Synthesis of functionalized polyketone- and polyphosphazene-based materials for ionic conduction applications

The objective of this work is the synthesis and characterization of new anion exchange polymers based on polyketone and polyphosphazene. The starting polymers were chosen for the possibility of undergoing easy chemical modification in mild conditions. Modulating the degree of their chemical functionalization allowed to manipulate the properties of the final polymers, which were characterized in terms of structure, chemical and thermal stability, and ion conductivity. Particular attention was given to the synthetic approach and its green and environmental aspects. In the case of polyketone based polymers, a polyethylene-propylene-ketone terpolymer was chosen as starting material for its excellent mechanical, chemical and thermal resistance. Its functionalization with specific primary amines via Paal-Knorr cyclization was exploited to incorporate a variety of N-functional groups (pyridinium, piperidinium, and morpholinium) and to modulate the conductivity properties of the final polypyrrole-co-polyketone polymers. This reaction is characterized by a high atom economy and the absence of undesired by-products since the incorporation of the amine in the polymer leads to the loss of only two equivalents of water. Moreover, the possibility to implement alternative and greener heating sources, like microwave and ultrasound, was investigated, with a drastic reduction in reaction time (from 72 to less than 3 hours) and an energy saving up to 41.5 times. The functionalized polyketone ionomers obtained by methylation (in iodide form) and subsequent anion exchange (in hydroxide form) possess chemically stable backbones when stored in alkaline media (1 M KOH(aq)), good thermal stabilities (up to around 200 °C in iodide form) and promising ion conductivities (up to σT = 12.42 mS cm-1 at 80 °C in hydroxide form). Polyphosphazenes, the second class of polymers investigated, were selected due to their inorganic backbone to further improve the alkaline stability, avoiding or limiting the degradation processes affecting organic polymers. A flexible synthetic route based on four steps, all conducted at room temperature was developed, leveraging on the ability of the cationic polymerization synthesis to overcome the problems related to the common ring opening polymerization approach (i.e., high reaction temperature and high by-products production). Polydichlorophosphazene is the important intermediate which allows the easy introduction of a wide range of functional groups by nucleophilic substitution of the P-bound chlorine atoms. The stepwise addition of different nucleophiles allows the creation of hybrid polymer structures, bearing both hydrophilic quaternizable amino groups and hydrophobic moieties (e.g., tert-butoxy and trifluoroethoxy) with which to tune the solubility of the final polymer. The proposed approach allowed the synthesis of several anion exchange polymers with high yields. After the structural characterization through 31P and 1H NMR and ATR-FTIR, their thermal and alkaline stability were evaluated, showing promising results, which are compatible with the electrochemical application.