(5) Fujita, A,, Numata, K., J . Vitamino[. 4, 299 (1958). (6) Green, J., Biochem. J . 49, 45 (1951). (7) Havinga, E., Verloop, A., Koevoet, A. I,., Rec. Trav. Chim. 75,371 (1956). ( 8 ) Huber, W., Ewing, G. W., Kriger, J., J . A m . Chem. SOC.67,609 (1945). (9) Inhoffen, H., Ann. 508,81 (1934). (10) Johnson, M. J., "Manometric Techniaues." R. H. Burris. W. 1%'. Umbreit. J. 'F. itauffer, eds., Burgess Publishing Co., RIinnea olis, Minn., 1959. (11) Jones, S. Morris, W. W., Wilkie, J. B., J. Assoc. Ofic. Agr. Chetnzsts 42, 180 (1959).
h.,
(12) Kodicek, E., Ashby, D. R., Biochem. J . 57, XI1 (1954). (13) Koevoet, 9.L., Verloop, A., Hwvinga, E., Rec. Trav. Chim. 74, 788 (1955). (14) 'Kritchevsky, D., Kirk, M : R . , J . A m . Chem. Soc. 74, 4484 (1952). (15) Mcllahon, J. M.,Davis, R. B., Kalnitsky, G., Proc. SOC.Exptl. Biol. M e d . 75, 799 (1950). (16) Xorman, A. W.,DeLuca, H. F., in preparation. (17) Shaw, R. H., Jefferies, J. P., Analyst 82,8 (1957). (18) Terepka, A. It., Chen, P. S.,
Jorgensen, B., Endocrinology 68, 996 (1961). (19) Trappe, W., Biochem. 2. 305, 150 (1940). (20) Iyindaus, A . , Ber. 36, 3752 (1903). (21) Ziffer, H , VandenHeuvel, W. J , Haahti, E. O., Homing, E. C., J . Am. Chem. SOC.82, 6411 (1960).
RECEIVED for review February 11, 1963. Accepted May 23, 1963. Published with the approval of the Director, Wisconsin Agricultural Experiment Station. Research supported by PHS grant ilM 05800.
Use of Phosgene in the Determination of Hydroxyl Groups in Polymers DAVID G. BUSH, LEO J. KUNZELSAUER, and STEWART H. MERRILL Research laboratories, Eastman Kodak Co., Rochester 4, N. Y.
b
A new analytical procedure is described for the determination of organic hydroxyl groups. Because the method i s capable of high precision and accuracy, it has been applied particularly to the determination of terminal hydroxyl groups in polymers. The hydroxyl-containing material reacts with phosgene to form the chloroformate. After removal of the excess reagent by volatilization, the chloroformate is hydrolyzed with aqueous alkali. The chloride content of the hydrolyzate, which is equivalent to the original hydroxyl content, is titrated with silver using potentiometric techniques. Water does not interfere. The relative error of the method is less than
2%.
