Tandem gas chromatogram combines separation and analysis chromatograms

usually operates at 650° or 700° C , and peaks pass through the pyrolyzer in 10 to 20 seconds.

Using the tandem gas chromato-graph as an analytical system requires the pyrolysis to be reproducible and the second chromatograph to be fast enough that the pyrograms of the com­pounds separated by the first chroma­tograph do not overrun each other. Many workers have used a gas chro­matograph to analyze the fragments emerging from a pyrolysis oven and have found the pyrolysis reproducible.

Dr. Wolf and Mr. Walker have cut the analysis time of the second chro­matograph to four or five minutes for each compound by programing the column flow rate. In many cases, how­ever, it is necessary to use stop-start (interrupted) elution, in which the analysis of the first chromatograph is stopped when necessary by depres-surizing the helium carrier gas in the column while the analysis of the py­rolysis product continues. The col­umn can be shut down for as long as three hours without affecting the sepa­ration, the McDonnell-Douglas work­ers say. They are working on an auto­mated stop-start procedure.

The information obtained from the tandem gas chromatograph is similar to that from the gas chromatograph-mass spectrometer combination. The second chromatography pyrogram is analogous to, but less complex than, the mass spectrometer's fragmentation patterns. Following mass spectrom­etry practice, the peak areas of the analysis chromatographs (the pyro­grams) are normalized with respect to the largest single peak in the spectra. Since the molecular fragments are stable, unlike the mass spectrometer's ion fragments, they can be collected and analyzed by other techniques, if desired.

When Dr. Wolf and Mr. Walker analyzed n-decane in the tandem gas chromatograph, they found that 5 to 10% of the compound is degraded by the pyrolysis. The analysis chromat­ograph shows 10 peaks, most of them alpha-olefins, with a nearly equal dis­tribution of fragments up to the C9 olefin. Pyrolysis of an aliphatic C1 0 alcohol and the methyl ester of a C9 acid yields 11 peaks, but the ratios be­tween the peaks are different. A mix­ture of a C9 acid methyl ester and a C10 alcohol may not be separated by the first chromatograph, but the pres­ence of the two compounds can be de­duced from the pyrogram. Dr. Wolf and Mr. Walker have used the tandem gas chromatograph on paraffins, ole­fins, alcohols, and methyl esters, and are now studying amino acids. The system can analyze compounds with as many as 20 carbon atoms.

Pendent hydrogen atoms degrade polymer «ι Γ Λ ACS NATIONAL MEETING Ι Ο τ Τ Η Polymer Chemistry

Although ordinarily quite unreactive, aromatic hydrogen atoms can be sur­prisingly labile and may participate in polymer degradation even in inert atmospheres. If oxygen is present, C-H bonds are even more susceptible to attack. In designing thermally stable and oxidation-resistant poly­mers, it follows that it would be de­sirable to minimize the number of hydrogen atoms and other pendent groups that do not directly contribute to the strength of the polymer chain.

To test this idea, Dr. Stephen S. Hirsch of Chemstrand Research Cen­

ter prepared an aromatic polymer totally devoid of pendent groups and compared its thermo-oxidative stability with a corresponding material contain­ing pendent hydrogen. It was desired to eliminate the available valency en­tirely, and not merely replaco pendent hydrogen with another group. For example, = C H groups can be replaced with nitrogen atoms, and methylene groups with either oxygen or sulfur. Such modifications lead to a hetero­cyclic series being partially organic and partially inorganic and having no bonds other than those necessary to propagate the chain. The result is a polymer with no "handles" of any kind that enter into degradative reactions at low temperatures.

The composition chosen for investi­gation was the product resulting from the reaction of pyrazinetetracarboxylic dianhydride(PTDA) with 2,5-di-amino-l,3,4-thiadiazole(DATD). The new polyimide has been designated PTDA-DATD PL

A polyimide was chosen because it represents a class of materials already possessing excellent thermal proper­ties. In addition, aromatic dianhy-drides, with four points of attachment to the center ring, have few remaining positions to be deprived of hydrogen, and synthesis is not too hard.

The standard material for compari­son was the corresponding polymer from pyromellitic dianhydride con­taining two pendent hydrogen atoms per repeating group in the chain. Tests on other materials comparable with PTDA-DATD PI were not re­ported.

Films of the standard material and PTDA-DATD PI were maintained in air at 400° C. for 25 hours and in­spected periodically. PTDA-DATD PI appeared to undergo no change other than a slow decrease in size while the standard suffered charring, bubbling, wrinkling, embrittlement, and cracking.

To determine whether the high-tem­perature-air resistance was equivalent to that in nitrogen, thermogravimetric analyses (DTA) and zero strength measurements were made on PTDA-DATD PI in both media. Up to 600° C , the TGA curves were nearly superimposable. The zero strength curves showed no difference due to air oxidation. In contrast, the hydro­gen-containing analog charred exten­sively above 320° C. in either air or

48 C&EN SEPT. 25, 1967

Chlorinated Solvents Sales, The Dow Chemical Company, Midland, Michigan 48640.

