Polystyrenes charge-transfer complexes

Recently chloromethylated polystyrene (CMS), a highly sensitive, high resolution electron resist with excellent dry etching durability, was developed. Very recently reactive intermediates in irradiated polystyrene, which is a starting material of CMS, have been studied and the transient absorption spectra of excimer (2-4), triplet states (2,5), charge-transfer complexes, and radical cations (6) of polystyrene have been measured. The present paper describes the cross-linking mechanism of the high sensitivity CMS resist and compares it to that of polystyrene on the basis of data on reactive intermediates of polystyrene and CMS. 

The charge transfer complexes of linear polymers of polystyrene, poly(acenaphthalene) and poly(vi-nyl naphthalene) with tetracyano-ethylene and other electron acceptors have been studied (99). 

Chloromethylated polystyrene and chloromethylated poly(a-methyIstyrene) are negative type resists having high sensitivity and high resolution. In the pulse radiolysis of solid films of this polymer, the absorption spectra of substituted benzyl-type polymer-radical and the charge transfer complex between phenyl rings and chlorine atoms were observed (Fig. 17) [59], The benzyl-type radical may be produced by the dissociative electron attachment to the benzyl part of chloromethylated polystyrene (CMS). 

Tetrathiafulvalene electrodes — Tetrathiafulvalene (TTF) and many of its derivatives are easily oxidized to form cations of the type TTF+. With various anions, most prominent is the tetracyanoquinodimethane anion (TCNQ-), these cations form - charge-transfer complexes, i.e., salts with metal-like conductivities (- molecular metals). These salts are used in electrochemistry as electrodes or to modify the surface of electrodes, for the purpose of achieving desirable electrocatalytic properties (- electrocatalysis). Tetrathiafulvalene-substituted polystyrenes have also been synthesized and used as modified electrodes [ii]. 

It is well known that several monomers,such as styrene, < ( methylstyrene,isoprene,vinyl acetate (jj) have shown formation of oharge-transfer complexes in the presence of oxygen. Polystyrene peroxide is formed by photoirradiation of charge-transfer complex in the initial stage of polymerisation and the further photoinduced decomposition of the polystyrene peroxide initiates the polymerisation of styrene. On the other way,the reaction between excited state of styrene and oxygen may induce the formation of an alternating copolymer with peroxide groups -0-0- in-backbone. 

Polystyrene Direct photoexcitation/photoxidation of phenyl rings. Charge-transfer complex with oxygen. -C=0 and -OOH groups from processing or from oxidation. Interactions with singlet oxygen. [32,95-101]... 

Photochemical addition of MA to polystyrene has been observed in dioxane solvent, using photosensitizers and radiation with A >340 It is thought that the sensitized photoaddition proceeds via an attack by the excited anhydride triplet state on the charge-transfer complex between the anhydride and the aromatic nucleus. Photochemical reaction of benzene and alkyl substituted aromatics with MA are known to give a variety of potentially useful products (see Chapter 6). 

The absorptions at both 500 nm and 320 nm follow first order kinetics with a lifetime of 420 ns. This absorption species is neither the excimer of polystyrene nor free cationic species of polystyrene. Although the excimer of polystyrene has an absorption band around 500 nm, the lifetime is only 20 ns. Further the free cationic species of polystyrene should live for a longer time in this solution, and the absorption band should exist in a longer wavelength region (6). These considerations of lifetime and absorption spectrum lead us to conclude that the absorption spectrum shown in Figure 12 is due to the charge transfer-radical complex between polystyrene and Cl radical (2,4,17). A very similar... 

The transient absorption spectrum obtained in the pulse radiolysis of polystyrene solution in CC1 is shown in Figure 13. The spectrum is very similar to the charge transfer radical complex (PS4+C14-) species. The lifetime is about 200 ns. Consideration of the absorption spectrum and the lifetime suggest that this species is (PS4+C14-)-. The processes leading to formation of this species in liquid CC14 can be written as follows (4,7). 

Reaction Scheme of CMS Resists. The transient absorption spectrum shown in Figure 6 and observed for irradiated CMS films is mainly composed of two components as based on pulse radiolysis data of solid films of CMS and polystyrene, and CMS and polystyrene solutions in cyclohexane, chloroform, and carbon tetrachloride. An absorption with a maxima at 320 nm and 500 nm as due to the charge transfer radical-complex of the phenyl ring of CMS and chlorine atom (see Figure 14) and an absorption with maxima at 312 and 324 nm is due to benzyl type radicals (see Figure 11). 

Radicals and Charge-Transfer Radical Complexes of Polystyrene... 

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