The heats of polymerization for a number of monomers are shown in the Appendix. Since ΔHp= ΔE − P ΔV, if the volume change can be neglected, the heat of polymerization becomes equivalent to the change in internal energy of the molecule. The internal energy may be reduced in a number of ways, for example, by a release of steric strain or by a loss of π-electron energy.