Block Copolymers of Styrene and Ethylene Oxide Submitted by: J. J. O'Malley and R. H. Marchessault 1 Checked by: D. J. Worsfold 2 1. Procedure Synthesis of polystyrylpotassium in tetrahydrofuran solution is carried out in the apparatus shown in Figure 1. After it is carefully dried and evacuated to 10-5 - 10-6 torr, it is removed from the vacuum line by sealing the constriction. Figure 1. Apparatus for synthesizing polystyrylpotassium 342 Block Copolymers of Styrene and Ethylene Oxide 343 A red cumylpotassium initiator solution (4.0 ml, 0.095 N, Note 1) in tetrahydrofuran (Note 2) is added to the reaction flask via the breakseal which is broken with a glass-covered iron nail. Any initiator solution remaining in the ampoule can be washed into the flask by cooling the ampoule and condensing solvent in it. After stirring is started, the monomer bulb containing 7.6 g (0.073 mol) of styrene (Note 3) in 38 ml of tetrahydrofuran is sprayed into the initiator solution at 0o through a 0.5 mm bore capillary tube. The nonterminating polymerization is completed almost instantaneously and the red polystyrylpotassium solution is transferred into ampoules, which are sealed from the apparatus at the constrictions and stored in the refrigerator until used (Note 4). The molecular weight of polystyrene prepared in this manner is 20,000.3 The polystyrene-poly(oxyethylene) block copolymer is prepared by adding purified ethylene oxide (Note 5) to a tetrahydrofuran solution of polystyrylpotassium. Caution! Ethylene oxide is very toxic and must be handled with extreme care. Ampoules of polystyrylpotassium (17 ml, 7.6 x 10-3 N in active end-groups) and ethylene oxide (2.5 g, 0.057 mol) are attached to the apparatus shown in Figure 2. After the apparatus has been dried and evacuated on the vacuum line, it is removed by sealing the constriction. The polymer solution is then emptied into the thick-walled Pyrex tube (Note 6) and the tube is cooled to -78o. After the addition of the ethylene oxide the reaction tube is sealed from the apparatus and the contents shaken for a few minutes while the tube warms to ambient temperature. During this warming period, the characteristic red color of the polystyrylpotassium disappears, an indication that copolymerization has started. To complete the ethylene oxide polymerization the tube is kept at 70o in a sand bath for 4 days (Caution! Note 7). At the end of this period the polymer solution (Note 8) is neutralized with acetic acid and the contents are isolated by precipitation into 10 volumes of heptane followed by centrifugation and decantation. Figure 2. Apparatus for synthesizing block copolymers. Copolymers prepared by the procedure described above contain varying amounts of both hompolymers because of transfer reactions; (Note 4), and the polymeric product should be purified to remove unwanted materials. Polystyrene homopolymer is eliminated by dissolving the precipitate in benzene and titrating the solution with diethyl ether to precipitate the block copolymer 344 Macromolecular Syntheses, Collective Volume 1 and any oxyethylene homopolymer present. After the nonsolvent mixture has been decanted, the precipitate is dissolved in a little benzene and the solvent is distilled in a rotary evaporator to form a thin polymeric film on the walls of the round-bottomed flask. Repeated washing of this film with distilled water will remove salts and any polyoxyethylene present (Note 10). The pure copolymer is dried by lyophilization of a 1% benzene solution. The yield of pure block copolymer is usually about 80%. 2. Characterization The block copolymer contains 45% polystyrene and has a number average molecular weight of 46,000. The calculated molecular weight, based on the molecular weight of the polystyrene segment (20,000) and the copolymer composition, is 44,000. The intrinsic viscosity of the copolymer in toluene at 35.0o is 0.33 dl/g. The semicrystalline block copolymer exhibits a melting temperature of 59o (the polyoxyethylene block) and a glass transition temperature of 90o (the polystyrene block) as determined by differential scanning calorimetry. 