The ring-opening polymerization of e-caprolactone (B-CL), initiated by carboxylic acids such as benzoic acid and chlorinated acetic acids under microwave irradiation, was investigated; with this method, no metal catalyst was necessary. The product was characterized as poly(epsilon-caprolactone) (PCL) by H-1 NMR spectroscopy, Fourier transform infrared spectroscopy, ultraviolet spectroscopy, and gel permeation chromatography. The polymerization was significantly improved under microwave irradiation. The weight-average molecular weight (M-w) of PCL reached 44,800 g/mol, with a polydispersity index [weight-average molecular weight/number-average molecular Weight (M-w/M-n)] of 1.6, when a mixture of epsilon-CL and benzoic acid (25/1 molar ratio) was irradiated at 680 W for 240 min, whereas PCL with M-w = 12,100 and M-w/M-n = 4.2 was obtained from the same mixture by a conventional heating method at 210 degreesC for 240 min. A degradation of the resultant PCL was observed during microwave polymerization with chlorinated acetic acids as initiators, and this induced a decrease in M-w of PCL. However, the degradation was hindered by benzoic acid at low concentrations.