Depolymerization

Depolymerization (or depolymerisation) is the process of converting a polymer into a monomer or a mixture of monomers.^[1] This process is driven by an increase in entropy.

Depolymerization of polystyrene via radical elimination mechanism

Ceiling temperature

The tendency of polymers to depolymerize is indicated by their ceiling temperature. At this temperature, the enthalpy of polymerization matches the entropy gained by converting a large molecule into monomers. Above the ceiling temperature, the rate of depolymerization is greater than the rate of polymerization, which inhibits the formation of the given polymer.^[2]

Ceiling Temperatures of Common Organic Polymers

Polymer	Ceiling Temperature (°C)[3]	Monomer
polyethylene	610	CH2=CH2
polyisobutylene	175	CH2=CMe2
polyisoprene (natural rubber)	466	CH2=C(Me)CH=CH2
poly(methyl methacrylate)	198	CH2=C(Me)CO2Me
polystyrene	395	PhCH=CH2
Polytetrafluoroethylene	1100	CF2=CF2

Applications

Depolymerization is a very common process. Digestion of food involves depolymerization of macromolecules, such as proteins. It is relevant to polymer recycling. Sometimes the depolymerization is well behaved, and clean monomers can be reclaimed and reused for making new plastic. In other cases, such as polyethylene, depolymerization gives a mixture of products. These products are, for polyethylene, ethylene, propylene, isobutylene, 1-hexene and heptane. Out of these, only ethylene can be used for polyethylene production, so other gases must be turned into ethylene, sold, or otherwise be destroyed or be disposed of by turning them into other products.^[4]

Depolymerization is also related to production of chemicals and fuels from biomass. In this case, reagents are typically required. A simple case is the hydrolysis of celluloses to glucose by the action of water. Generally this process requires an acid catalyst:

H(C₆H₁₀O₅)_nOH + (n − 1) H₂O → n C₆H₁₂O₆

See also

• Thermal depolymerization
• Polymer degradation
• Polymerisation
• Ceiling temperature
• Chain scission