f
M
are available for the determination of the hydroxyl content of organic compounds (10). However, if a method is desired for the analysis of terminal hydroxyl groups in a polymer, many of these general methods are no longer satisfactory. This arises from the fact that as the molecular weight increases, the terminal hydroxyl content decrease5 and a highly precise micromethod is necessary for terminal hydroxyl analysis. Some of the general hydroxyl methods have been applied to hydroxyl endgroup analysis in polymers. Several worker5 (5,7 , 8 ) have measured the hydroxyl conce~itratioii i i i poly(propylerie glycols) by infrared spectrophotometry. Lithium aluminum hydride has been used to measure the hydroxyl content of epoxy resins (14). The infrared method and the hydride method both have the dislzdvantage that water interferes in the determina-
1250
ANY METHODB
ANALYTICAL CHEMISTRY
tion. Therefore, for precise results the sample must be anhydrous or the water content must be measured by a separate technique, usually the Karl Fischer procedure, and the final results corrected. Variations of the acetylation method of Ogg, Porter, and Willets (12) have been used by a number of workers. A variety of acylation reagents (2) were evaluated for use in determining the hydroxyl content of epoxy resins. Acidcatalyzed acetylation has been used to determine the hydroxyl number of polyoxyalkylene ethers (16). These methods are all indirect, the amount of hydroxyl is calculated from the difference between the blank and the sample titrations. As a result, high precision for samples of low hydroxyl content is difficult to achieve. Other methods involve the synthesis of a new compound by the reaction of the hydroxyl group with some reagent which contains an easily characterized group. This product is then isolated and analyzed. Some workers (8, 16) have exchanged the hydrogen in hydroxyl groups of poly(ethy1ene terephthalate) for deuterium. The hydroxyl in this same polyester was also measured from the bromine content after acylation with bromoacetyl bromide (6). Another method is based on the conversion of the hydroxyl end groups of polyesters to acid groups by reaction with succinic anhydride (4). These methods have the disadvantage t h a t the polymer may be fractionated with regard to molecular weight during isolation from the excess reagent. T h e present method is not limited by any of these disadvantages. The chloroformate of the polymer is formed by reaction of the hydroxyl group with
phosgene. The excess reagent is removed by volatilization and the chloroformate is hydrolyzed. The hydrolyzate is analyzed for its chloride content. Water does not interfere. No separation is required which might fractionate the polymer. The final argentometric chloride analysis is direct, accurate, and highly precise. EXPERIMENTAL
Reagents. 1,18-Octadecanediol, m.p. 96' to 98' C., was obtained by reduction of the corresponding diester (1). I-Octadecanol, Eastman Grade, was recrystallized from ethanol and from benzene at 10' C., m.p. 58' to 59" C. Triethylene glycol, Eastman Practical Grade, was fractionated, taking the product boiling a t 167" C. at 15 mm., n D 2 j = 1.4542. All organic solvents used were Eastman Grade. Unless otherwise noted, all chemicals used were reagent grade. Apparatus. T h e potentiometric titrations were made with a Beckman Model G pH meter. T h e indicator electrode was a Beckman silver billet type, k39261. The reference electrode was either a Beckman glass electrode, 40498, or a Beckman saturated calomel electrode, $39170, n ith the saturated solution of potas-ium chloride replaced by a saturated solution of potassium nitrate. Procedure. Weigh accurately a sample containing 0.3 t o 0.8 mmole of hydroxyl into a 100-ml. two-necked. round-bottomed flask. Add 50 ml. of a n aprotic solvent (such as benzene, dichloroethane, trichloroethane, or 1,2 dimethoxyethane) to the flask. .Idd a stirring bar, place the flask in a hood over a magnetic stirrer, and start the stirrer. Fill a dry-ice condenqer and place it in one neck of the flask Connect the other neck of the flask to a phosgene tank. Condense
almut 25 ml. of phosgene into the flask. Disconnect phosgene and stopper the flask. Keep dry ice in the condenser for about a n hour. (Kote: At this point the dry ice is allowed to sublime away and all phosgene not reacted or dissolved in the solvent evaporates.) The reaction normally remains untouched in the hood overnight, with the magnetic stirrer operating. Remove the condencier and stopper and pass nitrogen through the flask until all of the solvent and reagent have evaporated. Place flask containing the chloroformate in a vacuum desiccator over calcium hydride. Connect the desiccator to a vacuum pump through a tube of Ascarite and pump for 1 hour to remove any last traces of phosgene and hydrogen chlorid If the sample is w holuble, add 25 ml. 0.1S sodium hydroxide in water (or alcohol) immediately after removal from the desiccator. If the sample is hydrophobic, add 20 mi. of tetrahydrofuran, 3 nil. of tetra-n-imtylammonium hydroxide (25% in methanol, Eastman Practical Grade) and :I! nil. of water. Stir until sample is dissolved. If the sample is a polyester, add I gram of potassium h y d r x i d e and 50 nil. of methanol. Stopper one neck of the flask and place a mter-cooled condenser in the other neck. Using a heating mantle, bring the solution to a boil and reflux it until the chloroformate of the uolvester klas completely dissolved. Aieidifv bv dronn-i:,e addition of glacial aceti; acid.' Insert the silver and reference electrodes into the solution and titrate it pc'tentiometrically with 0.05A- st,andard silver nitrate. Determine the chloiide content, if any, of the original matwial by titration of a separate sample wvi-:hsilver nitrate. Correct the final analj.sis accordingly. Run a reagent blank in the manner just described, omitting only the sample. Suhtract the reagent blank and calculate the hydroxyl content of the sample.
