Abstract
Controlled polymerizations are often used to synthesize polymers with low dispersity, which involves expensive initiators, constrained atmospheres, and multi-step purifying processes, especially with water soluble monomers. These drawbacks make the synthesis very expensive and of little industrial value. In this report, an inexpensive free radical polymerization of sodium 4-styrenesulfonate, using benzoyl peroxide as initiator in water/N,N-dimethylformamide solutions, is presented. After polymerization, an easy fiber precipitation method is implemented to extract and purify the polymer, obtaining conversions up to 99%, recoveries up to 98%, and molecular weight dispersities in the range of 1.15–1.85, where the pseudo-controlled behavior is attributed to a thermodynamic limiting molecular weight solubility. Three different methods were used to bring the polymer to its acid form, obtaining Ion Exchange Capacities as high as 4.8 meq/g. Finally, polymeric membranes were prepared and reached conductivities up to 164 mS/cm, which makes them good candidates as proton exchange membranes in fuel cells.
Highlights
Water soluble homopolymers and copolymers of sulfonated polystyrenes [1] are of great interest in the renewable energy field, being used as hole transporters in organic solar cells [2,3], ionic exchangers in polymer-lithium batteries [4,5], and proton conductors in fuel cells [6,7,8]
These drawbacks and the fact that styrenesulfonic membranes can be degraded in the aggressive operating conditions of fuel cells [12,13], as the oxyradicals produced in the cathode and the high acid concentrations, resulted in this homopolymer being rapidly abandoned as a proton exchange membrane
We report an inexpensive free radical polymerization of NaSS in water/DMF
Summary
Water soluble homopolymers and copolymers of sulfonated polystyrenes [1] are of great interest in the renewable energy field, being used as hole transporters in organic solar cells [2,3], ionic exchangers in polymer-lithium batteries [4,5], and proton conductors in fuel cells [6,7,8]. A sulfonation process can be performed on pre-synthesized nearly monodisperse polystyrene [9,10], but the reaction is far from 100% efficient, suffers from high crosslinking, uses very aggressive reagents (as concentrated sulfuric or chlorosulfonic acids), and requires controlled atmospheres [11] These drawbacks and the fact that styrenesulfonic membranes can be degraded in the aggressive operating conditions of fuel cells [12,13], as the oxyradicals produced in the cathode and the high acid concentrations, resulted in this homopolymer being rapidly abandoned as a proton exchange membrane. N,N-dimethylformamide [19,20] (DMF), where benzoyl peroxide (BPO) is commonly used for industrial applications because of its low cost and well-known half-life time In this type of polymerizations, the reaction is not kinetically controlled and polymers can reach high dispersities even at low molecular weights. We report an inexpensive free radical polymerization of NaSS in water/DMF solutions, using BPO as initiator, which resulted in a polymerization with high molecular weight and low dispersity, and a methodology to purify and protonate the polymer to be used as ion exchange membranes
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