By tomorrow, somebody'll be spending less than you do for propellant. Unless, of course, you're the first to use new Aerothene™ chlorinated solvents. As vapor pressure depressants. Or (when used with a compressed gas) to replace f luor inated hydrocarbon propel lant systems. Either way, they work better. And cost less. Who saves first depends on who calls first.

that extra molecule of CARE

Jon Bob Thompson supervises a forty-three minute car wash No three-minute wash jobs here. Not when you're preparing a 20,000-gallon epoxy-lined tank car for a shipment of polyester grade ethylene glycol. First a thorough steam and detergent bath, then every inch of this giant cylinder is carefully wiped dry with a chamois (no rags are used . . . they might leave lint deposits). No possible contaminant is left to chance. That's why Jon Bob, Quality Control Supervisor, asks J. D. Harris to re-chamois a suspicious area.

We couldn't make money running a car laundry this way, but we sure save a lot of $25,000 cargoes and, more importantly, the goodwill of our custo­mers. Quality Control gets a great deal more than lip service at Jefferson. Less than one quality com­plaint per million dollars of sales is pretty good proof . . . wouldn't you agree. Jefferson Chemical Company, Inc . , P . O. Box 53300, Houston, Texas 77052.

Jefferson 071

New reaction scrambles halves of olefins

(50%) (25%) (25%)

Tungsten catalyst probably forms tetragonal pyramid with olefins in transition state

GOODYEAR CHEMISTS. John Ward, Dr. Eilert Ofstead, Dr. Nissim Calderon, Dr. Kenneth Scott, and Dr. Hung Yu Chen (left to right), of Goodyear, track salient features of proposed transition state for olefin metathesis reaction

nitrogen, attesting to the influence of the labile hydrogen atoms.

Although the new polymer exhibits good thermal and oxidative stability, it possesses only moderate strength and molecular weight. Further de­velopment at Chemstrand could yield a new class of materials having higher strengths combined with the superior stability of the prototype.

Tungsten catalyzes olefin metathesis

ACS NATIONAL MEETING TH Organic Chemistry

A new reaction--olefin metathesis--has been disclosed by Dr. Nissim Calderon and his associates Dr. Hung Yu Chen and Dr. Kenneth W. Scott of Good­year Tire & Rubber Co. They have found that a tungsten catalyst breaks carbon-carbon double bonds in in­ternal olefins and randomly rejoins the fragments. For example, 2-pentene becomes a statistical mixture of 2-bu­tene (25%), 2-pentene (50%), and 3-hexene (25%). Similarly, a mixture of 2-butene and 2-butene-d8 produces only one more olefin--2-butene-d4.

The latter example suggests that alkylidene groups, RCH=, rather than alkyl groups, R- are exchanging. Therefore, carbon-carbon double bonds, not single bonds, are breaking and reforming. This is remarkable, Dr. Calderon comments, and is a novel route in olefin chemistry.

Perhaps even more remarkable, the reaction takes place at room tempera­ture and reaches equilibrium in seconds. Catalyst components are tungsten hexachloride, ethanol, and ethylaluminum dichloride. The re­action is versatile, Dr. Calderon says, and does not fit into any of the pre­viously known classes of general olefin reactions. Until now, catalytic skeletal transformations of olefins required high-temperature cracking conditions.

Dr. Calderon investigated the stereochemistry of the new reaction with Dr. Eilert A. Ofstead and John P. Ward. They find that cis and trans products form in thermodynamically predictable ratios. For example, 2-butene provides a mixture containing 28% ds-2-butene and 72% trans-2-butène regardless of the cis-trans com­position of the starting material.

Dr. Calderon speculates that the transition state for the reaction prob­ably contains the four ethylenic car­bon atoms of the two reacting olefins arranged to form the square base of a tetragonal pyramid, with the tung­sten atom at the pinnacle. The olefinic bonds are probably delocalized, as in

the analogous structure of norborna-dienyl-tungsten-tetracarbonyl.

Furthermore, Dr. Calderon and Dr. Ofstead suggest, the olefins probably do not leave the coordination complex in pairs. Rather, one of the product olefins remains coordinated, then re­acts further with an incoming third olefin. Leaving in pairs would re­strict product variety, they reason, particularly in the early stages of the reaction.

They illustrate the point by citing the sequence of events in the early stages of the metathesis of pure cis-2-pentene. Two molecules of cis-2-

pentene would react to form either two new cis olefins or two new trans olefins. If both new olefins deco-ordinated at the same time, the se­quence would produce equal amounts of cis-butene and cis-hexene, and equal amounts of trans-butene and irans-hexene.

Experiments show that these pairs of compounds do not form initially in equal amounts. Retention of one of the olefin products would allow a greater variety of configurations in the transition state, hence greater variety in early product ratios, as is observed.

SEPT. 25, 1967 C&EN 51

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