Sonochemical polymerization of benzene derivatives: the site of the reaction

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Abstract

Sonochemical polymerization of benzene and halogen-substituted benzenes has been studied. The difference of absorption spectra of polymerization products can be explained qualitatively using bond energies of the primary products. The relative rate constant of the polymerization reaction is apparently proportional to the inverse of the vapour pressure of the liquids. Using this relation, we analysed the relative rate constant of the polymerization in benzene/chrolobenzene mixtures. From this, we conclude that sonochemical polymerization proceeds in the vapour phase of a bubble.

Introduction

Sonochemical reactions in liquid proceed by the cavitational collapse of a bubble. The cavitational collapse of a bubble produces a hot spot in which temperature can exceed 5000 K [1]. Accordingly, the primary reaction of the sonochemical process can be regarded as pyrolysis at the hot spot. This model is called the `hot spot model and' is often used to explain the sonochemical reactions [2]. In the primary step, molecules are decomposed into small fragments or atoms at the hot spot. Actually, Flint and Suslick have observed luminescence spectra due to small fragments [3], such as C2, CN and Cl2, during irradiation of ultrasound in various organic liquids. After collapse of the bubble, the temperature surrounding the hot spot decreases very rapidly. During this quenching step, the fragments evolve into more stable products (primary product), such as molecules and polymers, immediately. In this step, relatively stable molecular radicals can also be produced. These radicals diffuse into the bulk liquid and react with the solvent or other radicals (secondary reaction).

Recently, the sonochemical process has been applied to material chemistry [4]. In particular, the sonochemical process can be used to produce fine particles. This is because only a few molecules in a hot spot can be decomposed and the subsequent quenching is very efficient. For liquid benzene, the sonochemical decomposition has been studied [5]. After irradiation of ultrasound, benzene liquid turns yellow and this is ascribed to polymerization. At the hot spot, benzene molecules may be decomposed into atoms or small fragments. This implies that the sonochemical decomposition of liquid benzene and its derivatives can be applied to synthesis of carbon materials, such as fullerenes and fine particles of carbon. Recently, we reported production of C60 by the ultrasonic irradiation of liquid benzene [6]. For control of the reaction, we need to study the primary step of the reaction at the hot spot. There are two actual sites for a reaction, the vapour phase inside a bubble and the liquid layer surrounding a bubble, and the site of the reaction is important for understanding the primary step. Here we study the sonochemical polymerization of liquid benzene and its derivatives in detail to understand the primary reaction as well as the site of the reaction.

Section snippets

Experimental

Samples were irradiated with a ultrasonic homogenizer (SMT, UH-600) equipped with a titanium tip 20 mm in diameter. The ultrasonic irradiation was carried out in an open glass vessel with Ar gas bubbling through the liquid. The volume of the liquid sample was 50 ml. The glass vessel was cooled using an ice bath. The homogenizer was operated at 600 W, 20 kHz in a pulsed mode. The temperature of the sample was controlled by the pulse period of irradiation. For example, during the pulsed irradiation

Emission and absorption spectra of sonochemical product

After irradiation of ultrasound, benzene turned yellow and showed blue luminescence when irradiated with UV-light. Fig. 1 shows the absorption and luminescence spectra of the irradiated benzene after 1.5 h irradiation. Luminescence spectra excited in various wavelengths (330, 360, 390 and 430 nm) are normalized at the peak intensity. It seems that the absorbance increases monotonously toward shorter wavelength and does not contain any vibrational structure. These structureless spectrum shows that

Acknowledgements

We thank Dr. S. Murata for the use of his fluorescence spectrometer. RK gratefully acknowledges helpful discussion with Dr. E. Sekreta on several points in the paper.

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