Zipper mechanism, degradation

Computer programs have been used to correlate the degradation kinetics of poly(vinyl halides) assuming a kinetic model that is based on the zipper mechanism. Best fit values of the parameters of the kinetic model allow reproduction of a degradation with an error that is usually less than 0.6% per point. 

The two kinetic models that effectively correlate data for the degradation of vinyl polymers were developed on the basis of the zipper mechanism and differ only in the approximations used to account for the premature termination of zip chains. Although the equations are complex in appearance (3), they are based on the relatively simple assumptions of the zipper mechanism. Chains are initiated as a certain fraction of chains per second, k., and unzip at a certain fraction of a started chain per second, R.. 

Before the implications of the zipper mechanism in terms of reproducible data were fully appreciated, efforts to attain reproducibility commensurate with the quality of the apparatus and the method of operation led to hundreds of degradation runs. Some of these runs... 

The "zipper" mechanism is confirmed by the work of Baum and Wartman (27) who found that ozonolysis followed by hydrolysis of partially degraded PVC did not appreciably reduce its molecular weight. If the HCl loss had occurred in a random fashion within the polymer chain, a considerable drop in molecular weight would have been expected. In addition, when the resin was mildly chlorinated... 

Vinylidene terminal groups can be formed by disproportioning they cause degradation by a zipper mechanism that reaches its maximum degradation rate at 280 °C [620]. Small amounts of statistically incorporated methyl acrylate disrupt this mechanism and increase thermal stability. 

At elevated temperatures, statistical chain cieavage is the dominant reaction its radicais initiate secondary reactions. Intramoiecuiar radical-hydrogen transfer that leads to depolymerization ( zipper mechanism ) releases monomers, dimers, and trimers. Figure 5.172. Intermolecular hydrogen transfer causes degradation in another polymer chain. Figure 5.173. 

When pure polymethyl methacrylate is heated to temperatures above 180 °C, it depolymerizes in a zipper-like mechanism. Its degradation rate is temperature... 

Polyoxymethylene is processed at mass temperatures of 180 to 230 °C, at which it may aiready degrade, Fig. 4.58. The degradation of poiyoxymethyiene ieads to formaldehyde cleavage and proceeds in a zipper-like mechanism, s. Fig. 5.197. This effect is most pronounced in the pure homopolymer. To avoid degradation, care must be taken to keep thermal loading as short as possible. 

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