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

A family of pyridine dipyrrolide bismuth complexes ( Mes,Ph L)MX (1-6) (M = Bi, X = O-2,6-Me-C 6 H 3 = OXyl (1); M = Sb, X = OXyl (2); M = Bi, X = O-2,6- i Pr-C 6 H 3 = ODipp (3), O-2,6- t Bu-C 6 H 3 = OAr t Bu (4), O t Bu (5) and OCMe 2 Et = OAm (6), N(SiMe 3 ) 2 = N'' (7) and CH 2 Ph (8)) have been prepared and investigated as initiators for the ring-opening polymerisation of lactide monomers. Bismuth lactate complexes ( Mes,Ph L)Bi{OC(H)(Me)C(O)OR} were prepared as models for the propagating species (R = t Bu (9), Me (10), i Pr (11)). The first insertion of the lactide monomer is rate limiting and the second and subsequent insertions are more rapid ( k init ≪ k LA2 < k prop ), 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 (( Mes,Ph L)Bi{OC(H)(Me)C(O)} 4 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-, rac - and meso -lactide are observed.