Synthesis and anhydrous proton conductivity of poly(5-vinyltetrazole) prepared by free radical polymerization

5-Vinyltetrazole (VT)-based polymer is mainly produced by ‘click chemistry’ from polyacrylonitrile due to the unavailability of 5-vinyltetrazole monomer, which usually produces copolymers of VT and acrylonitrile rather than pure poly(5-vinyltetrazole) (PVT). In present work, VT was synthesized from 5-(2-chloroethyl)tetrazole via dehydrochlorination. A series of PVT with different molecular weight were synthesized by normal free radical polymerization. The chemical structures of VT and PVT were characterized by 1H NMR and FTIR. PVT without any doped acid exhibits certain proton conductivity at higher temperature and anhydrous state. The proton conductivity of PVT decreases at least 2 orders of magnitude after methylation of tetrazole. PVT and PVT/H3PO4 composite membranes are thermally stable up to 200 °C. The glass transition temperature (Tg) of PVT/xH3PO4 composite membranes is shifted from 90 °C for x = 0.5 to 55 °C for x = 1. The temperature dependence of DC conductivity for pure PVT exhibits a simple Arrhenius behavior in the temperature range of 90–160 °C, while PVT/xH3PO4 composite membranes with higher H3PO4 concentration can be fitted by Vogel–Tamman–Fulcher (VTF) equation. PVT/1.0H3PO4 exhibits an anhydrous proton conductivity of 3.05 × 10−3 at 110 °C. The transmission of the PVT/xH3PO4 composite membrane is above 85% in the wavelength of visible light and changes little with acid contents. Thus, PVT/xH3PO4 composite membranes have potential applications not only in intermediate temperature fuel cells but also in solid electrochromic device.