Rapid Determination of Primary Hydroxyl Content of Poly(Oxyalky1ene) Glycols and PoIy (Oxya Iky Iene) So rbito Is by Pseudo First-Order DifferentiaI Reaction Kinetics FRISO WILLEBOORDSE and R. L. MEEKER Chemicals Division, Union Carbide Corp., Technical Center, South Charlesfon, W. Vu.
The conventional procedure for the determinaton of primary hydroxyl content in polyols, the logarithmic extrapolation method based on the differential kinetic analysis of the hydroxyl group reacting with either acetic anhydride or phenyl isocyanate, is cumbersome with respect to duration of analysis and method of calculation. A differential pseudo first-order reaction rate approach is described which only requires a fraction of the analysis time of the extrapolation method, and does not need a calculation procedure. The primary hydroxyl content of the polyol is obtained by determination of the effective fractional life time in the urethanation reaction and usage of a predetermined calibration curve. The procedure can be applied to any other binary system of activehydrogen-containing compounds when simultaneous reactions with phenyl isocyanate occur under sufficiently differential rate constant conditions.
A
of analytical techniques for the determination of mixtures of organic compounds which contain the same functional group have been based upon differences in reaction rates with a common reagent ( 5 , 7'). With reference to mixtures of hydroxyl compounds this approach was recently successfully applied under irreversible second-order reaction conditions to determine binary mixtures of alcohols and primary/secondary hydroxyl contents in polyether polyols with reagents such as acetic anhydride (3) or phenyl isocyanate (9). In addition, three nonkinetic methods have been developed. One approach employing the reaction between primary alcohols and trityl chloride is quite time-consuming and does not resolve secondary and tertiary alcohols. Another one involves an esterification reaction of primary alcohols with 3-nitrophthalic anhydride ( d ) , but secondary hydroxyl groups interfere because of partial esterification. A third method (1) is based on the N INCREASING NUMBER
(e)
854
ANALYTICAL CHEMISTRY
absorption spectra of nitrites of hydroxyl compounds in the ultraviolet region. However, the absorption band maxima of the nitrites of primary alcohols differ only by 1-2 mp from those of secondary alcohols. For polyglycols the method has to be carried out under nitrogen, and includes a filtration step and a correction for background absorption. A recently published kinetic method (9) will determine the primary hydroxyl content in w,w'-polyether glycols and in mixtures of isomeric alcohols. This method is not restricted to two-component systems. The technique presented in this paper is based on the same reaction-Le., between the hydroxyl compounds and phenyl isocyanate-which is followed by observing the disappearance of the NCO band a t 4.42 microns in the infrared region of the spectrum. However, instead of employing the second-order logarithmic extrapolation technique (9), the present method is based on pseudo-first order kinetics, thus circumventing the two inconvenient features of the extrapolation technique-Le., length of reaction time and the calculation procedure. Apart from the analytical desirability and convenience of the pseudo first-order approach, the method can be utilized to determine (by extrapolation) the reaction rate constants of single functional groups in polymeric substances. I n the underlying case, it was possible to determine the rate constants of the primary alcohol groups and those of the secondary alcohol groups in poly(oxyalky1ene) compounds with phenyl isocyanate in the urethanation reaction, and with acetic anhydride in the acetylation reaction. However, this approach is by no means limited to the above-mentioned cases, but can generally be applied for any other type of system where the simultaneous reactions of various molecules with the same functional group can lead to differential kinetic analysis ( 5 ) . The pseudo first-order approach has in addition the advantage of overcoming
nonintegral kinetic orders in reaction rate studies which has been recognized and utilized long ago by Roberts and Regan (8). The present method is extremely rapid, requiring 10 minutes per analysis, and is not subject to interference by finite mixing times. Reaction conditions are mild and do not effect fragmentation of polyether linkages. The procedure is applicable to binary mixtures of alcohols whose rate constant ratios are as low as 1.97 t o 1, and to glycol-based or sorbitol-based polyhydric alcohols in their entire molecular weight region between 200 and 5000. The procedure requires the construction of a predetermined calibration curve employing either the two constituents or, in the case of polyols, known mixture compojitions. The method should be industrially of significant applicability for routine use since the end-result is obtained by simple read-out from the calibration curve. EXPERIMENTAL
Reagents and Apparatus. N , N Dimethylformamide ( D M F ) . Redistill or dry D M F (Du Pont) with Linde Nolecular Sieve, Type 5A, until the water content is less than 0.01%. Store in a capped bottle. Toluene. Redistill the solvent from stannous octoate (catalyst T-9 from M and T Corp.), 100 grams per gallon of toluene. The water content should be less than 0.008%. Store in a capped bottle. Phenyl Isocyanate, reagent grade, boiling point 60" to 62" C. a t 20 mm. (Matheson Coleman and Bell). Add 10.0 ml. (10.95 grams) of phenyl isocyanate to 50 ml. of dry toluene, dilute to 100 ml. with additional toluene, and mix thoroughly. Store in the volumetric flask. Discard the reagent if any crystalline precipitate is formed upon standing. Alcohols. Use reagent grade chemicals as standards. Catalyst. Add 4.480 grams of 1,4diazabicyclo(2.2.2)-octane (DABCO, Houdry Process and Chemical Co.),
I
I
0
0.2
I
I
0.4
0.6
I
I
O 0 .G 56I c
d 4.
-
- 4
0.4-
4-
0,
C 0
o
e
0.3-
8 n
a
0.20.8
1.0
Bo/(Ao+ Bo1
o.\ .-
Figure 2. Calibration curves for binary mixtures of alcohols A = Faster reacting component B = Slower reacting component A = 1-PrOH; B = 2-PrOH A A = 1-BUOH; B 5 2-BuOH (catalyzed by 0.200M DABCO)
0+ c
Mixing Period
Figure 1 . sorbance
Ti me Rate curve for decrease of isocyanate abv5. time A I / A ~ = Aa/An = e (A1 and AS arbitrarily selected)
also known as triethylenediamine, to 100 ml. of dry toluene, dilute to 200 ml. with additional toluene, and mix thoroughly. The concentration of this solution is 0.200M. The amine content is determined by titrimetric analysis with perchloric acid in glacial acetic acid medium. The catalyst solution can be kept without any detrimental effect for three days a t the most. Infrared Spectrophotometer. Use a Beckman Model I R 5A or equivalent. The absorption cells are 0.10-nim. CaF2 cells. CaF2 cells are preferred rather than S a C l cells because of the elimination of possible complications due to the water content in polyols. Principle. Reilley, et al. (6) presented a generally applicable method for simultaneous reactions of two components A and B with a common reagent R , where the concentration of R with time was followed and where R may be detected with high sensitivity. If the concentration of A and B are each much greater than that of R , the rate expression for an irreversible second-order reaction becomes that of a pseudo first-order reaction. The rate of disappearance of R is given by:
where and k~ and ka are the second-order rate constants of A and B reacting with R . The latter part of Equation 1 is valid only when the values of [ A ] ,and [B],, the initial concentrations, are
equal to A and B , respectively, throughout the entire reaction period-i.e., when [R],
