Chain scission mechanisms

Materials that exhibit enhanced solubility after exposure to radiation are defined as positive resists. The mechanism of positive resist action in most of these materials involves either main-chain scission or a polarity change. Positive photoresists that operate on the polarity change principle have been widely used for over three decades in the fabrication of VLSI devices and they exhibit high resolution and excellent dry etching resistance. Ordinarily, the chain scission mechanism is only operable at photon wavelengths below 300 nm where the energy is sufficient to break main chain bonds. 

The pyrolytic process of a repetitive polymer frequently takes place with the formation of small volatile molecules and has a polymeric chain scission mechanism, as described in Section 2.6. Considering a polymer with a degree of polymerization (DP) n, the end scission reaction can be described by the chemical equation ... 

The polymeric chain scission mechanism is terminated as shown in Section 2.6 by one or more of the following types of termination reaction ... 

Selected examples of recycled resins are listed in Table 16.9. Polystyrene degradation during processing proceeds by the chain scission mechanism. 

It should be pointed out that Norrish-type 1 degradation is not efficient in PMMA, hence its rather low sensitivity to ionizing radiation, DUV radiation, and electron-beam radiation. For instance, it requires a dose of 3400 mJ/cm at 240 nm to get it fully exposed and developable. Experiments by Guillet and co-workers " have revealed that the cause of the inefficiency is the a-fission step in the chain scission mechanism. 

In another example, mechanical force can induce chain cleavage of perfluor-ocyclobutane (PFCB), shown in Fig. 14, generating trifluorovinyl esters (TFVE), which are also reformed by cycloreversion [61]. The chain cleavage proceeds via a stepwise mechanism with 1,4-diradical intermediates. This thermal remending is similar to that of other DA chain scission mechanisms, but is shown here to proceed through a reactive intermediate which can participate in intermolecular radical addition reactions. 

Figure 4. Proposed PET polymer chain scission mechanism by amide bond formation...

Scheme 1 Chain scission mechanism for PMMA e-beam resists.

Based on both experimental and computed thermo chemical and kinetic data. Corns (2008) developed two chain scission mechanisms the sulfonyl radical mechanism and the hydrogen radical... 

FIGURE 4.8 Sulfonyl radical initiating chain scission mechanism, proposed by Corns. (Redrawn from Corns, F. D. 2008. ECS Transactions 16 235-255.)... 

FIGURE4.10 Side-chain scission mechanism. (Redrawn from Chen, C.andFuller.T.F. 2009. Po/ymerDegradation... 

Polymers with only one substituent on alternating carbon atoms degrade according to the depolymerization and the random chain scission mechanisms [7]. Thus, Fig. 9.1 shows that the maximum quantity of styrene evolved by pyrolysis of polystyrene is attained at 725°C, whereas PMMA with two substituents on alternating carbon atoms degrades much faster and the maximum quantity of monomer released by pyrolysis is reached at a much lower temperature, as shown in Fig. 9.2 [7]. 

Geuskens G, Kabamba MS (1982) Photo-oxidation of polymers—part V a new chain scission mechanism in polyolefins. Polym Degrad Stab 4 69-76... 

The discoloration chemistry of SAN is complex ami not limited to one factor or mechanism. Both the monomer unit sequence distribution and small molecule residues in the polymer contribute to the discoloration. The SAN backbone chromophores appear to be located primarily on the chain-ends and a radical chain-scission mechanism involving nitrile cyclization may be responsible for its formation. 

Kim, H.M., Urban, M.W. Molecular level chain scission mechanisms of epoxy and urethane polymeric films exposed to UV/H2O. Multidimensional spectroscopic studies. Langmuir 16, 5382-5390 (2000)... 

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