Reaction types and mechanisms in pyrolysis

Reactions of several types may take place in pyrolysis (see e.g. [3]), the most common ones being eliminations and rearrangements. Other reactions such as oxidations, reductions, substitutions, or additions aiso are possible. 

In an elimination reaction, fragments of a molecule are removed and form a new molecule. Elimination involving a free radical mechanism operates in many pyrolytic reactions. First, an initiation occurs by pyrolytic cleavage, followed by 

As further shown, polymer chain scission or side scission of many linear polymers takes place through this mechanism. At higher temperatures (600-900° C) this type of reaction is also common for small molecules, which explains in some cases the formation of unsaturated or aromatic hydrocarbons from larger aliphatic ones. 

The free radical formation and propagation process usually occur with the dissociation of the weaker bonds. Besides the weaker bonds, other bonds also can be dissociated, which happens mainly when there are small differences between the bond dissociation energies. The strength of the bond being broken is not always known, and in some cases can be derived from tabulated heats of formation as shown in Section 2.2. 

The -elimination with two groups lost from adjacent atoms is another common reaction in pyrolysis, usually taking place with an Ei mechanism and not involving free radicals. An a-atom is the atom bound to a specific group or bond, and any atom adjacent to it is indicated as a p-atom. p-Eliminations or 1,2-eliminations involve, for example, the elimination of a group from a-atom and a hydrogen from the p-atom. For polymers where the pyrolysis takes place in condensed phase, E2 and Ei mechanisms are not excluded. The Ej mechanism involves a cyclic transition state, which may be four-, five- or six-membered [4]. No discrete intermediate is known in this mechanism (concerted mechanism). Two examples of reactions with E mechanism involving different sizes of cyclic transition state are shown below [3]  

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