Sterically rigid bismuth pincer complexes; observation of the growing polymer chain in polar monomer polymerisation.

A family of pyridine dipyrrolide bismuth complexes (L)MX (1-6) (M = Bi, X = O-2,6-Me-CH = OXyl (1); M = Sb, X = OXyl (2); M = Bi, X = O-2,6-Pr-CH = ODipp (3), O-2,6-Bu-CH = OAr (4), OBu (5) and OCMeEt = OAm (6), N(SiMe) = N'' (7) and CHPh (8)) have been prepared and investigated as initiators for the ring-opening polymerisation of lactide monomers. Bismuth lactate complexes (L)Bi{OC(H)(Me)C(O)OR} were prepared as models for the propagating species (R = Bu (9), Me (10), Pr (11)). The first insertion of the lactide monomer is rate limiting and the second and subsequent insertions are more rapid ( ≪ < ), leading to a significant induction period. The sterically demanding, rigid pincer ligand affords a well-defined coordination environment at the metal centre and allows for the enchainment of two lactide monomers to be differentiated spectroscopically ((L)Bi{OC(H)(Me)C(O)}OX (12-X)), with this species also implied to be the true initiator for the regime of propagation with first order kinetics. Well-controlled first order kinetic data for the polymerisation of L-, D-, - and -lactide are observed.