Polypivalolactone
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Polypivalolactone Submitted by: R. C. Blume1 Checked by: C. I. Poser2a, M. Tirrell2b, and O. Vogl2c 1. Procedure a. Pivalolactone In a 1 L, round-bottomed flask suitable for a spinning band column (Notes 1 and 2) are placed 117 g of hydroxypivalic acid (1.0 mol, Note 3), 182 g of trimethyl orthobenzoate (1.0 mol, Note 3), and 300 mL of benzene. The reaction flask is slowly heated with stirring in an oil bath at 100o, and the suspension becomes homogeneous when the pot temperature reaches ca 85o. The mixture begins to reflux gently at a head temperature of 58o, the boiling point of the methanol-benzene azeotrope. The reflux ratio of the distilling mixture is adjusted to ca 10:1 and the azeotrope is collected (Note 4). After 6 - 8 h, during which the azeotrope of methanol and benzene is continuously collected, the head temperature begins to rise, being allowed to reach 80o to ensure complete removal of methanol. Typically 140 -160 g of the azeotrope are collected. The heat is now discontinued, the oil bath removed, the pot allowed to cool to room temperature, and Amberlyst 15 (5 g, Note 5) is added (Note 6). The reaction flask is again attached to the spinning band column, the pressure is reduced to 90 mm, and the remaining benzene is removed at room temperature, at a reflux ratio of ca 5:1 (Note 7). After the benzene is completely removed, the oil bath is heated until the temperature of the distillation pot reaches 135 - 145o and the head temperature of the spinning band column reaches 90o, the reflux temperature of pivalolactove at 90 mm. The fraction boiling o 25 at 90 - 91 is collected to yield 35 - 38 g of pure pivalolactone, nD 1.4040. Higher boiling fractions should also be collected (e.g., fraction II, 18 g, bp 91 -105o; III, 38 g, bp 105 - 120o; IV, 163 g, bp 120 - 150o). Because essentially pure methyl benzoate begins to distill at 125 - 130o/90 mm, the upper temperature limit is not critical and the final yield of pivalolactove cannot be substantially improved by collecting material that distills above 130o. The pot temperature is finally raised to 190o. Fractions II, III, and IV can be redistilled to yield and additional 26 - 30 g of pure pivalolactone. A total combined yield of 61 - 68 g (61 - 68%; Note 8) is obtained. 1 2 Macromolecular Syntheses, Collective Volume 2 b. Polypivalolactone Ten mL of pivalolactone (0.1 mol, Note 9), 100 mL of dry hexane, and 100 ìL of 1M tetrabutylammonium hydroxide in methanol are added to a dry 300 mL, three-necked flask equipped with a mechanical stirrer, a reflux condenser, and a slow (1-4 cm3/min) nitrogen purge. The solution is now heated to reflux. After ca 5 min, a fine, white suspension of polypivalolactone begins to appear. The amount of polymer increases as the polymerization progresses. After 2 h at reflux, the suspension is cooled, suction-filtered through a medium porosity sintered glass funnel, washed three or four times with hexane, and dried overnight in a vacuum oven at 60o/1 mm. The yield of polypivalolactove is 8.0-8.3 g (80-83%). The inherent viscosity of the polymer, measured as a 0.5% solution in trifluoroacetic acid at 25o, is 1.6-1.75 dL/g. The crystalline mp (by DTA) is 235o (Note 10). 2. Notes 1. The design and operating conditions of the spinning band column that proved effective for the checkers are as follows: column length 40 cm, column bore 8 mm, band speed ca 2000 rpm. No column heating was used. Other types of distillation columns with comparable separation efficiency may be used instead of a spinning band column. 