Photochemically initiated depolymerization

Depolymerization is the reverse reaction to polymerization it consists of unzipping the monomeric units after initiation of the reaction either at random or at the chain ends. The initiation can be the consequence of a photochemical reaction but depolymerization itself is a purely thermal reaction which is discussed in Chapter 1 of this book. It is usually negligible at room temperature. Photochemically initiated depolymerization has been studied in the case of polymethylmethacrylate [9], poly-a-methylstyrene [10] and polymethylisopropenylketone [11]. 

On exposure to 253.7 nm radiation polymethylmethacrylate degrades rapidly to monomer at temperatures above 130° C in vacuo [9]. The depolymerization process involves photochemical initiation, depropagation and mutual termination of reaction chains. The over-all quantum yield is greater than 102. 

TABLE IV Depolymerization of TBMS Initiated by Thermal and Photochemical Generators of Acid at 120 C... 

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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