Diphenylbismuth ethoxide, Ph2BiOEt, initiates the ring-opening polymerization of epsilon-caprolactone via a coordination-insertion mechanism. The matrix assisted laser induced desorption and ionization-time-of-flight (MALDI-TOF) mass spectra proved the formation of one ethyl ester and one CH2OH endgroup. The reactivity of this initiator is high enough to allow even for polymerizations at 20 degrees C. At 120 degrees C the reactivity of Ph2BiOEt is lower than that of tin(II) 2-ethylhexanoate (SnOct(2)) + ethanol, whereas below 90 degrees C, the reactivity of Ph2BiOEt is higher. Small amounts of cyclic oligoesters are even formed at 20 degrees C. At 120 degrees C the number average molecular weights (M(n)s) parallel the monomer-initiator ratios, and M(n)s Lip to 115 000 Da were obtained. The time-conversion curves and the crystal structure indicate association of Ph2BiOEt via O -> Bi donor-acceptor interactions.