At constant temperature, the reversibility of polymerization can be determined using the Gibbs free energy equation: where
In the Gibbs free energy equation, the entropy term is negative.
[3] The thermodynamic explanation is due to Frederick Dainton and K. J. Ivin, who proposed that the chain propagation step of the polymerization is reversible.
[4][5] At the ceiling temperature, there will always be excess monomers in the polymer due to the equilibrium between polymerization and depolymerization.
Steric hindrance is significant in polymers derived from α-methylstyrene because the phenyl and methyl groups are bonded to the same carbon.
These steric effects in combination with stability of the tertiary benzylic α-methylstyryl radical give α-methylstyrene its relatively low ceiling temperature.
When a polymer has a very high ceiling temperature, it degrades via bond cleavage reactions instead of depolymerization.
A similar effect explains the relatively low ceiling temperature for polyisobutylene.