Unzippering reaction

In other instances the reactions appear to occur in sequence down the chain, for example in the depolymerisation reaction of polyformaldehyde (polyacetal) and polymethyl methacrylate which are referred to as zippering or sometimes unzippering reactions. In other cases cyclisation reactions can occur such as on heating polyacrylonitrile ... 

The composition of thermal degradation products was previously shown to be greatly affected by the processing temperature (34)-PVC is inherently unstable because of the presence of allylic chlorine atoms throughout the polymer. These chlorine atoms are easily removed by exposure to minimal heat and/or light which results in the well-known unzippering reaction (35). 

One such factor is the presence of weak links which can seriously affect the heat resistance. One polymer that is particularly affected in this way is polyvinyl chloride where stepwise dehydrochlorination can occur by means of an unzippering reaction initiated from a weak link. In rubbers the weak link, as has been mentioned, is rather more important in causing degradation under the more common oxidative conditions to be dealt with later. 

The next step in the process consists of removing the unstable terminal hemiacetal and formal groups responsible of depolymerization reactions which may easily occur by unzippering mechanism (—CH2CH2O—) - (-CH2-0-) H. 

This reaction is sometimes called unzippering and is thought to originate at certain structural defects within the polymer [23]. 

When the PVC unzippers, it discolors and becomes brittle. To prevent this dehydrochlorination reaction, heat stabilizers are added. Lead stabilizers are effective and a major type of heat stabilizer. Data from 2003 [24] shows lead stabilizers to represent more than 50% of the world PVC stabilizers, but less than 20% in the U.S. where tin stabilizers predominate. The use of lead stabilizers is on the decline and being phased out due to concerns about the health effects of lead. Nonetheless, the news occasionally reports that lead has been found in a toy or other PVC article imported from a country such as China. The source of lead is likely due to the stabilizer. Another type of heat stabilizers is the mixed metal packages which are based upon mixtures of different... 

Depolymerization is characterized by the scission of the main polymer chain backbone. A polymer molecule can be broken down into fragments by (i) random degradation, where bonds in the chain are broken at random (2) depolymerization, where chain scission is the reverse of addition polymerization and monomer units are released successively from a chain end or at a weak link (often called depropagation, unzippering, or unzipping) and (3) weak-hnk degradation, where chain scission occurs at weak links distributed at random along the chain, which are more easily ruptured than normal bonds (Table 1). These types of reactions may occur separately or in conjunction with each other and are initiated thermally, photochemically, by ultrasonic irradiation, mechanical action, or some other method. 

Chain reaction (depropagation, unzipping, unzippering, chain depolymerization). Stepwise reaction. 

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