2. In the checkers' first attempt to prepare pivalolactone from hydroxypivalic acid and trimethyl orthobenzoate, polymerization of pivalolactone in the spinning band column was observed during the distillation of pivalolactone. This was presumably caused by the basicity of the surface of the glass column. This problem was subsequently eliminated by rinsing the entire reaction-distillation system several times with 1N HCl, followed by rinsing with acetone and thorough drying before use. 3. Potassium hydroxypivalate, prepared by potassium hydroxide saponification of commercial neopentylglycol monohydroxypivalate (Eastman Chemical Products, Inc., Kingsport, TN), serves as a convenient source of hydroxypivalic acid. Hydroxypivalic acid prepared from the potassium salt has a mp of 122-124o when recrystallized from ethyl acetate. Trimethyl orthobenzoate is prepared by reaction of sodium methoxide with benzotrichloride.3 4. The methanol-benzene azeotrope is completely removed by slow distillation over a 6-8 h period; consequently, the exact reflux ratio should be adjusted to achieve the removal of he azeotrope during this time period and not substantially faster. 5. This strongly acidic cation exchange resin is available from Rohm and Haas, Independence Mall, West Philadelphia, Pennsylvania 19105. 6. The rate of pivalolactone formation is only modestly increased by the addition of the Amberlyst acid catalyst.4,5 The use of acid is recommended mainly to prevent adventitious initiation of polymerization of pivalolactone. 7. 2-Phenyl-2-methoxy-5,5-dimethyl-4-keto-1,3-dioxane is the main reaction product at this point.4,5 It is easily hydrolyzed and should be protected from atmospheric moisture if the reaction mixture is stored before completion of the preparation. 8. The checkers performed the monomer preparation also at one-tenth scale and obtained 1.9 g of pivalolactone on redistillation of the higher boiling fractions for a total yield of 54%. 9. Pivalolactone polymerizes readily in the presence of a wide variety of nucleophiles. It is best stored in bottles that have been acid-washed and carefully dried. The purity of pivalolactone that has been stored for a long period of time may be assured by filtering it through Woelm neutral alumina (10 cm X 25 mm column for 100 mL of pivalolactone), and then stirring over calcium hydride for several hours before redistilling it into a dry receiver. 10. The checkers obtained yields of 68% and 85% in two trials of this preparation. The polymer obtained had an inherent viscosity of 1.95 dL/g and a melting endotherm at 233o (Perkin-Elmer DSC II at a heating rate of 10o min). Polypivalolactove 3 3. Methods of Preparation Pivalolactone has been prepared by ring closure of chloropivalic acid6 and bromopivalic acid7,8 salts and by condensation of dimethylketene with formaldehyde.9 Other á,á-dialkyl-â-lactones have been prepared by diazotization of the â-aminopropionic acids.10 The polymerization of â-lactones can be initiated by a wide variety of nucleophiles and electrophiles.11,12,13,14,15 The present initiator is recommended because of its accessibility as an analytical reagent. 4.References 1. Textile Fibers Department, Pioneering Research Laboratory, Experimental Station, E. I. du Pont de Nemours & Co., Inc., Wilmington, DE 19899. 2. a. Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA 01002; b. Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455; c. Polytechnic University, Brooklyn, NY 11201. 3. McElvain, S. M; Venable, J. T. J. Am. Chem. Soc., 1950, 72, 1661. 4. Blume, R. C. (I. I. du Pont de Nemours & Co., Inc.), U.S. Patent 3 503 993 (1970). 5. Blume, R. C. Tetrahedron Lett., 1969, 13, 1047. 6. Lorenz, C. E. (E. I. du Pont de Nemours & Co., Inc.), U.S. Patent 3 291 810 (1966). 7. Hagman, S. Dissertation. Lund, Chem. Abst., 1924, 18, 2497. 8. Fischer, N. Thesis, Paris (1959). 9. Hasek, R. H.; Nations R. G. (Eastman Kodak Co.) U.S. Patent 3 004 906 (1960). 10. Testa, E.; Fontanella, L.; Mariani, L. J. Org. Chem., 1960, 25, 1812. 11. Reynolds, R. J. W.; Vickers, E. J. (Imperial Chemical Industries), British Patent 766 347 (1957). 12. Hall, H. K.; Schneider, A. K. J. Am. Chem. Soc., 1958, 80, 6409. 13. Thiebaut, R.; Fischer, N.; Etienne, Y.; Coste, J. Ind. Plast. Mod. (Paris), 1962, 14, No. 2, 13. 14. Tietz, R. F. (E. I. du Pont de Nemours & Co., Inc.), U.S. Patent 3 345 343 (1967). 15. Wilson, D. R.; Beaman, R. G. J. Polym. Sci., 1970, A-1,8, 2161..