S. LARACH, W. H. MCCARROLL AND R. E. SHRADER
604
irradiation long after the vinylidene groups have reacted. It seems likely, however, that the initial vinyl and vinylidene do contribute to crosslinking in the early stage of irradiation as is evidenced by the work of CharlesbyZ2who showed that in long chain hydrocarbons the energy requirement per crosslink drops from 32 e.v. in saturated, unbranched paraffins to 19 e.v. where terminal (ie., vinyl) unsaturation is present. The rapid disappearance of vinylidene which, as Dole suggests, does appear to be quite specific, might be explained by transfer of energy (not necessarily free radical migration) toward the vinylidene group or by a high reactivity of this group toward free radical attack. Finally, the general observation that normal, saturated paraffins (e.g., octacosane and polymethylene) crosslink indicates. that initial unsaturation is not a prerequisite for crosslinking. For the crosslinking of polymethylene-type hydrocarbons we suggest the following general mechanism :
*+
+ H.
Vol. 60
about 0.1.28 I n radiation chemistry (2) might be especially favored if in reaction (1) the H. atom is expelled with excess kinetic energy (i.e., a (‘hot” atom). -CHZCHZ- MI+H)C=C
