Weak links, depropagation

A polymer is thermally stable untill the decomposition process starts. Two (main) types of thermal decomposition processes are usually recognised for polymers chain depolymerisation and random decomposition. Chain depolymerisation is the release of monomer units from a chain end or at a weak link and is essentially the reverse process of polymerisation. It is often called depropagation or unzipping. Random degradation occurs by chain rupture at random points along the chain, giving a disperse mixture of fragments. These two different processes may occur separately or in combination the latter case is rather normal [2]. Both processes cause sample mass losses which can be measured with a TGA. 

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. 

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