Poly(3,3-dimethylselenetane) Submitted by: E. J. Goethals and E. Schacht 1 Checked by: B. Lindgren2 1. Procedure a. 3,3-dimethylselenetane Caution! Selenium and its compounds are highly poisonous. Liquid ammonia is dried by adding potassium until the blue color persists. Then 0.7 L of the ammonia is distilled into a thoroughly dried three-necked reaction flask provided with an inlet for dry nitrogen. With stirring, 29 g of potassium and 29 g of selenium are added alternately in small portions under a stream of nitrogen. After the ammonia has been evaporated under an atmosphere of dry nitrogen, 58 g of potassium selenide is obtained. Oxygen-free absolute ethanol, 700 mL, is distilled into the same reaction vessel. Then 60 g of 2,2-dimethyl-1,3-dibromopropane (Note 1) is added slowly to the boiling, stirred alcoholic solution. After 4 h of boiling, the reaction mixture is allowed to cool in contact with air. A precipitate of potassium bromide and selenium is filtered, and the filtrate is poured into 2 L of water. The suspension obtained is extracted 3 times with methylene chloride. The combined organic extract is washed with water, dried over anhydrous sodium sulfate, and concentrated by distillation. Vacuum distillation yields 24 g of 3,3- dimethylselenetane (50%) with bp of 30o at 29 mm. Before polymerization, the monomer is dried with calcium hydride and distilled just prior to use. b. Cationic Polymerization A thoroughly dried 20 mL reaction tube flushed with dry nitrogen through a lateral stopcock is charged with 1.49 g (0.010 mol) of 3,3-dimethylselenetane and 4 mL of dry methylene chlorid (Note 2). The tube is closed with a rubber cap, and the stopcock is closed and disconnected from the nitrogen supply. The reaction tube is placed in a water bath held at 20o, and a solution of 0.5 mmol of triethyloxonium tetrafluoroborate (Note 3) in methylene chloride is added with a hypodermic syringe through the rubber cap. After 24 h at 20o the reaction mixture is poured into a 50 mL, round-bottomed flask containing 10 mL of dry methanol. The solvents are removed on a rotary evaporator. The residue is dissolved in 10 mL of benzene (Caution! Carcinogen, Note 4), the solution is frozen, and the benzene is removed by freeze-drying, leaving 0.93 - 1.05 g of polymer (63 - 70%) having an average molecular weight (determined with a Mechrolab vapor pressure osmometer) of 5100. The intrinsic viscosity measured in benzene at 25o is 0.07 dL/g. c. Anionic Polymerization In a thoroughly dried 20 mL reaction tubewith a lateral stopcock are introduced 1.49 g (0.010 mol) of 3,3-dimethyl selenetane and 4 mL of dry n-hexane under a countercurrent of dry nitrogen. 1 2 Macromolecular Syntheses, Collective Volume 2 The flask is closed with a rubber cap and cooled in liquid nitrogen. With a hypodermic syringe a solution of 0.25 mmol of butyllithium (Note 5) is added to the mixture, the stopcock is connected to a vacuum line at a pressure of 0.01 mm, and the reactants are degassed by several freeze-thaw cycles. The reaction tube is then kept at -78o for 24 h. The reaction tube is opened, benzene is added to dissolve the precipitated polymer, and the solution is poured into a 50 mL, round- bottomed flask containing 10 mL of dry methanol. The polymer is isolated as described above. A yield of 0.54 g polymer (36%) with an average molecular weight of 34,000 is obtained (Note 6). The intrinsic viscosity measured in benzene at 25o is 1.8 dL/g. The poly(3,3-dimethylselenetanes) obtained are viscous oils or waxy substances. 2. Notes 1. 2,2-Dimethyl-1,3-dibromopropane is synthesized by reaction of 2,2-dimethyl-1,3-propanediol (Fluka AG., Buchs, Switzerland) with phosphorus tribromide.1 The reaction is performed by adding phosphorous tribromide to 2,2-dimethyl-1,3-propanediol preheated to 70o. After the addition of phosphorous tribromide, the reaction mixture is heated for 18 h at 160o followed by 2 h at vigorous reflux. This pattern of heating the reaction mixture is crucial for a successful synthesis. If the reaction mixture is allowed to stand for 1 h at 70o after addition of the phosphorous tribromide, pure 3-bromo-2,2-dimethylpropanol is obtained. If the reaction mixture is allowed to stand for 18 h at mild reflux (ca 125o) plus 2 h at vigorous reflux, a mixture of 2,2-dimethyl-1,3-dibromopropane and 3-bromo-2,2-dimethylpropanol, containing 64% of the dibromide is obtained. This mixture cannot be separated by distillation. To get pure dibromide, the reaction mixture must be heated at 160o for 18 h and then for 2 h at vigorous reflux. During reaction, a gummy orange residue is produced, which must not be discarded. The work-up of the reaction mixture has been described.3 2. Methylene chloride is distilled (bp 42o), dried on calcium hydride, and distilled prior to use. 3. Triethyloxonium tetrafluoroborate is prepared as described by Meerwein4, and stored as a solution in methylene chloride in closed glass vials at -80o. To determine the concentration, 1 mL of the solution is poured into 20 mL of water and the acid formed (HBF4) is titrated potentiometrically under vigorous stirring with 0.1N sodium hydroxide. Other cationic catalysts such as boron trifluoride etherate or dimethyl sulfate can also be used to initiate polymerization.5 4. It may take several hours to dissolve the polymers obtained with cationic catalysts. Small amounts of undissolved material are removed by filtration through a pourous glass filter. 5. The commercial product, a 20% solution in hexane (Fluka AG., Buchs, Switzerland), is used without purification. 6. The checkers also obtained polymer with an intrinsic viscosity of 1.8 dL/g but a molecular weight of only 14,700 (obtained with a Knauer vapor pressure osmometer). At a reaction temperature of 20o a yield of 80% is obtained, but the intrinsic viscosity of the polymer is lower. 3. References 1. Laboratory of Organic Chemistry, Rijksuniversiteit Gent, B-9000 Gent, Krigslaan 281-54, Belguim. 2. Institute of Chemistry, University of Uppsala, Uppsala, Sweden. 3. Whitmore, F. C.; Popkin, A. H.; Bernstein, H. I.; Wilkins, J. P. J. Am. Chem. Soc., 1941, 63, 127; Shortridge, R. W.; Craig, R. A.; Greenlee, K. W.; Derfer, J. M; Boord, C. E. J. Am. Chem. Soc., 1948, 70, 948. 4. Meerwein, H.; Battenberg, E.; Gold, H.; Pfeil, E.; Willfang, G. J. Prakt. Chem., 1939, 154, 38; Org. Syn., Coll. Vol. V, 1973, 1080. 5. Goethals, E. J.; Schacht, E.; Tack, E. J. Polym. Sci., 1972, A-1,10, 533..