Poly(ionic liquid)-based polyurethanes having imidazolium, ammonium, morpholinium or pyrrolidinium cations

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Abstract

The synthesis of cationic polyelectrolytes based on condensation-derived backbone is rarely performed due to the difficulty obtaining of the respective ionic monomers in high purity. Despite such an approach is favorable as it results in ionic polymers with well-defined chemical structure and ionic group distribution. In this work two efficient methods are presented for the synthesis of ionic diols in high purity, namely the technique with pyranyl protection of OH-groups and the direct quaternization of tertiary amine alcohols. Applying these methods five novel ionic diols bearing various cations, namely, 1,1-bis(2-hydroxyethyl)pyrrolidin-1-ium bromide, 4,4-bis(2-hydroxyethyl)morpholin-4-ium bromide, N,N-bis(2-hydroxyethyl)-N-methylethanammonium, 1,1′-(pentane-1,5-diyl)bis(1-(2-hydroxyethyl)pyrrolidin-1-ium) dibromide, and 3-(2-hydroxyethyl)-1-(5-(3-(3-hydroxypropyl)-1H-imidazol-3-ium-1-yl)pentyl)-1H-imidazol-3-ium dibromide, were synthesized in high purity and high yields. The tin(II) mediated solution polycondensation of ionic diols with commercial hexamethylene diisocyanate or 4,4′-methylenebis(cyclohexyl isocyanate) resulted in a series of ionic, high molecular weight (Mw = 2.3 × 104 −8.0 × 104) polyurethanes (PUs). The influence of various reaction parameters including reaction temperature and time, catalyst concentration and solvent nature upon PUs molecular weight was investigated. After the exchange of bromide to (CF3SO2)2N- anion the obtained poly(ionic liquid)s exhibit high thermal stability with onset mass loss above 225°C and demonstrate glass transition temperatures in the wide range from −22°C to 76°C depending on the nature of ionic diol used. Ionic PUs present excellent solubility in most organic solvents and are capable to form tough, flexible films with tensile strength up to 29.7 MPa.

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