ALKANES AS INITIATORS FOR THE FREE-RADICA L POLYMERIZATION

Alkanes with highly substitued carbon-carbon bonds can be cleaved in f ree radicals by homolytic dissociation under the influence of thermal energy. The energy of dissociation is mainly determined by the type an d size of the respective substituents. For tetraarylbutane dinitriles the temperatures of decomposition are in the range of other technicall y used initiators. But nevertheless, these compounds are up to now rat her seldom used to initiate free radical polymerizations. This is conn ected with the special mechanism of initiation: the formed highly subs tituted alkyl radicals are relatively stable and show only a rather lo w reactivity against the usually used monomers. Therefore, in the begi nning of the polymerization the radical concentration is rather high w hich leads not only to addition to monomers but also to a pronounced p rimary radical termination. In some cases this termination process is reversible which results in an unusual polymerization kinetics. Depend ing on the type of monomer and the degree of reversibility of the prim ary radical termination, either a period of ''dead-end'' polymerizatio n follows or the cleavage of the oligomers at the chain end results in a re-initiation process with a ''normal'' polymerization with increas ed rate. The reaction mechanism, kinetics and some possible applicatio ns of such initiating alkanes are reported.