Preparation and Optical Characterization of Two Photoactive Poly(bisphenol A ethoxylate diacrylate) Copolymers Containing Designed Amino-Nitro-Substituted Azobenzene Units, Obtained via Classical and Frontal Polymerization, Using Novel Ionic Liquids as Initiators Javier Illescas, 1,2 Jes  us Ortı´z-Palacios, 1 Jair Esquivel-Guzm an, 1 Yessica S. Ramirez-Fuentes, 1 Ernesto Rivera, 1 Omar G. Morales-Saavedra, 3 Antonio A. Rodrı´guez-Rosales, 3 Valeria Alzari, 2 Daniele Nuvoli, 2 Sergio Scognamillo, 2 Alberto Mariani 2 1 Instituto de Investigaciones en Materiales, Universidad Nacional Aut  onoma de M exico. Circuito exterior Ciudad Universitaria C.P. 04510 M exico DF 2 Dipartimento di Chimica e Farmacia, Universit a di Sassari, and local INSTM Unit, Via Vienna 2, 07100 Sassari, Italy 3 Centro de Ciencias Aplicadas y Desarrollo Tecnol  ogico, Universidad Nacional Aut  onoma de M exico. Circuito exterior Ciudad Universitaria C.P. 04510 M exico DF Correspondence to: A. Mariani (E-mail: mariani@uniss.it) or E. Rivera (E-mail: riverage@iim.unam.mx) Received 1 December 2011; accepted 25 January 2012; published online 23 February 2012 DOI: 10.1002/pola.25962 ABSTRACT: The frontal polymerization (FP) of bisphenol A ethoxylate diacrylate (BPAEDA) was carried with and without the presence of two different azobenzene comonomers by means of an external heating source. The first azomonomer (MDR-1) is a derivative of disperse red-1, N-ethyl-N-(2-hydrox- yethyl)-4-(4-nitrophenylazo)aniline, whereas the second (E)-2- (4-((4-nitrophenyl)diazenyl)phenyl)-5,8,11-trioxa-2-azatridecan- 13-yl methacrylate (4PEGMAN) comes from the azo-dye N- methyl-N-{4-[(E)-(4-nitrophenyl)diazenyl]phenyl}-N-(11-hydroxy- 3,6,9-trioxaundecas-1-yl) amine. In this work, an ionic liquid tri- hexyltetradecylphosphonium persulfate was used as initiator. This compound produced stable propagating polymerization fronts with good velocities and moderate maximum tempera- ture values. Moreover, this initiator prevented bubble forma- tion and was found to be the most efficient when it was used in lower amounts with respect to other initiators, such as ben- zoyl peroxide, 2,2 0 -azobisisobutyronitrile, aliquat persulfate V R , and tetrabutylphosphonium persulfate. The thermal properties of the obtained polymers and copolymers were determined by thermogravimetric analysis and differential scanning calorime- try. The nonlinear optical (NLO) characterizations of the devel- oped BPAEDA/MDR-1 and BPAEDA/4PEGMAN copolymers were performed according to the Z-Scan technique in film sam- ples prepared by classical polymerization. It has been proven that samples with higher 4PEGMAN content (0.26 mol %) exhibited outstanding cubic NLO-activity with positive NLO-re- fractive coefficients in the promising range of n 2 ¼þ3.2  10 4 esu. V C 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 1906–1916, 2012 KEYWORDS: azopolymers; frontal polymerization; initiators; NLO; radical polymerization; Z-Scan INTRODUCTION Azopolymers have been considered as highly versatile materials, due to their light response, nonlinear op- tical (NLO) effects and the photoinduced motions which occur on them, when they are irradiated with polarized laser light. 1 Several reviews covering most of the implications of azobenzene in polymer structures have been published. 1–4 At the beginning, research on azobenzene molecules focused mainly on the development of dyes. However, since the dis- cover of the trans–cis photoisomerization of azobenzene in the 1950s and its potential applications in optics, this research field has evolved significantly. 5 According to Rau, azobenzenes can be divided into three main categories: ‘‘azobenzenes,’’ ‘‘amino-azobenzenes,’’ and ‘‘pseudostilbenes.’’ 6 This classification is based on the relative energies of the n–p* and p–p* transitions. 7,8 Azobenzene molecules show an intense p–p* band in the UV region at 350 nm followed by a weak n–p* band at 440 nm. In the case of the amino-azobenzenes, the p–p* band is red-shifted to the proximity of the n–p* band, which is insensitive to substituent effects, thereby causing a partial overlap of both bands. 9 Unlike the preceding categories, pseudostilbenes are substituted with strong electron-donor and electron-with- drawing groups. Therefore, they are also named push–pull molecules; the p–p* and the n–p* are inverted in the energy scale and totally overlap so that only one absorption band is observed. Moreover, for this kind of azobenzenes, the V C 2012 Wiley Periodicals, Inc. 1906 JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 1906–1916 ARTICLE WWW.POLYMERCHEMISTRY.ORG JOURNAL OF POLYMER SCIENCE