Polymethylstyrene

Synthesis, Characterization and Lithographic Evaluation of Chlorinated Polymethylstyrene... 

Harita et al. (3) obtained a very sensitive deep-UV resist by chlorination of anionically polymerized polymethylstyrene. Although details of their chlorination procedure were not reported, preliminary studies showed that chlorination took place not only on the methyl group attached to the aromatic ring but also on the main chain. 

Chlorination can be effected by a variety of methods most of which involve free-radical mechanisms (8). In the present work, a chlorinating agent was sought that would preferentially attack the methyl group of polymethylstyrene so that a comparison with polychloromethylstyrene could be made. Both sulfuryl chloride (S02C12) and t-butyl hypochlorite (t-BuOCl) were suitable reagents for this purpose. 

The work reported in this paper concerns the synthesis of a series of chlorinated polymethylstyrenes, prepared by chlorination of both anionically and free radically synthesized polymethylstyrene, and their subsequent characterization by H NMR, 13C NMR, IR and elemental analysis. The preliminary lithographic evaluation of the chlorinated materials is also discussed. 

Polymethylstyrene (PMS) and PCMS (both prepared from mixtures of meta and para isomers) were prepared by free radical polymerization of the respective monomers in toluene initiated by benzoyl peroxide at 85°C (12). Chlorination of P-p-MS was carried out in carbon tetrachloride (CC14) at 60 °C in presence of AIBN using the desired amount of S02C12 (13). Chlorination of PMS was carried out in CC14 at 50 °C under irradiation from a 60W incandescent light with the requisite amount of t-BuOCl (14). 

Table I. Principal IR Bands and Assignments for the Series of Chlorinated Polymethylstyrenes...

Figure 14. DSC spectra of two chlorinated polymethylstyrenes with 0.56 Cl/m.u and 1.20 Cl/m.u respectively.

In the experiments which will be mentioned, the polymer is generally atactic polystyrene we shall also discuss experiments done with polymethylstyrene and polydimethylsiloxane. Polystyrene dissolves easily at room temperature. The good solvents which are used in this case, are... 

PS is miscible with several polymers, viz. polyphenyleneether (PPE), polyvinylmethylether (PVME), poly-2-chlorostyrene (PCS), polymethylstyrene (PMS), polycarbonate of tetramethyl bisphenol-A (TMPC), co-polycarbonate of bisphenol-A and tetramethyl bisphenol-A, polycyclohexyl acrylate (PCHA), polyethylmethacrylate (PEMA), poly-n-propyl methacrylate (PPMA), polycyclohexyl methacrylate (PCHMA), copolymers of cyclohexyl methacrylate and methyl methacrylate, bromobenzylated- or sulfonated-PPE, etc. Other miscible blends are listed in Appendix 2. 

In addition to main chain scission or cross-linking, gas formation is also observed as a result of irradiation. The gas in the hydrocarbon-based polymers mostly consists of hydrogen. The amount of gas produced depends on the nature of the polymer and also on dose, temperature, type of radiation, etc. In the case of polyethylene the G-value of gas production is high, G 0.32 pmol comparable to the gas yields observed in the radiolysis of hquid n-alkanes G 0.5-0.6 pmol In the radiolysis of polystyrene and polymethylstyrene the yield of gaseous products is only G 0.01 pmol 1 , that value is typical of aromatic compounds. The benzene rings attached to the main chain exert a protective effect against both the C-H and the C-C decompositions in the chain. 

Light scattering measurements based on turbidimetric titration have been applied to a wide range of polymers including PE [164], PP [165], PS [166-170], polymethylstyrene [171], PVC [172], chlorinated PVC-styrene [165], polyethylene oxide [173], polyphenylene oxide [174], PET [175,176], PEG [166], PS [177-179], styrene-methyl methacrylate copolymer [180], water-soluble acrylic acid and acryloamide copolymers [181], poly-a-methylstyrene [182], PS-block-polyisoprene [183], and poly(esterurethane) [184]. 

The choice of suitable initiating system allows preparation of polystyrene containing ARs on one (n-bntyl lithium) or two (naphthalate sodium) ends of macromolecules. By this method, aminoxyl-containing polymethylstyrene, poly-2-vinylpiridine and a copolymer of styrene with methylstyrene have been obtained [61]. 

---