Poly(ethylene oxide)
Submitted by: F. E. Bailey, Jr., and H. G. France 1 Checked by: C. C. Price, R. Spectro and Y. Atarashi 2
A. R2Zn-CH3OH as Catalyst
1. Procedure Under a nitrogen atmosphere, 15 ml of dry toluene and 15 g of ethylene oxide are charged to a suitable Pyrex® pressure tube (Note 1) followed by addition of 1 ml of a toluene solution of methanol-modified dibutyl zinc. Caution! Dibutyl zinc is pyrophoric and highly reactive; ethylene oxide is poisonous (Note 2). The tube is capped with a rubber septum and placed in a Dry Ice- acetone bath. The tube is flame-sealed (Note 3), then warmed slowly to about 50oC and placed in a constant-temperature bath (Note 4) at 90oC for about 24 h. After this time the tube is cooled (Note 5) and opened, and the solid polymer is dissolved in 300 ml of benzene. The polymer is precipitated by slowly adding the benzene solution to 2 l of heptane which is rapidly agitated with a suitable stirrer. The white, tough, solid polymer is recovered from the heptane by filtration and is dried under vacuum at 40oC for 12 h; yd. 50-70%. The polymer has a reduced viscosity in acetonitrile of 1.8-3.3 dl/g (Note 6).
2. Notes 1. A suitable pressure tube can be prepared from a 9 in piece of 22-25 mm Pyrex® tubing that is sealed at one end. To the other end is attached a 3 in piece of 8 mm glass tubing. The monomer, diluent, and catalyst are charged to the polymerization tube with a thistle tube that passes through the 8 mm neck and extends to the bottom of the tube. Caution! Because these tubes are under pressure during polymerization, they should be properly annealed, and extreme care should be exercised in handling them; use safety sheilds. 2. Dibutyl zinc can be prepared as described in Organic Syntheses.3 The methanol modification involves dissolving 10.5 g of dibutyl zinc in 141 ml of dry toluene and slowly adding 0.96 g of methanol. This solution is stored at temperatures below 0oC to retard degradation of the catalyst. Diethyl zinc can also be used as the catalyst. Use of different catalysts and different degrees of modification alter yields and properties of the polymer products. The molecular weight can be controlled by changes in catalyst concentration, monomer/solvent ratio, and the ratio of modifier in the catalyst preparation. 3. The polymerization tube is cooled in Dry Ice-acetone for a period of time sufficient to ensure good sealing because of the partial vacuum generated. This vacuum can be determined by observing the decrease in pressure on the rubber septum used to cap the tube.
228 Poly(ethylene oxide) 229 4. Any suitable bath that can be maintained at 90oC and that allows gentle rocking or rotating of the polymerization tube can be used. 5. The tube should be cooled to Dry Ice-acetone temperature to reduce the pressure in the tube prior to opening. 6. The reduced viscosity is determined at a concentration of 0.2 g/100 ml in acetonitrile at 30oC.
B. FeCl3-C3H7O Complex as Catalyst
1. Procedure A Pyrex® polymerization tube is charged in a manner similar to that in part A, except that 0.15 g of a ferric chloride-propylene oxide (Note 1) complex is used as catalyst. The tube is sealed and placed in a constant-temperature bath at 90oC for 24 h, after which time the polymer is recovered as in part A. There is obtained a 70-100% yield of polymer having a reduced viscosity in acetonitrile of 1.2-1.6 dl/g (Note 2).
2. Notes 1. The ferric chloride-propylene oxide complex catalyst is prepared as described in the literature.4 2. Variations in catalyst preparation and changes in the monomer-solvent ratio alter both yield and viscosity of product.
C. Ca(NH2)2-C2H4O as Catalyst
1. Procedure Under a nitrogen atmosphere, poly(ethylene oxide) can be prepared at room temperature using an ethylene oxide-modified calcium amide catalyst (Note 1). A 2 l, four-necked flask equipped with stirrer, thermometer, a gas inlet tube which extends to the bottom of the flask, and a reflux condenser is charged with 1 l of dry heptane and a quantity of catalyst suspension (Note 1) equivalent to 1 g of contained calcium. This heptane/catalyst suspension is stirred rapidly, and ethylene oxide is bubbled into the reaction mixture at a sufficient rate to give a gas evolution rate of not less than 40 bubbles/min. The temperature increases from 25o to 45oC in about 10-15 min and is maintained by cooling with an ice bath at a temperature below 50oC (Note 2) for about 7 h. The resulting finely divided polymer is recovered by filtration and dried under vacuum at 40oC for 12 h. A white granular polymer (about 70-80 g) having a reduced viscosity in acetonitrile of about 20-30 (Note 3) is obtained.
