Effect of microwave energy on chain propagation of poly(ε-caprolactone) in benzoic acid-initiated ring opening polymerization of ε-caprolactone

Microwave-assisted ring opening polymerization of ε-caprolactone (ε-CL) initiated by benzoic acid was investigated. The molar ratio of ε-CL to benzoic acid was 5, 15 and 25. The mixtures of ε-CL/benzoic acid were heated under microwave irradiation and the temperatures were self-regulated to equilibrium from 204 to 240 °C with microwave power ranging from 340 to 680 W. The polymer chain propagated fast between 160 and 230 °C, within which the higher the temperature, the faster the propagation. However, when the temperature was over 230 °C, the resultant poly-(ε-caprolactone) (PCL) degraded. The advantage of microwave-assisted polymerization was that the propagation of PCL chain was significantly enhanced but the formation of growing center at the beginning stage of the polymerization was greatly inhibited. With this metal-free method, PCL with weight-average molar mass (M_w) over 4×10⁴ g/mol was prepared.