flexible spacers have been less studied, and only a few examples of main-chain PLC with mesogen linked bilaterally to the main chain have been reported. 1-3 This type of PLC may exhibit inter- esting biaxial phase properties because bilateral attach- ment of spacers onto the mesogens will lead to a restriction of their rotational motions along the long molecular axis. As a result, the existence of additional orientational ordering with respect to the short axes is highly likely. Biaxial nematic phases are of great experimental and theoretical interest. In the biaxial nematic state, ori- entational ordering of molecules exists along the long molecular axis and also along the short axes. The biaxial nematic phase has been identified in side-chain PLCs with side-on fixed mesogens. 4-6 The existance of orientational ordering of molecules along both long and short molecules has been realized in rigid-rod polymers with flexible side chains. 7,8 Krone and Ringsdorf 2 re- ported that main-chain polymers composed of bilaterally linked three-ring mesogenic cores showed mainly the monotropic nematic phase, and rapid crystallization during cooling precluded the exhibition of the biaxial nematic phase in these polymers. By extending the mesogenic unit to a four-ring core, the mesomorphic stability of bilaterally linked mesogens should be en- hanced and realization of the biaxial nematic phase might be possible. Krone and Ringsdorf 2 demonstrated that PLC with bilaterally linked mesogens composed of a four-ring core indeed showed a wide enantiotropic nematic phase. In their case, however, the presence of cis/trans isomers in the azo groups on the mesogenic core may have introduced additional perturbation on the molecular packing order, and biaxial phase behavior could not be detected. We believe that by incorporating a four-ring core with ester linkages instead of azo groups in bilaterally linked mesogens, the advantage of high mesomorphic stability can be retained without complication from the presence of isomers and realization of the biaxial nematic phase may still be possible. On this basis, we have synthe- sized and characterized a new series of liquid crystalline polyester with bilaterally linked mesogens. Experimental Section The starting materials were purchased from Aldrich Chemi- cal Company, Inc., and were used without further purification. Column chromatography was performed on silica gel (Aldrich Chemical, 70-230 mesh, 60 Å). The 1 H NMR (200 MHz) spectra were recorded on a Varian Gemini-200 spectrometer. The IR spectra were recorded on a Nicolet Magna-IR 550 spectrometer. Elemental analyses were performed by Oneida Research Service, Inc., Whitesboro, NY. Differential scanning calorimetry (DSC) of the polymers was performed on a Perkin- Elmer DSC7 system equipped with TAC 7/DX thermal analysis controller. The experiments were conducted under nitrogen atmosphere; the scanning rates were 10 °C/min in all cases. Thermogravimetric analyses (TGA) were performed on a TA Instruments TGA 2950 system under nitrogen atmosphere with a heating rate of 20 °C/min. The anisotropic textures of the mesogenic compounds were analyzed with a Nikon OP- TIPHOT2-POL polarizing microscope equipped with Mettler FP52 hot stage and a Mettler FP5 heating controller. Gel permeation chromatography (GPC) was performed on a Waters 510 HPLC instrument with a Waters 410 differential refrac- tometer. The experiments were done using tetrahydrofuran (THF) as solvent (1 mL/min, 35 °C), with polystyrenes as standards. The X-ray diffraction patterns were recorded from a Siemens X-1000 system with an area detector using graphite monochromator to select CuK R radiation. An in situ magnetic field of ∼3 kG was applied to orient the nematic director. Actual sample alignment depended on the competition between the applied field and surface effects. Synthesis. The synthesis of monomer 7 is shown in Scheme 1. Compound 3 was obtained via Pd-catalyzed cross- coupling reaction 9 of the arylboronic acid 2 with 1. Deprotec- tion of the methoxy group with BBr3 10 yields 4. Selective protection of the phenolic groups at 2 and 2′ positions in 4 to give 5 was achieved with 1-cyclohexenyloxytrimethyl- silane, using trimethylsilyl triflate (TMSOTf) 11 as catalyst. Compound 6 was obtained by 1,3-dicyclohexylcarbodiimide (DCC) esterification of 5 and 4-hexyloxybenzoic acid with 4-dimethylaminopyridine (DMAP) 12 as catalyst. Deprotection of 6 under mild acidic condition yields monomer 7. Monomer 7 was then polymerized with the aliphatic dicarboxylic acid chlorides having different spacer length (Scheme 1) to give the polymers 8a-f. † Liquid Crystal Institute and NSF ALCOM Center. ‡ Department of Physics and NSF ALCOM Center. * To whom correspondence should be addressed. E-mail: CHIEN@SCORPIO.KENT.EDU. 3537 Macromolecules 1998, 31, 3537-3541 S0024-9297(97)01772-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/05/1998