Ceiling Temperature
Definition and meaning of Ceiling Temperature in chemistry.
Ceiling temperature (Tc) is the temperature above which polymerization of a given monomer becomes thermodynamically unfavorable, so depolymerization dominates and no net polymer forms.
In more detail
Polymerization is typically exothermic (ΔH negative) but entropically unfavorable (ΔS negative), since ordering monomers into a chain reduces disorder. Using ΔG = ΔH − TΔS, rising temperature eventually makes ΔG positive, at which point depropagation outpaces propagation and the equilibrium shifts back toward free monomer. Because polymerization is reversible, Tc depends on monomer concentration: Tc = ΔH°/(ΔS° + R ln[M]), so a more dilute monomer solution has a lower ceiling temperature. This concept explains why some monomers simply cannot be polymerized to high molecular weight above a certain temperature, regardless of catalyst or initiator used.
Key facts
| Field | Physical Chemistry |
|---|---|
| Governing equation | Tc = ΔH°/(ΔS° + R ln[M]) |
| Also known as | Tc, ceiling point |
| Classic example | Poly(alpha-methylstyrene), Tc ≈ 61°C at 1 M |
Alpha-methylstyrene has a ceiling temperature of about 61°C at 1 M bulk monomer concentration; above this temperature its polymer spontaneously depolymerizes back to monomer, which is why it is difficult to homopolymerize under ordinary conditions.
Frequently asked questions
Does every monomer have a ceiling temperature?
In principle yes, since all addition polymerizations are reversible equilibria, but for many common monomers like ethylene the ceiling temperature is so high (well above normal processing temperatures) that it is rarely a practical concern.
What happens to a polymer above its ceiling temperature?
Above Tc, the depropagation rate exceeds the propagation rate, so any polymer present is thermodynamically driven to unzip back into monomer until equilibrium is reestablished.