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Ind. Eng. Chem. Res. 2002, 41, 5554-5557
APPLIED CHEMISTRY A Novel Thermal Latent Cationic Initiator: S-Alkylsulfonium Salts Obtained by Desulfurization of Light Oil Yasuhiro Shiraishi,* Kenya Tachibana, Takayuki Hirai, and Isao Komasawa Department of Chemical Science and Engineering, Graduate School of Engineering Science, and Research Center for Solar Energy Chemistry, Osaka University, Toyonaka 560-8531, Japan
S-Methylsulfonium salts, obtained by a desulfurization process for light oil based on the methylation of sulfur-containing compounds using CH3I and AgBF4 and subsequent precipitation, were used as a thermal latent initiator for cationic ring-opening polymerization of epoxide (glycidyl phenyl ether, GPE). The sulfonium salts, obtained from the light oil, showed good thermal latency and demonstrated significantly high initiation activity for the polymerization of GPE. The activity of the salts was rather higher than that obtained using the salts of pure dibenzothiophenes (DBTs) and benzothiophenes (BTs) as the model sulfonium salts. The salts, obtained from light oil, are derived from highly alkyl-substituted DBTs and BTs in light oil, of low nucleophilicity (low electron density on the sulfur atom), and therefore are unstable against thermal stimulation, thus giving the high initiation activity for the polymerization. Introduction
Scheme 1
Thermal latent polymerization catalysts, which reveal no activity at ambient temperature but become active at higher temperature, are becoming important in the chemical industry for simplifying the polymerization processes. Various kinds of S-alkylsulfonium salts have been reported to serve as the efficient thermal latent initiator for cationic polymerization of epoxides and olefins.1-3 The polymerization is initiated by the elimination of the alkyl group from the sulfonium salts, such that the unstable salts that were derived from less nucleophilic sulfides reveal higher initiation activity. Dibenzothiophene (DBT) and its derivatives have lower nucleophilicity than alkyl and aryl sulfides. The Smethylsulfonium salts of DBTs, therefore, show significantly high initiation activity for cationic polymerization of glycidyl phenyl ether (GPE) and curing of epoxy resins.2-4 For industrial applications, a simple and mass synthetic process for the salts of DBTs is therefore required. As is well-known, a large amount of alkyl-substituted DBTs and benzothiophenes (BTs) is contained in light oil, and their removal from the oil is urgently required for protection of the environment.5,6 In a previous work, a novel desulfurization process for light oil has been investigated based on the methylation of sulfur-containing compounds using CH3I and AgBF4 and subsequent precipitation of the resulting sulfonium salts.7 The DBTs and BTs in light oil are methylated by the alkylating agents to form the corresponding sulfonium salts. These salts are highly polarized and insoluble to nonpolar light oil, such that they can be removed from the light oil as precipitates. The reaction proceeds under conditions of * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +81-6-6850-6273. Tel.: +81-6-6850-6271.
room temperature and atmospheric pressure, and the sulfur content of light oil is decreased to less than 0.005 wt %, which is the value limited strictly in Sweden and Switzerland. By employment of the process, a large amount of the sulfonium salts of DBTs and BTs is obtained from light oil as the desulfurization product. In the present work, the sulfonium salts, obtained by the desulfurization of light oil, were used as a thermal latent initiator for cationic polymerization of GPE (1; Scheme 1). The initiation activity for the salts was compared with that obtained using the salts of pure DBTs and BTs, and the possibility of the salts as a new polymerization initiator was examined. Experimental Section 1. Materials. 1 was purchased from Wako Pure Chemical Industries, Ltd., and was used without further purification. The other reagents were the same as those used in a previous paper.7 The model S-methylsulfonium salts of DBTs (3a-c; Scheme 1) and BTs 4a-c were synthesized according to the procedure of the
10.1021/ie0204437 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/16/2002
Ind. Eng. Chem. Res., Vol. 41, No. 23, 2002 5555 Table 1. (i) Electron Densities on Sulfur Atom for (a) DBTs and (b) BTs and (ii) the Mole Fraction of the S-Methylsulfonium Salts of DBTs and BTs Obtained by Desulfurization of Light Oil (a)
(b)
speciesa
(i)
(ii) (mol %)
(i)
(ii) (mol %)
C6 total
0.940b
14.7c
0.781d
0.913 0.889 0.876 0.861 0.854 f
9.4 3.9 1.6 0.5 33.6 63.7
0.745 0.741 0.732 0.721 0.719g
10.9e 6.1 7.5 5.1 1.8 4.9h 36.3
a C denotes the total number of carbon atoms of the alkyl group, n substituted at any position of the DBT and BT molecules. b Average value obtained by the calculation of C - and C -DBT. 0 1 c
