High Molar Mass Poly(ε-caprolactone) by Means of Diphenyl Bismuth Ethoxide, a Highly Reactive Single Site Initiator

Diphenylbismuth ethoxide, Ph2BiOEt, initiates the ring-opening polymerization of e-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 °C. At 120 °C the reactivity of Ph2BiOEt is lower than that of tin(II) 2-ethylhexanoate (SnOct2) + ethanol, whereas below 90 °C, the reactivity of Ph2BiOEt is higher. Small amounts of cyclic oligoesters are even formed at 20 °C. At 120 °C the number average molecular weights (Mns) parallel the monomer-initiator ratios, and Mns up to 115 000 Da were obtained. The time−conversion curves and the crystal structure indicate association of Ph2BiOEt via O → Bi donor–acceptor interactions.