Steric Interaction and the Heat of Polymerization

Steric Interaction and the Heat of Polymerization.—The heats of formation at 25°C of the gaseous normal paraffin hydrocarbons H—(CH2)n—H, with the exception of methane, are adequately represented by the formula  

An estimate of the energy of steric repulsion may be obtained by comparing the observed heat of polymerization with the value calculated on the assumption that the interaction energy between substituents is zero. For the latter purpose, we consider the following steps 

The heats of hydrogenation for a number of unsaturated compounds are available from the work of Kistiakowsky and collaborators, and in a few cases from accurate heats of formation of the monomer and the reference compound can be estimated 

Monomer Structural unit AHe calc d -AHh observed -AHp calc d 

If X is an alkyl group and Y = H, the reference compound XIII is a straight chain hydrocarbon, and the second process above consists in its transformation to a branched unit (XIV) having a single substituent. Hence 

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