Poly(2-cycloalkyl-2-oxazoline)s: high melting temperature polymers solely based on Debye and Keesom van der Waals interactions

The synthesis of new 2-cycloalkyl-2-oxazoline monomers, namely 2-cyclobutyl (cBuOx), 2-cyclopentyl (cPentOx) and 2-cyclohexyl-2-oxazoline (cHexOx) is described. Their microwave-assisted cationic ring-opening polymerisation allowed the synthesis of well-defined homopolymers. The resulting homopolymers are crystalline and have high chemical resistance to organic solvents. The highest melting point was registered for cPentOx (306 °C), followed by cHexOx (251 °C) and cBuOx (243 °C). The crystal structure of the homopolymers was put into evidence by X-ray diffraction and appeared to be similar to that of the poly(2-isopropyl-2-oxazoline). The absence of any hydrogen bonding or π–π interactions, makes these compounds a special class of high performance polymers which possess high Tm solely based on Debye and Keesom van der Waals interactions of the side-chains. The copolymerization of cPentOx with 2-ethyl-2-oxazoline (EtOx) and cBuOx, respectively are also reported indicating the formation of near ideal random copolymers. Furthermore, the copolymers of cPentOx with cBuOx with compositions in the range from 0 to 100% revealed a linear dependence of melting temperature with the weight fraction of comonomer. For cPentOx-EtOx copolymers containing up to 25 wt% EtOx a linear decrease of the melting temperature with composition was registered, most likely due to the disturbance of the cPentOx crystalline domains. Further, increasing the EtOx wt% revealed a complex non-linear dependence of glass transition temperature on composition, whereby the glass transition temperature of some copolymers was lower than for pure PEtOx.