3. Notes 1. Cumylpotassium is prepared by Ziegler's method4 from methyl cumyl ether and sodium- potassium alloy. A course fritted-glass filter should be used to remove the insoluble sodium methoxide formed in this reaction. Finer filters may be used thereafter if deemed necessary. The concentration of cumylpotassium is determined by addition of a known volume of solution to distilled water, followed by titration with 0.01 N hydrochloric acid to the phenolphthalein end- point. 2. Tetrahydrofuran is purified by heating under reflux for 24 h over potassium metal and collecting the middle fraction of the distillate at 66o. Sodium metal, benzophenone and a stirring bar are then introduced into the distilled solvent which is degassed on the vacuum line. The solvent gradually takes on the purple color of sodium benzophenone dianion when the tetrahydrofuran is dry. All solutions are prepared by distilling tetrahydrofuran from this reservoir. 3. Styrene is purified by two distillations on the vacuum line from calcium hydride. Residence time on calcium hydride between distillations is 24 h. Pure monomer is stored in a refrigerator and used as soon as possible. 4. Proton abstraction reactions, reported5 to occur at room temperature, result in the formation of a 1,3-diphenylallyl anion at the end of the polymer chain. 5. Commercially pure (99.7%) ethylene oxide is purified by distilling three times at -40o over fresh calcium hydride under high vacuum. Because of its low boiling point (10.7o), ethylene oxide must be kept cold during these operations. 6. A Carius combustion tube is convenient for this purpose. 7. Pressure buildup caused by the ethylene oxide will occur and necessary precautions should be taken. 8. At high ethylene oxide concentrations, the block copolymer solution may exist as an amber- colored gel at room temperature or below.6,7 9. The presence of homopolymer may be detected by size-exclusion chromatography or density gradient ultracentrifugation. 10. Some block copolymer will be lost in this step, especially if the copolymer is rich in ethylene oxide. In the latter case other solvent-nonsolvent systems may be preferred, but the final choice is governed by the chain length of the respective sequences. 4. Methods of Preparation The block copolymer described above is A-B type. Copolymer structures of the type A-B-A have been prepared8,9 with a difunctional anionic initiator, where B corresponds to polystyrene and A to Block Copolymers of Styrene and Ethylene Oxide 345 poly-oxyethylene. Anionic monopolystyrene terminated with phosgene to form an acid chloride end-group has been condensed with polyoxyethylene to synthesize copolymer structures of type 10 B-A-B. If the dianionic polystyrene is used in the latter scheme, then A-(B-A)n structures should result. A comparable synthesis of styrene-isoprene block copolymers has been described.11 5. References 1. Chemistry Department, State University of New York, College of Forestry, Syracuse, NY 13210; current address O'Malley - Xerox Corporation, Research Laboratories, Webster, NY 14580; Marchessault - University of Montreal, Montreal Quebec, Canada. 2. Applied Chemistry Division, National Research Council, Ottawa, Canada. 3. Waach, R.; Rembaum, A.; Coombes, J. D.; Szwarc, M. J. Am. Chem. Soc., 1957, 79, 2026. 4. Ziegler, K.; Dislich, H. Ber., 1957, 90, 1107. 5. Spach, G.; Levy, M.; Szwarc, M. J. Chem. Soc., 355. 6. Skoulios, A.; Finaz, G.; Parrod, J. Compt. Rend., 1960, 251, 739. 7. O'Malley, J. J. Ph.D. Thesis, State University of New York, College of Forestry, 1967. 8. Richards, D. H.; Szwarc, M. Trans. Faraday Soc., 1959, 56, 1644. 9. Baer, M. J. Polym. Sci., 1964, A2, 417. 10. Finaz, G.; Rempp, P.; Parrod, J. Bull. Soc. Chem., France, 1959, 262. 11. Prud'Homme, J.; Roovers, J. E. L.; Bywater, S. European Polym. J., 1972, 8, 901..