HOW TO GET THE INSIDE STORY ON POLYMER FORMATION EPR Spectroscopy
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11 E - P- R AT W O R K
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(Electron Paramagnetic Resonance)
In the formation and cross linking of polymer chains, free radicals are key participants. EPR can distinguish among them — monomer and first and second polymeric radicals — and can often follow the kinetics of each individually. EPR Spectroscopy gains these specific types of information: (1) free-radical kinetics in polymer formation (2) kinetics versus antioxidant control (3) definition of radiation damage sites and the quantitative measurement of them and (4) free-radical kinetics in cross linking. In each case, EPR "observes" without damaging the sample or in any way disturbing the rate or manner of reaction. EPR is applicable to reactions as fast as a few milliseconds or as slow as hours and days. Typical EPR sample is 0.15 cc in a quartz tube, frozen or at ambient temperature. Also, a continuous flow of reacting material may be observed by passing a tube through the sample cavity. Concentration of the free radical can be as low as 10- 8 molar. Example below is one of a continuing series.
of a s e r i e s F R E E R A D I C A L I N T E R M E D I A LE S INTERPRETATION: An important parameter in polymer chemistry is the action of inhibitors. Aromatic amines and phenolic compounds which inhibit oxidation have been presumed to act through the removal of hydrogen from the hydroxyl or amino group. Recent alternative proposals for the terminating reaction involving these inhibitors have been made, and EPR has been used to test these. The free radical intermediate in the oxidation of octadecene inhibited by an initial
IN T H E ACTION O F OXIDATION I N H I B I T O R S 0.01 M of phenyl-oc-naphthylamine was monitored by electron paramagnetic resonance with the results shown in the graph below. The typical spectrum shown to the right' was obtained by sampling the reaction, freezing at liquid nitrogen and monitoring the thawed sample at room temperature at a later time. These results support the two consecutive reaction hypothesis of Boozer, Hammond et a l ! . For a more complete discussion the reader is referred to the published paper 3 .
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