2. Notes 1. Ethylene oxide-modified calcium amide is prepared by a procedure similar to that described in U.S. Patent 2,971,988. Calcium (5 g) is slowly added to 200 ml of liquid ammonia in a 1 l glass flask, in an efficient hood, avoiding exposure of the contents to the atmosphere. It is best to use a distilled granular calcium metal rather than calcium turnings or wire because the presence of calcium oxide reduces the activity of the catalyst. The ammonia should contain less than 0.1% water because the presence of water also diminishes catalyst activity. The calcium-ammonia solution can be filtered to remove the insoluble calcium oxide present. Caution! Calcium hexammoniate, which is formed on evaporation of the excess ammonia, is very pyrophoric. Ethylene oxide (3 g) dissolved in 50 ml of liquid ammonia is slowly added to the calcium-ammonia solution over a 15-30 min period. The ammonia solution turns from dark blue on addition of calcium to a dark grey color after the addition of the ethylene oxide. The reaction mixture is stirred and the excess ammonia is allowed to evaporate. After about 4-6 h 50 ml of dry heptane is slowly added to displace the last 50 ml of excess ammonia, and the 230 Macromolecular Syntheses, Collective Volume 1 catalyst is obtained as a fine greyish white suspension in heptane. On titration for calcium this suspension contains approximately 1 g of contained calcium per 10 ml of suspension. It is desirable to charge a glass jar containing small glass beads with the catalyst suspension and place it on a roller for about 1/2-1 h to obtain the catalyst as a very fine suspension. 2. The temperature on addition of ethylene oxide should not be allowed to exceed 50oC because the polymer will tend to agglomerate and decrease the catalyst activity. The temperature can be controlled by raising an ice bath under the reaction flask and maintaining the temperature between 45o and 50oC during the initial polymerization stage. The checkers found that some heating was needed in the latter stages to maintain the temperature above 40oC. 3. Polymers prepared by this method have extremely high molecular weight, and measurement of the reduced viscosity is difficult. The polymer dissolves readily in refluxing acetonitrile. The presence of moisture converts the catalyst residue into insoluble salts that appear as haze in the solution. The solution must be filtered before viscosity determinations, or samples of the polymer should be dissolved, filtered, and precipitated to remove the catalyst residue.
D. KOH as Catalyst
1. Procedure Anhydrous (Note 1) diethylene glycol (118 g) is added to a 2 l, three-necked flask equipped with reflux condenser, stirrer, thermometer, and gas inlet tube which extends below the surface of the liquid. Freshly fused potassium hydroxide (2.5 g) is added to the diethylene glycol and the mixture is stirred and heated to 100oC, at which point all the potassium hydroxide is dissolved. The heat source is removed and a cold water bath is placed in position beneath the flask. Dry ethylene oxide gas is fed into the flask. An extremely exothermic reaction occurs and the temperature of the reaction is controlled at 100-120oC with the water bath and the regulation of the ethylene oxide feed. The reaction is allowed to continue for about 8-9 h, or until no further absorption of ethylene oxide is observed (Note 2). The hot reaction mixture is poured from the flask into a 2 l beaker and allowed to cool to room temperature. A tan, waxy solid (about 700-900 g) is obtained that has a freezing point of about 25-30oC, indicating a molecular weight of about 600-700 (Note 3). When heated to 250o at 2-3 torr, this material shows no appreciable loss in weight, and no volatiles are distilled.
2. Notes 1. Diethylene glycol is dried for 16 h over calcium hydride and then distilled at 129oC at 10 torr. The reactants should be kept as nearly anhydrous as possible in order to obtain the required molecular weight ranges (see Note 3). 2. A bubbler connected to the exit side of the reflux condenser can be used to determine when the ethylene oxide ceases to react. The temperature of the reaction decreases as the absorption of ethylene oxide gas slows, and external heat is supplied to maintain the temperature at 100-120oC. The checkers found no decrease in reactivity in 9 h and interrupted the polymerization at that time. 3. Trace quantities of water present in the reaction decrease the molecular weight and yield of the product. The catalyst/diethylene glycol ratio also affects the molecular weight. Higher molecular weight polymers, up to about 6000, can be prepared by dividing the obtained product in half, adding more potassium hydroxide (2 g) and continuing the ethylene oxide feed. The temperature should be increased to about 130-140 oC. Poly(ethylene oxide) 231 3. Methods of Preparation This method, along with the other three given here, allow preparation of polymers with virtually any molecular weight desired. Additional procedures for polymerizing ethylene oxide have been described.5,6,7,8,9,10
E. References 1. Research and Development Department, Union Carbide Corp., Chemicals and Plastics, South Charleston, WV 25303. 2. Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104. 3. Noller, C. R. Org. Syn. Coll. Vol. 2 184, 1943. 4. Price, C. C.; Osgan, M. J. Am. Chem. Soc. 1956, 78 4787. 5. Price, C. C.; Carmelite, D. D. J. Am. Chem. Soc. 1966, 88, 4039. 6. Price, C. C.; Spector, R. J. Am. Chem. Soc. 1966, 88, 4171. 7. Bawn, C. E. H.; Ledwith, A.; McFarlane, N. Polymer 1969, 10, 653. 8. Solonvyanov, A. A.; Kozanskii, K. S. Vysokomol. Soyed. 1970, A12, 2114. 9. Kazanskii, K. S.; Solovyanov, A. A.; Entelio, S. G. European Polym. J. 1971, 7, 1421. 10. Nenna, S.; Figurello, J. E. European Polym. J. 1975, 11, 511.