I
WARREN J. MURBACH and ARNOLD ADICOFF Research Department, U. China Lake, Calif.
S. Naval Crdnance
Test Station,
linear Polyurethanes
olyalkylene Ether Glycols New copolymer diols derived from ethylene oxide and tetrahydrofuran yield polyurethanes with potentially superior physical properties
polypropylene glycol to prepare polyurethane elastomers suffers from two major disadvantages. First, polypropylene glycol contains significant amounts of terminal unsaturation which acts as a chain stopper in polymerization. Second, polypropylene glycol contains both primary and secondary hydroxyl groups, but most polyfunctional cross linkers contain predominantly primary groups. Differences in reaction rates of these groups with diisocyanates yields a nonideal, three-dimensional network with possible inferior mechanical properties. In the work described, new, polymeric diols were made by copolymerizing ethylene oxide and tetrahydrofuran, using boron trifluoride as the initiator and ethylene glycol as the co-caxalyst. Linear polyurethanes were then prepared by the reaction of these copolymers or Teracol 30 (7) with diphenylmethane-4,4’-diisocyanate (MDI). A low molecular weight polymeric diol containing virtually no unsaturation can be prepared by copolymerizing ethylene oxide with tetrahydrofuran in the presence of an equimolar ratio boron trifluoride to ethylene glycol catalyst. The 11 (ethylene oxide as M I ) and ra values for this polymerization a t 0°C. are 0.08 and 2.2, respectively. These polymeric diols can be further polymerizedusingMD1 to give high molecular weight polyurethanes of essentially linear character. The isocyanate-to-hydroxyl T H E USE O F
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INDUSTRIAL
AND ENGINEERING
ratios required to give the highest molecular weights are higher than those previously reported from mechanical behavior studies of cross-linked polyurethanes. I t is suggested that an additional way in which isocyanate is lost during polymerization is by a dimerization reaction. A rough calculation of the rate of dimerization of M D I indicates that this reaction is at least as rapid as the urethane hydrogen-isocyanate reaction. Because initially, a t least, the isocyanate concentration is much higher than that of the urethane hydrogens, more dimer is formed than allophanate through the urethane-isocyanate reaction. However, the total reaction product may be indistinguishable because the dimer may eventually react to form the same sort oi substituted allophanate. Experimental
Materials and Methods. Tetrahydrofuran was refluxed over lithium aluminum hydride and then fractionated in a 2.5 X 50 cm. packed column just prior to use (boiling point, 62.9’ C. a t 705 mm. of mercury). Matheson Co. tank ethylene oxide, having a stated minimum purity 99.770, was used without further purification. Ethylene glycol was distilled through a 30-plate Oldershaw column at reduced pressure and a center cut was collected. The 1 : l boron trifluoride-tetrahydrofuran com-
CHEAMSTRY
plex was prepared by the method of Osthoff and others ( 3 ) (boiling point, 73.lo-73.3OC. a t 4.8 mm. of mercury). M D I (Allied Chemical and Dye Corp., Nacconate 300) was dissolved in benzene, the solution filtered to remove the dimer, and the filtrate evaporated. The residue was distilled at reduced pressure just prior to use, a center cut being retained (boiling point, 138.OoC. at 0.3 mm. of mercury). Merck and Co. chromatographic grade aluminum oxide (basic form) was used throughout this work. Teracol 30 was used as received. All solvents were carefully dried and fractionated. The procedure used to determine molecular weights was modified after Siggia (4). Only 10 ml. of 1M phthalic anhydride was used and the final titration was made with 0.4N sodium hydroxide. The method used to analyze diisocyanates was similar to that described by Siggia (5) except that di-n-butylamine was substituted for n-butylamine and bromcresol green was used as the indicator. Intrinsic viscosity measurements were carried out a t 30.00 i 0.02OC. by successive dilutions in an Ubbelohde dilution viscometer. The particular viscometer used had a flow time for pure benzene of 111 seconds. The spectra of the liquid copolymers were obtained as 0.025-inch film on a Perkin-Elmer Model 21 double-beam
P L A S T I C S A N D E L A S T O M E R S IN R O C K E T S recording infrared spectrophotometer fitted with sodium chloride optics.
