Rice-Kossiakoff cracking mechanism

The liquid products of the pyrolysis of PP contain primarily olefins that resemble the molecular skeleton of PP (i.e. branched hydrocarbons). A distinguishing feature of PP pyrolysis is the predominant formation of a particular C9 olefin in the pyrolysis product. The level of this C9 compound identified as 2,4-dimethylhept-l-ene can be as high as 25%. Also present are C5 olefin, Cs olefin, several C15 olefins and some C21 olefins [2]. The tertiary carbon sites in PP allows for the facile chain cleavage and rearrangements according to the Rice-Kossiakoff cracking mechanism shown in Figure 15.2. The noncondensable gas from PP pyrolysis contains elevated levels of propylene, isobutylene and n-pentane. 

Figure 15.2 Rice-Kossiakoff cracking mechanism for polypropylene showing that the pyrolysis products of PP retain a branched structure... 

The product yields by carbon number are plotted in Figure 6. At all except the most severe conditions studied,the major component in the gas phase was n-butane. This observation is consistent with the a-ring opening and dealkylation mechanism proposed for tetra-and octa-hydrophenanthrene cracking. At the most severe conditions ethane was present in the greatest quantities. This can be explained by side chain cracking of n-butylbenzene according to the Rice-Kossiakoff mechanism or by secondary reactions of the n-butane. 

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