Communications to the Editor Effect of Branching and Molecular “Kinks” on the Properties of Main Chain Thermotropic Liquid Crystalline Polymers Containing Flexible Spacers Anil Kumar and S. Ramakrishnan* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560 012, India Received September 28, 1995 Revised Manuscript Received August 12, 1996 Main chain thermotropic liquid crystalline polymers (MCLCP’s), both wholly rigid rod systems and those that incorporate flexible spacers, have been a subject of intensive investigation for over two decades now. 1-3 The effect of various structural variations such as lateral substitutions, molecular kinks, comonomer incorpora- tion, etc. have been investigated, especially in the wholly aromatic systems, primarily as an approach to decrease the melting temperatures and facilitate processing. 1-3 In cases where the mesogenic units are incorporated into the polymer backbone along with flexible spacer segments, the structural variations have essentially focused on incorporation of different types of mesogens, such as biphenyl, 4-6 stilbene, 7,8 azobenzene, 9,10 phenyl benzoate, 11 etc., and also on the variation of the nature and length of the flexible spacers. Most of the ap- proaches for the preparation of such LCP’s with flexible spacers have utilized an A-A + B-B type condensation route. An A-B type self-condensation route, on the other hand, permits the generation of novel types of main chain liquid crystalline polymers with better control over some unique molecular structural features, such as configurational isomers, 12 and well-defined placement of lateral substituents 11 along the polymer backbone. One other interesting structural feature that can be introduced using the A-B type approach is the con- trolled degree of branching, which can be achieved by copolymerization of the A-B type mesogenic monomer with controlled amounts of an AB 2 type monomer. In this report, we describe the synthesis of a series of such branched liquid crystalline polyesters by copolymeriza- tion of a biphenyl containing A-B type monomer, ethyl 4-[4-(ω-hydroxydiethyleneoxy)phenyl]benzoate (BEO2), and an AB 2 monomer, ethyl 3,5-bis(ω-hydroxydiethyl- eneoxy)benzoate (EBHDB), based on 3,5-dihydroxyben- zoic acid. In addition to introducing branching, this AB 2 comonomer also introduces molecular “kinks” by virtue of the 1,3,5-connectivity on the aromatic ring. There- fore, copolymers with a kink-introducing monomer, ethyl 3-(ω-hydroxydiethyleneoxy)benzoate (EHDB), based on 3-hydroxybenzoic acid were also prepared for com- parison. Experimental Section. 1 H NMR (200 MHz) spectra were recorded on a Bruker ACF-200 instrument using the TMS/solvent signal as an internal reference. Dif- ferential scanning calorimetric studies were done using a Rheometric Scientific DSC Plus instrument at a heating rate of 20 °C/min under nitrogen. All the samples were heated to 250 °C and then quenched to room temperature before recording the first scans. Reproducibility was checked by running second heating/ cooling scans. Polarizing light microscopic studies were done on Leitz Ortho Lux 12POL-BK microscope at- tached to a Mettler FP82 HT hot stage. Viscosity measurements were made using an Ubbelohde viscom- eter in a constant temperature bath. All the monomers were synthesized using reported procedures starting from commercially available starting materials like 4-phenylphenol (for BEOn), 13 3-hydroxybenzoic acid (for EHDB), and 3,5-dihydroxybenzoic acid (for EBHDB). 14 Results and Discussion. Three A-B type biphenyl monomers (BEOn), where “n” represents the number of ethyleneoxy units, were prepared from the 4-(4- hydroxyphenyl)benzoic acid 13 and the appropriate monochlorooligo(ethylene glycol)s. The monomers (BEOn) were polymerized under standard transesteri- fication conditions using ca. 1 mol % of tetraisopropyl orthotitanate as catalyst. The polymerization was initially carried out under a nitrogen atmosphere at 200 °C for 2 h and then under dynamic vacuum (0.05 mbar) for an additional 15 h. The structures of the various polyesters are shown in Figure 1. Polymers PBEO2 and PBEO3 were soluble in solvents like p-chlorophenol, trifluoroacetic acid, tetrachloroethane, and/or their com- binations, but PBEO1 was insoluble. The inherent viscosity of PBEO2 (0.2% solution in p-chlorophenol at 50 °C) was found to be 0.53 dL/g and that of PBEO3 (0.2% solution in chloroform at 30 °C) was 0.51 dL/g, indicating that they are of moderately high molecular weights. The 1 H NMR spectra of the soluble polymers were in accordance with the expected structure. The DSC thermograms of the polymers PBEO2 and PBEO3 showed multiple endothermic transitions, while that of PBEO1 showed only a single endotherm. The various transition temperatures of the polymers are given in Table 1. Both PBEO2 and PBEO3 exhibited fluid birefringent patterns, when viewed under a polar- izing microscope at the expected mesophase tempera- tures, but the textures were not clearly identifiable. Of the three linear polyesters, PBEO2 with a diethyleneoxy spacer exhibits the largest mesophase range of 82 deg. Thus, in order to examine the effect of molecular kinks and branching, the monomer BEO2 was used for the copolymerization studies. Copolymerization of BEO2 with varying mole frac- tions of EHDB and EBHDB, under standard trans- esterification conditions, yielded two series of copoly- esters, PBEO2-M-X and PBEO2-B-X, respectively (where X is the mole percent of the comonomer in the feed) (Figure 1). Unlike the linear polyester PBEO2, the copolyesters were insoluble in most common solvents but swell in solvents such as p-chlorophenol. Hence, these polymers were extracted with p-chlorophenol, washed with acetone, and dried. Due to the insolubility of the copolyesters, their compositions could not be determined and therefore are taken to be equal to the feed compositions of the monomers. In wholly aromatic polyesters, Kricheldorf et al. utilized an AB 2 type 8551 Macromolecules 1996, 29, 8551-8553 S0024-9297(95)01464-1 CCC: $12.00 © 1996 American Chemical Society