Synthesis And Optical Properties Of Liquid Crystalline Polyacetylene Containing Bis(4-alkylphenyl)terephthalate As Pendant

A novel acetylene derivative, 2-propargyloxy-1,4-dimethyl terephthalate, was synthesized. The monomer acetylene with steric substitute was polymerized using WCl₆, WCl₆-SnPh₄ and [Rh(nbd)Cl]₂ as catalyst systems. The structure and optical properties were studied by means of nuclear magnetic resonance (NMR), infrared spectroscopy (IR), ultraviolet spectroscopy (UV) and photoluminescence (PL). The polymerization using WCl₆-SnPh₄ as catalyst afforded polyacetylene with only 60% of cis-content due to the steric substitute. The polymerization yield was strongly dependent on reaction temperature and solvent .High yield ang high trans-content of polyacetyl were obtained by plymerization using [Rh(nbd)Cl]₂ catalyst The steric effect of bulky substituete led to narrow conjugation of polyacetylene.Liquid crystalline monomers, 2-propargyloxy-bis(p-alkylphenyl) terephthalate, were synthesized. 2,5-Dimethylanisole was used as starting material. Through the oxidation by using potassium permanganate, 2-methoxylterephthalic acid was obtained. 2-Methoxyl- bis(p-alkylphenyl)terephthalate was synthesized through the esterification between 2-methoxylterephthalic acid and 4-alkylphenol. The reaction between tribromoborane and 2-methoxyl-bis(p-alkylphenyl)terephthalate produced 2-hydroxyl- bis(p-alkylphenyl)-terephthalate. The propargyl bromide was added into the above resultant to achieve the target monomer. By introducing different alkyl as tails of mesogen, the relationship between structure and liquid-crystallinity was studied by polarized optical microscopy (POM). The structure of the acetylene derivates was characterized by nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FT-IR), and Mass Spectra (MS). The monomer with alkyl tails (p-methyl and p-propyl) formed a smectic mesophase upon heating above the melting temperature, but the other one without tails could not exhibit liquid crystallinity at elevated temperatures. The lower melting temperature and wider mesomorphic temperature range of 2-propargyloxy- bis(p-propylphenyl)terephthalate (k 91.6 SmB 102.6 i) than 2-propargyloxy- bis(p-methylphenyl)terephthalate ( k 134.3 SmB 143.7 i) indicated that the long tail in mesogen played an important role in the packing order and stability of their liquid crystalline phase.Monosubstituted polyacetylenes containing lateral pendants of bis(p-alkylphenyl) terephthalate with spacer and alkyl tails [R, where R is CH₃, C₃H₇] were synthesized, and the effects of the backbone structure and alkyl tails on the properties were investigated. The polymerization of acetylene monomers were carried out with [Rh(nbd)Cl]₂ as a catalyst in toluene. The structures and properties of the monosubstituted polyacetylenes were characterized by the means of nuclear magnetic resonance, polarized optical microscopy, infrared spectroscopy, ultraviolet spectroscopy, photoluminescence, thermogravimetry and differential scanning calorimetry. The molecular weights of the polymers were measured by gel permation chromatograph. The two polymers could not exhibit liquid crystallinity at elevated temperatures. The steric effect of bulky liquid-crystallinity mesogens prevented the coplanar conformation of the polyene backbone, and thus, led to the lower absorption and emission of the polymers.The studies of structure-property relationship encourage us to design "jacket" liquid crystalline polyacetylenes with spacers in between mesogens and polyene backbone so as to reduce the electronic interactions and steric hindrance of side and main chains, hence enabling efficient light emission.