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Ceiling temperature ($T_c$) is a measure of the tendency of polymers to revert to their monomers. When a polymer is at its ceiling temperature, the rate of polymerization and depolymerization of the polymer are equal. Generally, the ceiling temperature of a given polymer is correlated to the steric hindrance of the polymer’s monomers. Polymers with high ceiling temperatures are often commercially useful.

Thermodynamics of polymerization

At constant temperature, the reversibility of polymerization can be determined using the Gibbs free energy equation:

$\mathrm{\Delta }{G}_p=\mathrm{\Delta }{H}_p-T\mathrm{\Delta }{S}_p$

where $\mathrm{\Delta }{S}_p$ is the change of entropy during polymerization. The change of enthalpy during polymerization, $\mathrm{\Delta }{H}_p$, is also known as the heat of polymerization, which is defined by

$\mathrm{\Delta }{H}_p=E_p-E_{dp}$

where $E_p$ and $E_{dp}$ denote the activation energies for polymerization and depolymerization, respectively.

Entropy is the measure of randomness or chaos. A system has a lower entropy when there are few objects in the system and has a higher entropy when there are many objects in the system. Because the process of depolymerization involves a polymer being broken down into its monomers, depolymerization increases entropy. In the Gibbs free energy equation, the entropy term is negative. Enthalpy drives polymerizations. At low temperatures, the enthalpy term is greater than the $T\mathrm{\Delta }{S}_p$ term, which allows polymerization to occur. At the ceiling temperature, the enthalpy term and the entropy term are equal. Above the ceiling temperature, the rate of depolymerization is greater than the rate of polymerization, which inhibits the formation of the given polymer. The ceiling temperature can be defined by

$T_c=\frac{\mathrm{\Delta }{H}_p}{\mathrm{\Delta }{S}_p}$

Monomer-polymer equilibrium

At the ceiling temperature, there will always be excess monomers in the polymer due to the equilibrium between polymerization and depolymerization. Polymers derived from simple vinyl monomers have such high ceiling temperatures that only a minuscule amount of monomers remain in the polymer at ordinary temperatures. The situation for α-methylstyrene, PhC(Me)=CH₂, is an exception to this trend. Its ceiling temperature is around 66 °C. 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 poly(isobutylene).

Ceiling temperatures of common monomers

Monomer	Ceiling Temperature (°C)	Structure
1,3-butadiene	585	CH2=CHCH=CH2
ethylene	610	CH2=CH2
isobutylene	175	CH2=CMe2
isoprene	466	CH2=C(Me)CH=CH2
methyl methacrylate	198	CH2=C(Me)CO2Me
α-methylstyrene	66	PhC(Me)=CH2
styrene	395	PhCH=CH2
tetrafluoroethylene	1100	CF2=CF2

References

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