might fractionate the sample. Since the chloroformate is hydrolyzed by water to alcohol, carbon dioxide, and hydrogen chloride, i t is essential to keep the system dry during volatilization of excess reagent and hydrogen chloride. Thus, dry nitrogen is used to sweep the system free of these gases. When a suitable solvent has been used the chloroformate is the only material which remains in the reaction flask after the synthesis. Because of the reactivity of phosgene, the method requires the use of an aprotic solvent during the chloroformate synthesis. I n addition, it is advantageous to use a solvent which boils below 100" C., so that the solvent is also rpmovpd h\- sweeping with nitrogen. Such solvents as benzene, dichloroethanr, trihloroethane, and 2-dimet hoxye t h a m have been used successfully. Depending on solubility requirements, the liquid phosgene itself may be used as a solvent for the reaction. Water does not interfere in the synthesis because, if present, it reacts with phosgene to form carbon dioxide and hydrogen chloride which are removed by the nitrogen stream. The reaction of phosgene with most. hydroxyl groups on polymers proceeds at 8" C., the boiling point of phosgene, without need of a catalyst. I n these Laboratories i t has been our practice, after placing the reactants together, to leave the reaction untouched in the hood overnight. However, the reaction time can undoubtedly be shortened if desired. No study of reaction times \vas made. There is no evidence for the rea,ction of the chloroformate with another hydroxyl to form the carbonate,
DISCUSSION AND RESULTS
under the conditions of the experiment. In several cases the usual order of addition of reagents was reversed-that is, the hydroxyl-containing material was added to the phosgene. There was no difference in the yield of chloride on hydrolysis. A number of favorable circumstances contribute to the precision obtainable by use of the method. The synthesis, hydrolysis, and titration with silver are all done in the same flask, eliniinating the possibility of losses during transfer. The argentometric titration of chloride is accurate even when dilute silver solutions are used. Unlike some methods, this method permits the sample size to be varied over a very wide range. If the hydroxyl content of the material is expected to be low, a larger sample may be taken so that the final titration is large enough for the desired accuracy. Seven poly(ethy1ene oxides) were analyzed, in quadruplicate, for their
.
I
Some methods for the determination of hydroxyl invohe reaction with a reagent to form a nen product, which is then analyzed. Others involve reaction with excess reagent and the amount of unreacted r2agent is measured. Inherently, i t e m be seen that the former is capable of greater accuracy than the latter. Yet these methods which involve the synthesis of a new product are usually plagued by the ~)roblemof obtaining the product uncontaminated by excess reagent. The phosgene method is free of such difficulties since the rfhagent and the reaction by-product are both volatile. 0
ROH
+ COC1,
=
ROd -C1
+ HCI
Another advantage of the volatile reagent is the fact that the sample is not subjected to a separation step which
Table 1.
Hydroxyl Content of Some Poly(ethy1ene Oxides) hf eq. Poly hY(ethylene droxyl/ Std. oxide)" gramc dev. Carbowax 600 3.344 *o 022 Carbowax 750b 1.372 =to 012 Carbowax 1000 1.949 1 0 019 Carbowax 1540 1.498 10.013 Carbowax 4000 0.717 f0.005 Carbowax 6000 0.332 =t 0,006 Carbowax 20,000 0.333 =tO 009 Products of Union Carbide Chemicals co. * Carbowax 750 is methoxypoly(ethy1ene oxide) reported by the manufacturer t,o have one terminal hydroxyl group per molecule. Average of four replicates.
(3
0
0
I/
Ib-OG-Cl
+ ROH
=
I/
'
BOCOR.
