The carbon-oxygen double bond presents certain simularities to the carbon-carbon double bond, and, on paper, it would seem that its polymerization should be possible. However, there are significant differences that make polymerization unsuccessful except in a few cases.
First of all, attempted radical polymerization fails because the possible propagating species are too unstable.
However, the carbonyl group behaves as if it had separated charges, as shown in the resonance structure:
Ionic polymerization looks as if it should work, and it does.
The carbon-oxygen double bond is much more stable than the carbon-carbon double bond. Therefore C=O polymerizations are less exothermic than C=C polymerizations. This means that they have less driving force in the Gibbs Free energy relationship, and the ceiling temperatures are low. The unsubstituted carbonyl (i.e., O=CH2, formaldehyde) has a moderately high ceiling temperature, but any substitution leads to steric crowding in the polymer, raising its energy and drastically reducing the ceiling temperature.
| Polymer | Ceiling Temperature (°C) |
 |
119 |
 |
-39 |
 |
11 |
For polymers with low ceiling temperatures, there is a wierd polymerization effect. You can mix the monomer and initiator at room temperature, and nothing happens. However, if you cool down the mixture, reaction ensues and polymer forms. Of course, the polymerization reverses on rewarming.
Polyformaldehyde (also called polyoxymethylene) is a commercially important engineering thermoplastic, sold as Delrin (Dupont) or Celcon (Hoechst-Celanses). It is highly crystalline, and very tough, so it can be used for bearings or other parts subjected to physical stress. However, its celing temperature is rather low, so it must be stabilized before use. This is accomplished by end-capping the polymer with an unreactive group, as in this example that puts an ester at the chain end (used for Delrin).
The ester is stable even well above the ceiling temperature, so it blocks the depolymerization reaction. Yes, the polymer is thermodynamically unstable above Tc, but end-capping makes the polymer kinetically stable because the mechanism for depolymerization is not availale. The activation energy for depropagation is simply too high.
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