Chain scission and depolymerization

Since the weakest bond is the C—Cl bond, a possible degradation mechanism involves homolysis of this bond as the first step, followed by chain scission and depolymerization. There is spectroscopic evidence for the CF2=CF— structures also formed in this mechanism (Scheme 28). Further macroradicals may result by attack of chlorine atoms. 

According to these results, we proposed the following mechanism of degradation by hydrolysis which involves simultaneous chain scissions and ftulher depolymerization together with ester group hydrolysis (Figure 8). 

A shown in Fig. 7.1, there are two hroad categories of chemical amplification resists based on their imaging mechanisms, namely, (i) those based on acid-catalyzed main chain scission and (ii) those based on functional group polarity switch brought about by acid-catalyzed deprotection of lipophilic pendant groups, depolymerization, and Claisen rearrangement. 

Degradation of polyacetals may also occur by oxidative attack at random along the chain leading to chain scission and subsequent depolymerization (unzipping). Oxidative chain scission is reduced by the use of antioxidants (see Chapter 1), hindered phenols being preferred. For example, 2,2 -methylene-bis(4-methyl-6-r-butylphenol) is used in Celcon (Celanese) and 4,4 -butylidene bis(3-methyl-6-t-butylphenol) in Delrin (Du Pont). 

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 ... 

Thermal degradation occurs when a polymer is exposed to an elevated temperature in an inert atmosphere under exclusion of other compounds. The resistance against such degradation depends on the nature and the inherent thermal stability of the polymer backbone. There are three main types of thermal degradation depolymerization, random chain scission, and unzipping of substituent groups. 

Some metal cations such as sodium and potassium in the feed increase racemization risk, while other metals (Al, Fe) are catalytically active in transesterification, resulting in competitive polylactide formation [68,69]. Through corrosion, metals may be released in the residue and will build up there [6, 75]. Some patents discuss the presence of acid impurities in the process [6, 7, 67, 78], Mono- and dicarboxylic fermentation acids are responsible for stoichiometric imbalance in the lactic acid polycondensation reaction. Consequently, the composition of the obtained lactic acid oligomer chains can differ from pure PLA, resulting in impeded and incomplete catalytic depolymerization of the oligomers into lactide. In PLA manufacture, degradation reactions play a role, mainly via intramolecular chain scission, and this may also affect lactide synthesis. 

It has been reported that the thermal degradation of PLA predominantly consists of random main-chain scission and unzipping depolymerization reactions. The random degradation reaction involves hydrolysis, oxidative degradation, c/s-elimination, and intramolecular and intermolecular transesterification. Almost all the active chain-end groups, residual catalysts, residual monomers, and other impurities enhance the thermal degradation of PLA. As a consequence... 

Figure 46 Examples of chain scission and cross-iinking resists that are possible based on copolymers of styrene and functionalized styrenes. Poly(a-methylstyrene) wiii efficiently depolymerize under ionizing radiation while addition of haiogen or halomethyi groups onto the styrene ring results in reasonably efficient cross-iinking of the poiymer.

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. 

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