+ H(:l
hydroxyl content. These results, which appear in Table I, indicate that the relative deviation in most cases is less than +2%. In Table I, Carbowax 20,000 has a hydroxyl content very close to that of Carbowax 6000. The reason for this result is apparent from the trade literature. Carbowax 20,000 is made by the reaction of Carbowax 6000 with a diepoxide to yield a product of higher molecular weight. 2HO( CHzCH,O),CHzCHzOH H H €I I1
I I H-C-C-It-C-C-H
Y
+
I 1
=
Y
IIO( CH,CH,O),CHzCHzO H H I
?Q
I ' ! I H OH OH H CHzCH,(OCHzCLIz),OH
Thus, the same number of hydroxyl groups per unit weight are found in Carbowax 20,000 as in Carbowax 6000 (if the weight of the connecting diepoxide is ignored). The internal hydroxyls in this polymer are secondary hydroxyl groups and the results indicate that they react readily with p ho-sgene . Unsaturated compounds interfere in this method by partial reaction with phosgene (see interferences below). Some investigators have reported the presence of vinyl ether terminal groups on certain poly(ethy1ene oxides) (11). To check for the presence of such groups in the products listed in Table I, Carbowax 600 and Carbowax 1540 were analyzed for unsaturation using the Hanus method ( I S ) . The former contains 0.018 mole of unsaturation per gram, the latter, 0.015. Since these values are of the same order of magVOL. 35, NO. 9 , AUGUST 1963
0
1251
-
~~~
Table
II.
Accuracy of the Phosgene Method for Hydroxyl Content
-Meq. h y d r o x y l / g r a L Eo.@ M. wt. Material 4 150.2 Triethylene glycol 1-Octadecanol 2 270.5 2 286.5 1,18-Octadecanediol a Number of replicates.
Table 111.
Hydroxyl Content
Calcd. 13.31 3.70 6.98
Found 13.20 & 0.06 3.73 0.01 6.88 f 0.04
*
riel. error -0.8% +0.8%
-1.4%
of Some Materials Using Phosgene Method
Meq. Material Source No. bI. wt. OH/gram Std. dcv. Poly(neopenty1 Tennessee 4 2160“ 0.401 10.011 isophthalate) Eastman 2 3000b 0.703 10.002 Poly(propy1ene glycol) Dow 3025 2.32 10.02 Union Carbide 3 610b Poly(ethy1ene glycol chloride) 610 2.S9 fO.O1 4 33gb Triton X-100 Rohm and IIaas Measured by Dr. C. Glover, Tennessee EasLman Corp., using ebullioscopic methods (6). This polyester was made by reaction of neopentyl glycol with excess butyl ester of isophthalic acid. In addition to hydroxyl end groups, butyl isophthalate terminal groups are probably present. b Trade literature values. 0
chloroformamide. This product is readily hydrolyzed by alkali and the resulting chloride may be titrated. Tertiary amines react to form the amine hydrochloride as a result of the decomposition of phosgene to hydrogen chloride. Unsaturated compounds react to some extent with phosgene to form a product R hich yieldschloride on alkaline hydrolysis. About 80% of the unsaturated ends on vinyl n-octadecylcther appear to react with phosgene, amuming 1 mole of chloride results from the phosgene reaction Rith a carbon-carbon double bond. Also, 1-octadecene was found to react to the extent of 25% under the conditions of the method. Aliphatic carboxylic acids like stearic and adipic acids do not react with phoqgene. The case for acid groupq on hydroxyl polybasic acids is lehs clear. I n exploratory experiments n ith tartaric and citric acids, phosgene reacted with tartaric to give more than the expected 2 equivalents of chloride per mole and with citric more than the expected 3 equivalents per mole. ACKNOWLEDGMENT
