Synthesis of a New Side-Chain Type Liquid Crystal Polymer Poly[dicyclohexyl vinylterephthalate]

Introduction. The properties and the chemical structure dependence of side-chain liquid crystal polymers have been extensively studied in recent years because of their potential applications in nonlinear optics, information storage, and display devices.1,2 A side-chain liquid crystal polymer often comprises three basic components: a polymer backbone, a mesogenic unit, and a flexible spacer that bridges the mesogenic unit and the polymer backbone. Its properties may be affected by varying the chemical structures of these components. Empirical rules have been developed for systematically producing side-chain liquid crystalline polymers.3,4 According to Finkelmann et al.,5 if mesogenic groups are directly attached to the polymer main chain, thermal motions of the polymer segments and mesogenic groups are directly coupled. When the temperature is above the Tg, the polymer tends to adopt statistical chain conformations that hinder the anisotropic orientation of the mesogenic groups. Therefore, flexible spacers should be inserted between the backbone and mesogenic units to decouple their interactions. In such conditions, the mesogenic side chains can be anisotropically ordered in the liquid crystal state even though the polymer main chains tend to adopt the statistical random coil conformations. This idea of Finkelmann has been proven by many experiments and becomes a useful guide for molecular design of side-chain liquid crystalline polymers. On the other hand, the mesogen-jacketed liquid crystalline polymers (MJLCPs) form a different class of liquid crystalline polymers, not described by Finkelmann model.6 In MJLCPs, the rodlike mesogenic units are connected at their gravity center (or a nearby position) to the main chain through no or only very short spacers. It has been proven that in this case the introduction of the flexible spacers to decouple the interactions of the main chain and the mesogenic side groups is not necessary for the polymer to form a liquid crystalline phase.6-8 However, the rigid mesogenic units are believed to be essential for these two classes of the side-chain type polymers to form a liquid crystal phase, although in some cases the mesogenic group itself cannot form a stable liquid crystal phase,9 and in few others the low molar mass liquid crystals can be flexible molecules.10 Here we report the synthesis of a new polymer poly[di(cyclohexyl) vinylterephthalate] in which neither flexible spacers nor conventional rigid mesogenic units are included. We found that this polymer is amorphous below Tg but forms a liquid crystal phase above Tg. To our knowledge, it is the first liquid crystalline polymer that consists of nonrigid mesogenic side groups laterally attached to the main chain through no decoupling spacers. Synthesis. The synthetic route of the monomer and polymer is shown in Scheme 1. Vinylterephthalic acid (0.77 g, 4.0 mmol) and triphenylphosphine (2.2 g, 8.4 mmol) were dissolved in about 10 mLof dried pyridine to obtain solution A. Dried cyclohexanol (0.84 g, 8.4 mmol) and hexachloroethane (2.1 g, 8.8 mmol) were dissolved in 10 mL of dried pyridine to obtain solution B. Solution B was then dropped into solution A slowly. The mixture was stirred at 60 °C for 5 h. After cooling to room temperature, the mixture was poured into dilute HCl and extracted with chloroform three times. The organic layer was dried over anhydrous sodium sulfate and chloroform was removed by vacuum distillation. The residue was purified using column chromatography on silica gel with chloroform as eluant to obtain a colorless sticky liquid, 0.91 g (yield 64%). The structure of the monomer was proven by mass spectroscopy (VG-ZAB-HS instrument), 1H NMR spectrometry (Bruker ARX 400), and IR (Vector 22 FT-IR) as follows: mass spectrum 356(parent), 274(base), 257,193,192,175; 1H NMR 1.30-1.96, m, 20H, 2× (5-CH2); 5.01-5.05, m, 2H, 2× (-OCH-); 5.39-5.77, 2d, 2H(d CH2); 7.36-7.50, q, 1H (sCHd); 7.88-8.24, m, 3H (phenyl); IR(cm-1) 1244 (C-O-C), 1717 (CdO), 2860 (CH2, νas), 2936 (CH2, νs). The polymerization of the monomer was carried out in THF solution at 60 °C using 0.3% mol equiv AIBN as initiator. The polymer was precipitated in and washed with methanol. The molecular weight of the polymer was determined by a Waters 201 GPC instrument with polystyrene as standard. Found: Mn ) 2.7 × 104, Mw/Mn ) 2.4. Results and Discussion. The monomer was an oily liquid at room temperature, and no mesophase was detected by means of polarizing optical microscope (POM). Its differential scanning calorimetry (DSC) curve showed also only one melting endotherm at -30 °C. The monomer was easily polymerized to a moderately high molecular weight polymer by way of convenient radical polymerization. The WAXD of powder samples showed that the polymer was amorphous at room temperature. The lack of any crystallinity of this and other homologous MJLCPs6 indicates that the bulky side groups hinder the crystallization of these polymers as what the phenyl group does for atactic polystyrene. Since the TGA analysis (at a heating rate of 20 °C/min in nitrogen atmosphere) had showed a 1% weight loss * To whom all correspondence should be addressed. Scheme 1. Synthesis and Chemical Structure of the Monomer and the Polymer 4494 Macromolecules 1999, 32, 4494-4496