Sulfur dioxide-butene copolymers

TABLE IX Preparation of 2-Butene-Sulfur Dioxide Copolymers"... 

Tihis chapter describes the manufacture of expandable beads in the copolymer 1-butene-sulfur dioxide by suspension polymerization. The processing and properties of expandable beads and of molded articles are examined briefly. 

Expandability. Polybutene sulfone is insoluble in 1-butene but is soluble in sulfur dioxide. The higher the concentration of 1-butene in 1-butene-sulfur dioxide mixture, the lower is the solubility of the copolymer in this mixture. Even when copolymerization takes place with an excess of sulfur dioxide, the two monomers react in equimolecular... 

Up to 1973 very little information had been published on these foams. France has produced polysulfone foams from expandable beads which resemble polystyrene beads in their processing. In a typical formulation 1- butene and sulfur dioxide are copolymerized in such a way that the copolymer contains excess unpolymerized 1-butene, which volatilizes to cause foaming. Water, ethyl hydroxyethyl cellulose, 1-butene, sulfur dioxide, and a solution of isoprophyl peroxydicaibonate in... 

It has been shown by Barb and by Dainton and Ivin that a 1 1 complex formed from the unsaturated monomer (n-butene or styrene) and sulfur dioxide, and not the latter alone, figures as the comonomer reactant in vinyl monomer-sulfur dioxide polymerizations. Thus the copolymer composition may be interpreted by assuming that this complex copolymerizes with the olefin, or unsaturated monomer. The copolymerization of ethylene and carbon monoxide may similarly involve a 1 1 complex (Barb, 1953). 

Another class of "chain scission" positive resists is the poly(olefin sulfones). These polymers are alternating copolymers of an olefin and sulfur dioxide. The relatively weak C-S bond is readily cleaved upon irradiation and several sensitive resists have been developed based on this chemistry (49,50). One of these materials, poly(butene-l sulfone) (PBS) has been made commercially available for mask making. PBS exhibits an e-beam sensitivity of 1.6 pC cm-2 at 20 kV and 0.25 pm resolution. 

PBS (Figure 30) is an alternating copolymer of sulfur dioxide and 1-butene. It undergoes efficient main chain scission upon exposure to electron beam radiation to produce, as major scission products, sulfur dioxide and the olefin monomer. Exposure results first in scission of the main chain carbon-sulfur bond, followed by depolymerization of the radical (and cationic) fragments to an extent that is temperature dependent and results in evolution of the volatile monomers species. The mechanism of the radiochemical degradation of polyolefin sulfones has been the subject of detailed studies by O Donnell et. al. (.41). 

Copolymers have also been prepared using mixtures of olefins with sulfur dioxide. Olefin pairs studied were butene with propylene [22-22b], butene with pentene [13], butene with isobutene [13b], butene with acrylonitrile [13,23], butene with vinyl acetate [24], butene with methacrylate esters [25], butene with acrylic esters [25], and butene with butadiene [24]. 

Vinyl acetate in the presence of other olefins reacts with sulfur dioxide to give copolymers [24]. For example 20-30% vinyl acetate in the presence of propylene or butene reacts with sulfur dioxide. 

Poly(olefin sulfones), alternating copolymers of sulfur dioxide and an olefin, are another important class of positive resists, which exhibit a very high sensitivity [35] owing to the weak carbon-sulfur bond (see Figure 6.19). Poly(butene-l-sulfone) shows the best properties (o = 1.6 pC/cm at 20 kV) and is commercially available. 

Poly(2-butane sulfone) (PBS) is decomposed by UV irradiation or EB exposure as shown in Figure 1.16. PBS is completely decomposed to give monomers, 1-butene, and sulfur dioxide. PBS is a positive EB resist. The copolymer of butane sulfone and 4-pentane-2-one is reported as highly sensitive and having high resistibility to the EB resist ° (Figure 1.17). 

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