The electronic environment about the main chain atoms in polystyrenes has been varied systematically by introducing electronegative or electropositive substituents (H3C, CH3O, Br, Cl, NC, and O2N groups) in the p‐position of the ring. Energy yields for γ‐initiated H2 formation and crosslinking, both a measure of the radiation sensitivity of the backbone, were determined above and below Tg from (1) mass spectrometric examination of the decomposition gases and (2) molecular weight changes. For most of the polymers, it is concluded that the backbones of p‐substituted polystyrenes are more radiation stable than that of polystyrene, and that main chain macroradicals formed early in the reaction are responsible for the major changes observed, i.e., hydrogen evolution and crosslinking. The role of the p‐substituent on the latter effects is interpreted qualitatively in terms of the contribution by the substituent to the resonance stability of these intermediate polymer radicals. In the case of poly‐p‐bromo‐ and poly‐p‐chlorostyrene, the inordinately high sensitivity to crosslinking and the absence of halogen atoms in the decomposition gases suggest the importance of a chain reaction in the radiation chemistry of these molecules.

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Recent reports on air pollution* 1 ' 2 " 3 *, particularly those concerned with the Los Angeles smog, have indicated that hydrocarbons might be important smog-forming compounds, and that automobile exhaust could be an important source of hydrocarbons in the Los Angeles area. Surveys have been made to determine the composition of automobile exhaust, and tests have been run attempting to simulate smog under controlled conditions by combining automobile exhaust with either ozone or oxides of nitrogen. Plants which are known to be susceptible to smog damage have then been exposed to these artificial "smogs," and attempts have been made to correlate plant damage with hydrocarbons in automobile exhaust (4). A broad program on automobile exhaust and air pollution has been active in our company for some time. We have conducted surveys of our products to determine composition of exhaust gas, we have developed several techniques for exhaust gas analysis, and we have tried several methods for reducing hydrocarbons in automobile exhaust. This paper is essentially a progress report describing some of the results we have obtained to date. Analysis Techniques All the data reported in this paper were obtained using a Consolidated Mass Spectrometer Model 21-103 B (Fig. 1). During the course of our mass spectrometric analysis of exhaust gas it became apparent that a significant amount of apparently unchanged gasoline is found in the exhaust. The relative proportion of unchanged gasoline to "cracked" or partly burned materials varies with the engine operating conditions, being highest on deceleration and lowest on cruising. This proportion, however, remains relatively constant for any given type of engine operation. The gasoline actually used in each test was scanned in

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