Scission reactions depolymerization

Depolymerization, elimination, and statistical chain-scission reactions can be used for polymer analysis. When the monomer is the main degradation product obtained, it can easily be identified by boiling point and refractive index. 

Properly end-capped acetal resins, substantially free of ionic impurities, are relatively thermally stable. However, the methylene groups in the polymer backbone are sites for peroxidation or hydroperoxidation reactions which ultimately lead to scission and depolymerization. Thus antioxidants (qv), especially hindered phenols, are included in most commercially available acetal resins for optimal thermal oxidative stability. 

Two limiting cases of degradation can be distinguished depolymerization and chain scission. In depolymerization, monomer M is split off from an activated chain end. This is the exact reverse of addition polymerization, taking the form of a kind of unzipping reaction ... 

Table 9.2 gives values for the heat of polymerization and the yield in monomer for well-known polymers. Obviously, polymers with a heat of polymerization aboye 80 kJ mol decompose according to scission reactions. Polymers having a low heat of polymerization show a high yield in monomer. The following polymers tend to depolymerize ... 

Example 4. Depolymerization under Pressure.62 PET resin was depolymerized at pressures which varied from 101 to 620 kPa and temperatures of 190—240° C in a stirred laboratory reactor having a bomb cylinder of2000 mL (Parr Instrument) for reaction times of 0.5, 1, 2, and 3 h and at various ratios of EG to PET. The rate of depolymerization was found to be directly proportional to the pressure, temperature, and EG—PET ratio. The depolymerization rate was proportional to the square of the EG concentration at constant temperature, which indicates that EG acts as both a catalyst and reactant in the chain scission process. 

Although some depolymerases act processively in cleaving their polymeric substrates, others act by what can be described as multiple attack which results in nonselective scission or random scission. The analysis of cleavage products during the course of enzyme-catalyzed depolymerization can provide important clues about the nature of the reaction. With random scission, the rate of bond scission must be proportional to the total number of unbroken bonds present in the solution. Thomas measured the rate of base addition in a pH-Stat (a device with an automated feedback servomotor that expels ti-trant from a syringe to maintain pH) to follow the kinetics of DNA bond scission by DNase. The number of bonds cleaved was linear with time, and this was indicative of random scission. In other cases, one may apply the template challenge method to assess the processivity of nucleic acid polymerases. See Processivity... 

The degradation reactions of polymers have been widely reviewed 525). In the absence of air, thermal reactions are the important degradation route. They may involve reactions of functional groups on the chain without chain scission, typified for example by the dehydrochlorination of PVC, or reactions involving chain scission, often followed by depropagation and chain-transfer reactions to yield complex mixtures of products. This latter route would be typical of the degradation of poly(methyl methacrylate), which depolymerizes smoothly to its monomer, and of polystyrene, which produces a wide range of tarry products. 

When the carbonyl group is located on C-2 (see Scheme 6.2 bottom equation) the chain scission will occur on C-4, producing a new chain with a reducing end at C-1. This new chain will behave according to the upper equation in Scheme 6.2, which means that the depolymerization reaction will propagate along the chain and consequently the hot alkali solubility will be high... 

This depolymerization was discovered by Dimroth et al. (6, 7, 8, 9) and Bamann, Trapmann, and Fischler (1) and has been investigated extensively recently in various laboratories 21, 23), including our own (2,12). It consists of the scission of the phosphodiester linkages between the ribose and phosphate in the backbone of the molecules. The reaction does not occur with DNA 1, 12), thus implicating the participation of the 2 -OH group of ribose and leading to a structural intermediate involving complex formation as follows ... 

Depolymerization of poly(a-methylstyrene) has been studied also in solution in various solvents ". The results have been interpreted in terms of a dhain reaction initiated by random scission, followed by a rapid depropagation, no transfer and a kinetic chain... 

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