6174
Macromolecules 2001, 34, 6174-6183
Novel Electron-Donating Quinonoid Compounds: 2,5-Bis(4′,5′-dimethyl-1′,3′-dithiolan-2′-ylidene)- and 2,5-Bis(1′,3′-dithian-2′-ylidene)-2,5-dihydrothiophenes. Synthesis and Polymerization in the Presence of Oxygen Takahito Itoh,* Koji Kada, and Masataka Kubo Department of Chemistry for Materials, Faculty of Engineering, Mie University, 1515 Kamihama-cho, Tsu-shi, Mie 514-8507, Japan Received August 28, 2000; Revised Manuscript Received June 21, 2001
ABSTRACT: Two novel electron-donating quinonoid compounds, 2,5-bis(4′,5′-dimethyl-1′,3′-dithiolan2′-ylidene)-2,5-dihydrothiophene (1b) and 2,5-bis(1′,3′-dithian-2′-ylidene)-2,5-dihydrothiophene (1c), were synthesized successfully. The homopolymerizations of 1b and 1c took place only in the presence of oxygen to give the corresponding polymers composed of three different structural units. The ratios of the structural units in the polymers were significantly dependent on the polarity of the solvents used for the polymerizations. On the basis of NMR analysis of the resulting polymers and of the reaction products of 1b and 1c with water, a possible polymerization mechanism via a radical-cation intermediate was proposed.
Introduction Previously, we prepared 2,5-bis(1′,3′-dithiolan-2′ylidene)-2,5-dihydrothiophene (1a)1 as an isolable and polymerizable electron-donating quinonoid compounds based on 2,5-dimethylene-2,5-dihydrothiophene and investigated its polymerization behavior. It has dithioacetal rings at the exocyclic positions that are capable of ring-opening polymerization, and its polymerization takes place only in the presence of oxygen to give polymers composed of three different structural units, one bearing two thioester groups obtained by ringopening (A), a second bearing one thioester group from ring-opening and one cyclic dithioacetal ring obtained without ring-opening (B), and a third bearing two cyclic dithioacetal rings obtained with no ring-opening (C).
compound and its polymer because of the lack of planarity. It is well-known that the ring size of the cyclic compound can also affect ring-opening polymerizability,2-4 and 2,5-bis(1′,3′-dithian-2′-ylidene)-2,5-dihydrothiophene (1c) was synthesized as an electrondonating quinonoid compound with a six-membered ring containing 1,3-dithiane units. In this work, the syntheses of 1b and 1c and their polymerizations in the presence of oxygen are described.
Experimental Section
The relatively poor solubility of 1a and its polymer toward conventional solvents prevented a detailed investigation of the polymerization of 1a. Further investigation of derivatives of 1 with higher solubilities was desired to study the mechanism of formation of the three different structural units. 2,5-Bis(4′,5′-dimethyl1′,3′-dithiolan-2′-ylidene)-2,5-dihydrothiophene (1b) is one candidate with improved solubility because the methyl groups on the 4 and 5 positions of the dithiolane rings should improve the solubility of the quinonoid * Author to whom correspondence should be addressed. Phone: +81-59-231-9410. Fax: +81-59-231-9410. E-mail: itoh@ chem.mie-u.ac.jp.
Materials. 2,5-Thiophenedicarbaldehyde (2) was prepared according to the method reported by Ferigna et al.5 2,5-Bis(1′,3′-dithiolan-2′-ylidene)-2,5-dihydrothiophene (1a) was prepared according to the procedure reported previously.1 p-Chloranil (Wako Pure Chemicals Ltd.) was purified by recrystallization from benzene. Chloroform, dichloromethane, 1,2-dichloroethane, benzene, hexane, diethyl ether, and chlorobenzene were refluxed over calcium hydride for 12 h and then distilled. Thiophene (TCI Co.), 2,3-butanedithiol (TCI Co.), 1,3propanedithiol (TCI Co.), trityl tetrafluoroborate (Aldrich Co.), boron trifluoride diethyl etherate (Aldrich Co.), butyllithium (1.6 M solution in hexane, Aldrich Co.), N,N,N′,N′-tetramethylethylenediamine (TMEDA, Aldrich Co.), and tetrabutylammonium perchlorate (Aldrich Co.) were used without further purification. Monomer Synthesis. 2,5-Bis(4′,5′-dimethyl-1′,3′-dithiolan-2′-yl)thiophene (3). 2,5-Thiophenedicarbaldehyde (2) (1.5 g, 11 mmol) and 2,3-butanedithiol (2.5 g, 21 mmol) were dissolved in 30 mL of chloroform, the resulting solution was
10.1021/ma001490i CCC: $20.00 © 2001 American Chemical Society Published on Web 08/03/2001
Macromolecules, Vol. 34, No. 18, 2001 cooled to 0 °C, and 1 mL of boron trifluoride diethyl etherate was added. Stirring was continued for 1 h. The reaction mixture was washed three times with 20 mL of saturated aqueous sodium bicarbonate, dried over anhydrous magnesium sulfate, and filtered. The filtrate was placed under reduced pressure to remove solvents and give a brown, viscous oil, which was then dissolved in a small amount of benzene. The resulting solution was passed through a silica gel column using a mixture of benzene and hexane as the eluent. A yellow elution band was collected and then placed under reduced pressure to remove solvents and give 3.6 g (99% yield) of 3 as an orange, viscous oil: IR (KBr) νC-H 2928, νC-H 2880, νCdC 1520, νC-S 808 cm-1; 1H NMR (CDCl3) δ 6.82 (m, 2H, thiophene ring), 5.88, 5.80, 5.76 (s, 2H, >CH-), 3.98, 3.81, 3.48 (m, 4H, dithiolane ring), 1.46 (m, 12H, Me); 13C NMR (CDCl3) δ 148.75, 148.52, 148.37, 147.37, 147.17, 147.02, 145.10, 144.87, 144.69 (3 position in thiophene ring), 125.30, 125.25, 125.14, 124.78, 124.67, 124.36, 124.27 (2 position in thiophene ring), 57.81, 55.81, 54.30, 53.73 (-CH
