Side-group reactions

Side group reactions are common during polymer pyrolysis, and they should take place before the chain scission [8], The presence of water and carbon dioxide as main pyrolysis products in numerous pyrolytic processes can be explained by this type of reaction. The reaction can have either an elimination mechanism or it can have a substitution mechanism. Side eliminations are common for many linear polymers. However, because these reactions generate smaller molecules but do not affect the chain of the polymeric materials, the heating continues to affect the chain. Therefore the initial reactions may be continued with chain scission reactions, and the end result appears as combined types of reactions. 

An example of side chain reaction is given below for poly(vinyl chloride)  

Similar reactions are given by polyvinyl acetate with elimination of acetic acid, poly(vinyl ethers) with elimination of alcohols, etc. The most common case for these reactions is the p-elimination with two groups lost from adjacent atoms, usually taking place with an E mechanism, although E2 and E, mechanisms are not excluded. 

The polyene chain further decomposes under the influence of heat, generating aromatic hydrocarbons such as benzene, styrene, naphthalene, etc. This type of reaction can be the source of some polycyclic aromatic hydrocarbons found in traces during the pyrolysis of certain vinyl polymers. The elimination of a HX molecule from a vinyl type polymer is favored by the presence of a p-double bond in a compound of the form -CH2-CHX-CH=CH-. For this reason the side reaction for vinyl polymers is slower for the intact polymer and accelerates as the polymer tends to decompose. 

The elimination of a HX molecule is also typical for a polymer such as poly(ethylene-a/f-chlorotrifluoroethylene). In this case the elimination reaction from the side chain takes place as follows  

Eliminations and other reactions do not necessarily take place only on the polymeric chain or only on the side groups. Combined reactions may take place, either with a cyclic transition state or with free radical formation. The free radicals formed during polymeric chain scission or during the side chain reactions can certainly interact with any other part of the molecule. Particularly in the case of natural organic polymers, the products of pyrolysis and the reactions that occur can be of extreme diversity. A common result in the pyrolysis of polymers is, for example, the carbonization. The carbonization is the result of a sequence of reactions of different types. This type of process occurs frequently, mainly for natural polymers. An example of combined reactions is shown below for an idealized structure of pectin. Only three units of monosaccharide are shown for idealized pectin, two of galacturonic acid and one of methylated galacturonic acid  

Two pyrolysis products that are formed during pectin pyrolysis are furfural (2-furancarboxaldehyde, 2-furaldehyde) and 4-(hydroxymethyl)-1,4-butyrolactone. The proportion of the butyrolactone compared to that of furaldehyde in the pyrolysis products of pectin was found to correlate with the methylation degree of pectin [6]. The formation of 2-furaldehyde from the galacturonic unit probably takes place with the following mechanism (hydrogens are shown with shorter bonds)  

The intermediate compound assumed to be 2-hydroxy-4,8-dioxabicyclo[3.2.1]octane-2,6-dione is not a stable molecule and continues to decompose, without being possible to be isolated, with the formation of another unstable molecule that eliminates water  

The reaction may take place preferentially at the end of the chain. This reaction explains the elevated levels of 2-furaldehyde in the pyrolysis products of pectin. 

Volatile product evolution has been observed in the photolysis of many vinyl polymers as a consequence of side-group scission. The nature of these volatile products is therefore related to the side group. Carbon dioxide, carbon monoxide and methyl formate are produced from polymethylmethacrylate [12], carbon dioxide, carbon monoxide, methane and acetic acid in the photolysis of poly vinylacetate [13], and hydrogen chloride in the photolysis of polyvinylchloride [14]. 

---

The side group reactions with water elimination take place at lower temperatures of about 350° C, while chain scissions are predominant at higher temperatures. The Ei water elimination reaction can be written as follows (see Section 2.1) ... 

Polymer degradation reactions are frequently categorized based on the site in the macromolecule structure where the reaction occurs. This leads to the following classification of scission reactions a) polymeric chain scission, b) side group reactions, c) combined reactions [5, 3]. These reactions follow one of the mechanisms described previously, but this different classification allows a better correlation of the nature of the reaction products with the structure of the polymer and provides more understanding regarding the expected pyrolysis products. 

A number of recently reported synthetic methods include the formation of organic polymers with pendent cyclic or linear phosphazene side groups (reactions 7-9) and a process for the preparation of linear polymers in which phosphazene rings are linked together by organic oligomer chains using acyclic diene metathesis (ADMET) techniques (reaction 10). ... 

The chemical methods depend almost exclusively on two principles chain scission or neighboring side group reactions. The methods that work on... 

The chemical methods depend almost exclusively on two principles— chain scission or neighboring side-group reactions. The methods that work on the chain scission principle use the fact that one of the two components of a bipolymer (or other copolymers) will be attacked by a specific reaction process, while the chain of the other component is stable. The copolymer of isobutylene with about 2 % isoprene,... 

The mechanisms of pyridine formation are poorly characterized. Vemin and Parkanyi [12] have proposed three mechanisms. The first two mechanisms are dependent upon aldol condensations to yield unsaturated aldehydes with side groups. Reaction then with ammonia or an amino add and ring closure would yield a nitrogen-containing heterocyclic. The oxidation of this heterocyclic would result in the formation of a pyridine. The third pathway involves the reaction of dialdehydes with ammonia followed by dehydration to produce pyridines. 

---