High Molecular Weight Poly( ε ‐caprolactone) by Initiation with Triphenyl Bismuth

Ring‐opening polymerizations of ε ‐caprolactone ( ε CL) were conducted in bulk at 120 °C with triphenyl bismuth, Ph 3 Bi, as an initiator or catalyst. Variation of the monomer–initiator ratio (M/I) allowed for a variation of the molecular weight, but not an accurate control. With an M/I ratio of 1 000:1 and ultra‐dry ε CL, a number average molecular weight (corrected $\overline M _{\rm n}$ ) of 285 kDa was obtained corresponding to a degree of polymerization around 2 500. Addition of tetra(ethylene glycol) resulted in incorporation of this coinitiator and allowed for a better control of the molecular weight. Time–conversion curves revealed a long induction period followed by a conspicuous acceleration upon addition of a coinitiator (tetraethylene glycol). Model experiments demonstrated that Ph 3 Bi is unstable at 120 °C in the presence of water, oxygen, or alcohols and slowly a precipitate is formed which mainly consists of (PhBiO) x . Ph 2 BiOR groups formed by side reactions seem to be the true initiators.