General Technique of Copolymerization A simple "semi-open" technique was used in all polymerizations. A typical experiment was carried out as follows: A 250-ml. flask fitted with a stirrer, thermometer, and gas delivery tube was charged with 36.1 grams (0.5 mole) of tetrahydrofuran, 3.10 grams (0.05 mole) of ethylene glycol, and 50 grams of 1,2-dichloroethane. This solution was cooled to -10' to -5'C. in an ethyl alcohol-dry ice bath and then 19.8 grams (0.45 mole) of ethylene oxide was added from a glass ampoule. T h e temperature was adjusted to -5' to OOC. and then a solution of 7.00 grams (0.05 mole) of BFsO(CH2)4 in 10 grams of 1,2-dichloroethane was added dropwise over 7 minutes. The reaction mixture was stirred 1 hour at -5' to O'C., 2 hours a t OOC., and then stored a t O'C. After 48 hours of reaction time, solvent and unreacted monomers were removed in vacuo on a rotating evaporator without external heating. The residue consisted of 56.0 grams (94.9%) of a clear, colorless, moderately viscous liquid. To remove BFa a benzene solution of the copolymer was stirred for 43 hours with 50 grams of A1203. T h e A1203 was filtered off and the solvent evaporated as before to yield 45.5 grams of pure copolymer which gave a negative test for fluoride ion with zirconium-alizarin reagent. A sample was submitted for analysis after additional drying for 24 hours a t 55OC. (< 1 mm.). Analysis. Found: C, 60.63, 60.79; H, 10.42, 10.43. The infrared absorption spectrum showed a medium intensity O H stretching mode at 2.90 microns. Characteristic C=O and -C=Cstretching vibrations in the 5.5- to 6.3-micron region were not observed indicating the absence of undesirable oxidation products and terminal unsaturation. The intrinsic viscosity, [ a ] , in benzene was found to be 0.0551 dl. per gram. The number average molecular weight, M,, as determined by end group analysis was calculated to be 1250, assuming difunctionality .
Batch-to-Batch Variation in the Copolymerization of Ethylene Oxide and Tetrahydrofuran The same general technique was employed except that the apparatus was scaled up to accommodate a 5-mole run. The properties of the copolymers obtained from two identical runs are summarized in Table I.
Table 1. Ba+ch-to-Batch Variation in the Copolymerization of Ethylene Oxide and Tetrahydrofuran Yield, [TI, Analysis, Mol. Expt. % Dl./G. %C Wt. 1 2
82.0 82.5,
0.0619 0.0632
60.09 60.45
1440 1440
Determination of Monomer Reactivity Ratios The cationic copolymerization of ethylene oxide ( M I ) and tetrahydrofuran (Mz) initiated by BF,O(CH,), in 1,2-dichloroethane solution at 0' C. was carried out using the semi-open technique described for the product studies. I n each experiment the total monomer charge amounted to 1 mole while 50 weight 7 0 of solvent was used in experiments employing 0.1 to 0.5 mole fraction of M I . At higher concentrations of Mi it was found necessary to use 100 weight % ' solvent due to the limited solubility of Mi in the solvent. In order to keep conversions below 5% only 2 drops of catalyst was used and polymerization was allowed to proceed for only 45 minutes. At the end of the reaction time the flask was cooled rapidly and solvent and unreacted monomem were rapidly removed in a high vacuum system while keeping the mixture a t about -20° C. The copolymer was weighted to ascertain the conversion and then BFs was removed by stirring a benzene solution of the copolymer with 3 grams of A1203. T h e copolymer was isolated and dried as previously described. Preparation of linear Polyurethanes A quantity of the polymeric diol under study (usually 0.002 to 0.005 mole) was introduced into five or more 5-dram, screw-cap vials and the samples were degassed overnight a t 60' C. (