nitude as the mean error, no correction was made on the hydroxyl values. A measurement of the accuracy of the method is difficult since standard polymeric substances of known low hydroxyl content are not available. However, two long-chain alcohols and triethylene glycol were analyzed to estimate the accuracy of the method. The results appear in Table 11. These results indicate that the relative error of the method is probably the same as the relative standard deviation, less than *2%. The method has been applied to additional hydroxyl-containing materials. These results appear in Table 111. It is obvious that a method which measures the hydroxyl content of polymers can be used t o estimate the number-average molecular weight of polymers that are known to terminate with hydroxyl groups, as with poly(ethylene oxide) glycols. Of course, these polymers must not contain other than terminal hydroxyl groups, as was the case with Carbowax 20,000. The hydroxyl results on three carbo-
Table IV. Comparison of Ebullioscopic and Phosgene Methods for Estimation of Molecular Weights
Molecular wt. Ebul!ioMaterial scopic Phosgene 595 i 30 600 =k 5 Carbowax 600 Carbowax 1000 1010 i 50 1030 f 10 Carbowax 1540 1300 f 50 1335 f 11
1252
ANALYTICAL CHEMISTRY
waxes listed in Table I have been calculated as molecular weight, i t being assumed that there are two hydroxyl groups per molecule. These are compared in Table IV with the ebullioscopic molecular weights of the same compounds. It is apparent that hydroxyl values obtained by the phosgene method can be used to estimate molecular weight in those instances where the polymer terminates exclusively with hydroxyl groups. Two problems associated with the application of the method should be recognized. First, phosgene is extremely toxic and care must be exercised in its use. However, its use presents no unusual hazards if the work is done in a n efficient hood. Secondly, because of the reactivity of phosgene, the chloroformate must be prepared in an aprotic solvent. This may present solubility problems for certain polymers. INTERFERENCES
Hydroxyl groups on aromatic nuclei react partially with phosgene under the anhydrous acidic conditions of the reaction. Therefore, such hydroxyls must be considered an interfering group. The extent of the reaction is variable and clearly dependent on the nature of the aromatic hydroxyl involved. Primary, secondary, and tertiary amines all interfere. Preliminary resuits indicate that under the conditions of the method both primary and secondary amines react to form the
The authors are indebted to E. P. Przybylowicz and C. W. Zuehlke for their interest and helpful suggestions as this work progressed. LITERATURE CITED
(1) Bennett, G. M., Gudgeon, H., J . Chem. Sac. 1938. 1679. ( 2 ) Bring, A., Kadlecek, Fr., Pluste Kautschuk 5, 43-8 (1958); C. A . 52, 12450h (1958). (3) Burns, E. A., Muraca, R. F., ANAL. CHEM.31, 397 (1959). 141 . , C o n k A.. Makromol. Chem. 26. 226 (1958). (5) Glover, C., Tennessee Eastman Corp., Kingsport, Tenn., private communication, March 1961. (6) Griehl, W., Nene, S., Faserfarsch. Testiltech. 5, 423 (1954); C. A . 49, 9359a (1955). (7) Hilton. C. L.. ANAL.CHEM.31. 1610 . ,(1959). ’ (8) Koepp, H. hl., Werner, H., 2CfakramaZ. Chem. 32, 79 (1959). (9) Kyriacou, D., ANAL.CHEM.33, 153 (1961). (10) Mehlenbacher, V. C., “Organic Analysis,” Vol. I, pp. 1-38, Interscience, New York, 1953. (11) Miller, R. A., Price, C. C., J . Polymer Sci. 34, 161 (1959). (12) Ogg, C. L., Porter, W. L., Willets, C. D., ANAL.CHEM.17, 394 (1945). (13) “Sc;tt’s Standard Methods of Analysis Vol. 11, p. 1767, Van Nostrand, New kork, 1939. (14) Stenmark, G. A., Weiss, F. T., ANAL. CHEM.28, 1784 (1956). (15) Stetzler, R. S., Smullin, C. F., Ibid., 34, 194 (1962). 53, (16) Ward, I. M., Trans. Faraday SOC. 1406 (1957). RECEIVEDfor review February 4, 1963. Accepted May 16, 1963. Division of Analytical Chemistry, 140th Meeting, ACS, Chicago, Ill., September 1961. Communication No. 2343 from the Kodak Research Laboratories. .
I
