Effect of the cyclic structures of p-tert-butylcalix[n]arenes on a bisoxazoline curing system

The thermal curing reactions of p-tert-butylcalix[n]arenes (Cn) (n = 4, 6, and 8) with 1,3-phenylenebis(2-oxazoline) (PBO) were performed. The obtained thermosets were characterized to determine the relationships between the ring size of the calixarenes and the properties of their network polymers. The samples were cured by heating at 160 °C and 180 °C for 1 h each and then at 200 °C, 230 °C, and 250 °C for 2 h each without a solvent and catalyst. For comparison, a corresponding linear four-nucleus novolac (L4) was cured with PBO under the same conditions. Dynamic mechanical analyses of the thermosets revealed that the glass transition temperature (T_g) increased in the following order: L4/PBO < C4/PBO < C8/PBO < C6/PBO. Model reactions with a monofunctional oxazoline compound indicated that the final crosslinking degree of the network polymers increased with increasing ring size. Conversely, the cyclic structures became increasingly rigid as the ring size decreased. Because of its moderate reactivity and rigidity, the network polymer derived from C6 exhibited the highest T_g.