Substituent Effect on Azobenzene-Based Liquid-Crystalline Organophosphorus Polymers S. KUMARESAN, P. KANNAN Department of Chemistry, Anna University, Chennai 600025, India Received 4 April 2003; accepted 19 July 2003 ABSTRACT: A new series of combined-type, azobenzene-based organophosphorus liquid- crystalline polymers were synthesized, and their photoisomerization properties were studied. The prepared polymers contained azobenzene units as both the main-chain and side-chain mesogens. Various groups were substituted in the terminal of the side-chain azobenzene mesogen, and the effects of the substituents were investigated. All the polymers were prepared at the ambient temperature by solution polycondensa- tion with various 4-substituted phenylazo-4'-phenyloxyhexylphosphorodichloridates and 4,4'-bis(6-hydroxyhexyloxy) azobenzene. The polymers were characterized with gel permeation chromatography, Fourier transform infrared, and 1 H, 13 C, and 31 P NMR spectroscopy. Thermogravimetric analysis revealed that all the polymers had high char yields. The liquid-crystalline behavior of the polymers was examined with hot-stage optical polarizing microscopy, and all the polymers showed liquid-crystalline proper- ties. The formation of a mesophase was confirmed by differential scanning calorimetry (DSC). The DSC data suggested that mesophase stability was better for electron- withdrawing substituents than for halogens and unsubstituted ones. Ultraviolet irra- diation studies indicated that the time taken for the completion of photoisomerization depended on the dipolar moment, size, and donor–acceptor characteristics of the ter- minal substituents. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3188 –3196, 2003 Keywords: liquid crystals; organophosphorus azobenzene polymers; UV studies; thermal studies; photoisomerization; substituent effect; irridation; liquid-crystalline polymers (LCP); polycondensation INTRODUCTION Photochemical reactions that occur in small mol- ecules can also be induced to occur in macromol- ecules. There are some environmental constraints on macromolecules that do not concern small mol- ecules, and applying the fundamental principle to macromolecules is rewarding. 1 The photochemi- cal response, a property of azobenzene in liquid- crystalline polymers (LCPs), is receiving atten- tion because of its potential applications. The photochemistry of azobenzene-containing main- chain and side-chain LCPs has been studied in detail in the past decades. 2,3 Literature on the physical and chemical properties of combined- type LCPs is scarce, and the research in this field is still not adequate. 4 Combined-type LCPs con- taining azo groups as both main-chain and side- chain mesogens have little been explored. 5 Termi- nal substitution on the mesogen plays a vital role in the mesophase and thermal stability by ex- tending or limiting the conjugation or by steric and polarity factors. 6 The effect of terminal sub- Correspondence to: P. Kannan (E-mail: pakannan@ annauniv.edu) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 41, 3188 –3196 (2003) © 2003 Wiley Periodicals, Inc. 3188