Sulfonating agents, polymers

Allcock s research led to the development of poly-phosphazene-based PEMs by his small molecule studies of the sulfonation of cyclic trimeric phosphazenes and the surface chemistry of polyphosphazene macromolecules. In a 1993 report, he described the sulfonation of aminophosphazenes with 1,3—propanesultone. While these specific materials are not necessarily ideal as PEMs, this study demonstrated a novel technique for creating sulfonated polyphosphazene materials that may provide more control over the sulfonated polymer product than wholesale sulfonation of a base polymer by a strong sulfonating agent. 

The second route to ionomers Involves modification of a preformed polymer. Sulfonation of EPDM, for example, permits the preparation of sulfonated-EPDM with a level of sulfonate groups in proportion to the amount of sulfonating agent(27). These reactions are conducted in homogeneous solutions permitting the direct neutralization of the acid functionality to the desired level. Isolation of the neutralized ionomer is effected by conventional polymer isolation techniques, such as coagulation in a nonsolvent or solvent flashing. These procedures are detailed in several patents and publications(28-31). 

The very mechanisms of sulfonation of high polymers have been reviewed by Kucera and Janc. The sulfonation reaction proceeds easily in the presence of groups, such as —Cl, —NH2, —OH, —SH, etc. In fact, the active agent in the sulfonation reaction is the SO3 cation. The sulfonation of aromatic compounds is a reversible reaction. Sulfonating agents can be classified into three groups, namely... 

Uzal, N. et al.. Optimization of Co + ions removal from water solutions by application of soluble PVA and sulfonated PVA polymers as complexing agents, J. Colloid Interface Sci. 362, 615, 2011. 

Figure 2.7 shows the different steps of the synthesis procedure. The commercial polysulfone Udel (Amoco) was dissolved in chloroform at a temperature of 25 °C and subsequently treated with trimethylsilyl chlorosulfonate (Aldrich) to produce a silyl sulfonate polysulfone. The amount of the intermediate product was cmitrolled by the molar ratio of the sulfonating agent to the polymer repeating unit and varied from 2 to 2.3. The reaction time was 24 h at a temperature of 25 0.1 °C. The obtained silyl sulfonate polysulfone polymer was then treated with a 30 wt% sodium methoxide/ methanol solution (2.26 mmol g of polysulfone) at 25 °C for 1 h, which was used to cleave the sUyl sulfraiate moieties, yielding the sodium... 

Molar ratio of sulfonating agent to polymer repeating unit reaction time 24 h at 25 °C SPSf-60 + 5 wt% Si02... 

Polysulfone (PSU, Fig, 11,4) is an interesting polymer for its low cost, commercial processability and availability. Two different procedures for the sulfonation of polysulfone are reported. In the first, a sodium-sulfonated group is intfoduced in the base polysulfone via the metalation-sulfination-oxidation process [90], and in the other, the trimethylsilyl chlorosulfonate is used as the sulfonating agent. However, following either of these two procedures, the result remains the same The proton conductivity of the sulfonated PSU membrane is lower than Nafion membrane, because the C-F chains in Nafion have high hydrophobicity and large-phase separation. 

In order to prepare advanced molecules of poly(arylene ether sulfones) for fuel cell apphcations without sacrificing their excellent physical properties, Noshay and Robeson developed a mild sulfonation procedure for the commercially available bisphenol-A-based poly(ether sulfone) [62,63]. The sulfonation agents that have been used for this polymer modification are chlorosulfonic acid and a sulfur trioxide-triethyl phosphate complex. Recently, Kerres and co-workers [102] reported an alternative sulfonation process of commercial polysulfone based on a series of steps, including metalation-sulfmation-oxidation reactions. 

Sometimes, when the aromatic groups of the polymer are too activated, simple post sulfonation can lead to crosslinking. The arylsulfonic add groups formed by sulfonation are then able to act as sulfonating agent themselves. 

Depending on the chemical composition of the polymer backbone and the desired degree of sulfonation, various sulfonating agents with different reactivities can be selected and are commercially available (Table 6). 

The starting anionically polymerized polystyrene usually employed is a commercial homopolymer purchased either from Dow Chemical (Styron 666, Mw/Mn = 2.05), from Pressure Chemical and Co. or from Polymer Laboratory Ltd. (Mw/Mn = 1.05). The sulfonation reaction is monitored in order to avoid a complete sulfonation as in the polyelectrolyte preparation. The most commonly employed reaction is performed in 1,2-dichloroethane at 50 °C using acetyl sulfate as sulfonating agent [41]. The sulfonate content is determined after reaction and purification. The polymer molecular weight ranges from 3500 to 1800000 and the sulfonate content from 0 to 10%. The nonpolar solvents commonly used are THF, toluene, xylene or low polarity solvents like cyclohexanone. The conditions of preparation and the aging of the samples were shown to be important parameters [28, 42]. 

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