Macromolecules 2003, 36, 8617-8620
8617
Investigation on Different Procedures in the Oxidative Copolymerization of a Dye-Functionalized Thiophene with 3-Hexylthiophene Alessandro Fraleoni-Morgera,* Carlo Della-Casa, Massimiliano Lanzi, and Paolo Costa-Bizzarri Dipartimento di Chimica Industriale e dei Materiali, Universita` di Bologna, Viale del Risorgimento, 4 I-40136 Bologna, Italy Received June 26, 2003; Revised Manuscript Received September 12, 2003
ABSTRACT: For the purpose of preparing materials for nonlinear optics applications, a 3-ethylthiophene bearing an NLO-active chromophoric group as a substituent at the end of the ethylic chain was copolymerized with 3-hexylthiophene. Different FeCl3-based oxidative polymerization conditions in solvents such as CH3NO2 and CCl4 and their effects on the copolymers characteristics and solubility in CHCl3 (i.e., the copolymer’s soluble fraction) were examined. This work allowed to achieve a satisfactory chloroform-soluble fraction of the copolymer, still containing a notable amount of NLO-active group, and to tune to some extent the main characteristics of the copolymer; moreover, the behavior of the monomerssolvent-oxidant system was investigated.
Introduction In the past years the area on nonlinear optics (NLO) applications has drawn considerable attention from chemists because of the possibility to obtain both very high NLO responses and a molecular design tailored for the desired application from organic materials; in particular, polymeric systems of this nature are being widely investigated.1,2 There are various methods for obtaining polymeric materials for NLO applications. Given the interesting electronic properties and the high thermal and chemical stability of the conjugated polythiophenes, our attention has focused on the grafting of a second-order NLO-active molecule as a side-chain pendant of a 3-alkylthiophene;3-5 it cannot be ignored, however, that the intrinsic properties of the NLO-active molecules make it difficult to obtain satisfactory processable materials. In fact, the solubility of polythiophenes containing chromophoric groups is often very low.6-11 The aim of the present study has hence been to overcome this problem so as to open the way for obtaining really application-suitable materials, through a systematic study aimed at optimizing the oxidative polymerization for achieving soluble products. In particular, our attention has focused on reagent and solvent addition modalities, as reaction results are deemed to be mainly affected by these parameters. For this study the monomer 3-[2-(4-(4′-nitrophenylazo)phenoxy)]etylthiophene (T2OAz) was taken as the reference dye-functionalized compound and was copolymerized with 3-hexylthiophene (T6H) to yield the polymer P1 (Scheme 1). More precisely, polymer P1 was prepared via FeCl3 oxidative polymerization by different methods mostly based on a two-solvent approach (CH3NO2/CCl4, where CH3NO2 is a solvent and CCl4 a nonsolvent for FeCl3), capable of giving better yields in terms of soluble polymer fraction than the usually employed CHCl3based one.12 The key step of this procedure involved the use of a dissolved form of FeCl3 which, upon contact with * Corresponding author: e-mail
[email protected]; Tel +39-051-2093672; Fax +39-051-2093673.
Scheme 1. General Synthetic Route to Copolymers P1, Obtained Keeping the Molar Ratio between T6H and T2OAz in the Feed Constant to 1:1a
a With reference to the soluble fraction of the copolymers, x was found variable between 0.21 and 0.48 (y ) 1 - x) depending on the reaction conditions used.
the nonsolvent CCl4, probably precipitates in the form of microcrystals, thus providing a wider surface for polymerization to begin and leading to the formation of a more chloroform-soluble polymer. By varying reaction times, as well as solvent and reactant addition modalities, and keeping constant reaction temperature (room temperature) and comonomer feed ratio (1:1), a number of copolymers with different characteristics were obtained. The presented results provide interesting information on the behavior of these systems, and the proposed synthetic methods permit to achieve acceptable yields in terms of soluble material. Results and Discussion The functionalized monomer T2OAz was made react with T6H in a 50:50 molar feed ratio under different reaction conditions. The operative parameters are shown in Table 1, where columns S1, S2, and S3 refer to the solvent placed in the reaction flask, the solvent used for delivering the reactants into the reactor, and the
10.1021/ma0348730 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/17/2003
8618 Fraleoni-Morgera et al.
Macromolecules, Vol. 36, No. 23, 2003 Table 1. Overview of the Main Operating Parameters
reaction
solvent S1a
solvent S2b
r1 r2 r3 r4 r5 r6 r7 r8
CCl4 CH3NO2 CH3NO2 CH3NO2 CH3NO2 CH3NO2 CH3NO2 CH3NO2
CH3NO2 CH3NO2 CCl4 CCl4 CH3NO2 CH3NO2 CH3NO2
solvent S3c
reaction time T1d (min)
CCl4 CCl4 CCl4 CCl4
13 20
