Ring‐opening polymerization of ε‐caprolactone via the bismuth‐2‐mercaptoethanol complex

A complex consisting of one Bi3+ ion and two 2‐mercaptoethanol units (BiME2) was used as initiator for the ring‐opening polymerization of ε‐caprolactone in bulk. A kinetic comparison showed that BiME2 is as reactive as initiator as Sn‐octanoate and more reactive than Bi‐hexanoate. The difference to BiHex3 decreased at higher temperatures and upon addition of an alcohol as coinitiator. When tetra(ethylene glycol) was used as coinitiator, it was completely incorporated into the poly(εCL) chain, so that telechelic polylactones having two OH‐endgroups were formed. In the absence of a coinitiator, 2‐mercaptoethanol or its disulfide were incorporated in the form of ester groups. Furthermore, it was found by MALDI‐TOF mass spectrometry that small amounts of cyclic oligolactones (detected up to a degree of polymerization of 17) were formed under all reaction conditions. Higher temperatures and longer times favored a higher content of cycles.