I NOTES Analysis of Dimers of Aromatic Diisocyanates C. B. Puchalsky UniRoyal Chemical, Division of UniRoyaI, Inc., Naugatuck, Conn. 06770
The dimers of aromatic diisocyanates, such as toluene and methylene dianiline diisocyanate (TDID and MDID) contain the uretidinedione ring, as well as two isocyanate groups. The ring is a blocked source of isocyanate functionality that is available for polymer synthesis a t elevated temperatures. Its analytical reactions are similar to the isocyanate group (1, 2) but a total isocyanate method of assay will be high due to the presence of unreacted monomeric diisocyanate. Reacting separate portions of the sample with an excess of diallylamine differentiates the uretidinedione ring from isocyanate functionality due to dimer and monomer. Diallylamine has insufficient basicity to react with the ring but measures total isocyanate functionality. Dibutylamine heated with the sample (in dimethylformamide) for 2 hours reacts completely with isocyanate and the ring. The difference between the two tests provides the dimer assay. The standard deviation of the test is 0.9'70.
EXPERIMENTAL Reagents. The diallylamine reagent was made by bringing 13 nil of a center cut of distilled diallylamine to 1 liter with dioxane that has been dried by storage over molecular sieves. Hydrogen chloride in isopropanol was prepared by adding hydrogen chloride gas to isopropanol until the gain in weight is 4 grams per liter. Dibutylamine reagent was made by bringing 14 ml of Analytical Grade dibutylamine to 1 liter with dimethylformamide that has been dried over molecular sieves. The bromcresol green-methyl red mixed indicator contains 0.2 gram of each dissolved in 100 ml of ethanol. Procedure. A . Diallylamine reaction. Into clean, dry 8 - 0 2 . Boston-round bottles, weigh 0.3 millimole of the well-mixed dimer sample. Add a magnetic stirring bar and 10 ml by pipet of the diallylamine reagent. Stopper immediately with screw-top caps containing conical polyethlene seals. Prepare a blank as above omitting the sample. Stir blank and sample solutions on a magnetic stirrer for 15 minutes, swirling back into the solution any portion of the sample that is thrown onto the bottle wall. Then add 20 ml by pipet of the hydrogen chloride reagent to each bottle, reseal, and mix for 5 more minutes. Add several drops of indicator and titrate samples and blank to the first reproducible color change of salmon pink to gray, with 0.1N sodium hydroxide solution. Record the volumes for sample and blank. B. Dcbutylamine Reaction. Into clean, dry Boston-round bottles, weigh 0.5 millimole of the dimer sample. Pipet 50 ml of the dibutylamine reagent into these bottles and into 2 bottles containing no sample to be used as blanks. Stopper tightly with screw caps containing conical polyethylene seals. Heat the blank and sample solutions a t steamplate temperatures (100 to 105 "C) or shallowly immersed in boiling water, for sufficient time to complete the reaction. TDID requires thirty minutes, while MDID requires 2 hours. When heating is completed, allow the bottles to cool for about 10 minutes, then chill to ice water temperature. Titrate with 0.1N sulfuric acid while still cold, using about 8 drops of indicator. The change is a sharp darkening of green color. Record blank and sample titration volumes. (1) S. Siggiaand J. G . Hanna, Anal. Chem., 20, 1084 (1948). (2) J. A. Vinson. Anal. Chem., 41, 1661 (1969).
Table I . Reaction with Diallylamine Theoretical meqjgram TDlD 5.75 MDID 4.00
Experimental meqlgra? 5.75,5.76 3.89.3.93
Table 11. Reaction with Dibutylamine at about 100 "C Theoretical Experimental meq/grarn, after heating for: meq/ gram 30 rnin 1 hr 2 hr 3 hr TDlD 11.49 11.40 11.43 ... ... MDID 8.00 6.86 7.42 7.82 7.84
Table Ill. Analysis of Samples Sample Melt range, " C TDlD 146-148 TDlD 142-145 MDID, Tech. 202-275 MDID. Tech. 176-267
Dimer, Oh 100.2 99.0 85.3 78.3
Calculations:
A = M e q i G r a m of dibutylamine
=
N ) blank - (M1 X N ) sample Sample Weight B = Meq /Gram of diallylamine = (M1
(M1
% Dimer =
X
X
,_I
11 I \-I
N ) sample - (M1 x N ) blank Sample Weight
(A
- B)
X
(2)
Molecular weight (3)
20
RESULTS AND DISCUSSION TDID and MDID were separately analyzed by both titration techniques to discover the minimum time and temperature requirements for complete reaction. The diallylamine reaction was quantitative in 15 minutes at room temperature. See Table I. Complete reaction of these dimers with dibutylamine required heating and longer reaction times (3). See Table IT. Table I11 shows results for representative samples.
ACKNOWLEDGMENT The author wishes to thank A. Schwarcz and D. McCanty, formerly of the Research Center of UniRoyal, for valuable information concerning the use of diallylamine. Received for review October 4, 1972. Accepted December 18, 1972. (3) P . Singh and J. L. Bowin. Can. J. Chem., 40, 935 (1962).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 4, APRIL 1973
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