Knowledge of vacuum pyrolysis mechanisms in biomaterials is most advanced for some classes of relatively simple compounds, such as amino acids (ref. 59). However, relatively little is known about these mechanisms for most biopolymers, with the exception of the thermal degradation of polysaccharides such as cellulose (ref. 90) or glycogen (ref. 53). Whereas some synthetic polymers such as polystyrene or polytetrafluoroethylene produce extremely simple pyrolysis mass spectra, provided that soft ionisation techniques are used (see Figure 1), the pyrolysis mass spectra of biopolymers are usually much more complex.
To some extent, this is caused by a general difference in pyrolysis mechanisms. In polystyrene and polytetrafluoroethylene the major degradation reaction is a straightforward depolymerisation by 3-bond scission (initiation followed by unzipping) (ref. 9). This yields styrene (MW 104) and tetrafluoroethylene (MW 100), respectively, and thus the low voltage electron impact spectra show almost exclusively the molecular ion of the monomers and, at a much lower intensity, the molecular ions of the dimers and trimers provided that the higher mass range is scanned.
None of the more common biopolymers possesses a structure wherein each of the monomeric building blocks contributes a two-carbon segment to the polymer backbone which could lead to simple depolymerisation by g-scission. Natural rubber, a poly-isoprene, is perhaps the only biopolymer to yield marked amounts of monomer under analytical pyrolysis conditions (see Section 3.5). Instead, most biopolymers decompose by a variety of mechanisms often characterised by the elimination of stable neutral molecules, such as H20, HCN, CHgO, CHgOH, H2S, CO, C02> C2H4 and Hg, accompanied by the break-up of the polymer chain into larger fragments. Fortunately, these fragments often retain characteristics of the original monomeric building blocks.
It should be noted that since the initial pyrolytic reactions are heterolytic, the course of the thermal degradation and the relative yields of the various pyrolysis products can be significantly influenced by the presence of acidic or alkaline catalysts or of salts (ref. 91).
In the following paragraphs the present state of knowledge of vacuum pyrolysis mechanisms for the major groups of biopolymers will be briefly discussed.
CH2 II 1 CH
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