Degradation by depolymerization

End-chain scission the polymer is broken up from the end groups successively yielding the corresponding monomers. When this polymer degrades by depolymerization, the molecules undergo scission to produce unsaturated small molecules (monomers) and another terminal free radicals. (Polymethylmethacrylate, polytetrafluorethylene, polymethacrylonitrile, polyethylstyrene, polystyrene, polyisobutene)... 

Because very rapid depolymerization occurred at higher temperatures, it was necessary to control the temperature within the narrow range of 50 10°C. Even so, the of the polymer was no greater than 15,000 because of rapid degradation by the living cationic end group. 

Methacrylic polymers in general can be easily identified by Py-GC/MS because at temperatures higher than 400°C they degrade by unzipping, a mechanism of depolymerization in which the polymer essentially reverts to monomers [65]. Thus the main pyrolysis product of Elvacite 2044 is BMA. 

Fig. 38.—13C-N.m.r. Spectrum of A, Partly Depolymerized O-Methylcellulose (d.s. 2.8) in CDC13 at 30° (R, signal due to reducing-end residue Me, O-methyl inset lines represent chemical shifts of corresponding carbon atoms in methyl hepta-O-methyl-jS-cellobioside) and of B, O-Methylcellulose (d.s. 0.7), Partially Degraded by Cellulase, in D20 at 30°. (S represents a 13C nucleus bonded to an OMe group inset lines give the chemical shifts of corresponding carbon atoms in methyl /3-cellobioside.)...

PBS (Figure 30) is an alternating copolymer of sulfur dioxide and 1-butene. It undergoes efficient main chain scission upon exposure to electron beam radiation to produce, as major scission products, sulfur dioxide and the olefin monomer. Exposure results first in scission of the main chain carbon-sulfur bond, followed by depolymerization of the radical (and cationic) fragments to an extent that is temperature dependent and results in evolution of the volatile monomers species. The mechanism of the radiochemical degradation of polyolefin sulfones has been the subject of detailed studies by O Donnell et. al. (.41). 

Kuran W, Gorecki P (1983) Degradation and depolymerization of poly(propylene carbonate) by dlethylzlnc. Makromol Chem 184 907-912... 

There is evidence that both ionic and free radical species are involved in the degradation and depolymerization of poly (olefin sulfone) s by high energy radiation (70). Thus, the yields of olefins from poly (1-butene sulfone) at 30 °C (the sample was heated to 70 °C during removal of the gaseous products) are shown in Table II. The butene is not solely 1-butene, but comprises significant proportions of all three isomers, 1-butene, 2-butene and isobutene. 

Unsaturated polyesters can undergo degradation by oxidation of their aliphatic segments, decarboxylation of esters and partial depolymerization of polystyrene chains. 

Figure 9.2. Lactide ring formation by depolymerization of low-molecular weight PLA. Reprinted with permission from J. Lunt, Polymer Degradation and Stability, Vol. 59, p. 145,1998, 1998, Elsevier Science Ltd.

There are several critical issues to consider in this research area. The kenaf fiber must not be degraded by the chemical modification procedures. To maintain the strength of the kenaf fiber, depolymerization or degradation of the cellulose must be avoided. 

These apparent contradictions can be rationalized in terms of a model which incorporates plasma-induced polymerization along with depolymerization. PBS has long been known to exhibit a marked temperature-dependent etch rate in a variety of plasmas. This is clearly seen in the previously published Arrhenius plots (3,7) for two different plasma conditions (Figure 1). This dependence is characteristic of an etch rate that is dominated by an activated material loss as would occur with polymer depolymerization. The latter also greatly accelerates the rate of material loss from the film. Bowmer et al. (10-13) have shown in fact that poly(butene-l sulfone) is thermally unstable and degrades by a depolymerization pathway. A similar mechanism had been proposed by Bowden and Thompson (1) to explain dry-development (also called vapor-development) under electron-beam irradiation. 

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