Catalytic-Site-Mediated Chain-End Control in the Polymerization of rac-Lactide with Copper Iminopyrrolide Complexes.

Reaction of copper(II) methoxide with N-R-2-iminopyrroles (LH) and pyridylmethanol (R′OH) provided the dinuclear complexes {LCu(μ-OR′)}2 (R = naphtyl, CHPh2, 2,6-xylyl, 2,6-diisopropylphenyl (diip), or p-bromobenzyl). All complexes crystallized as dinuclear compounds with a square-pyramidal coordination geometry around copper and either imine or pyridine (for R = diip) in the apical position. The naphthyl-substituted complex was inactive in rac-lactide polymerization at room temperature in benzene. All other complexes showed good activity with apparent rate constants of kobs = 0.16(1)-1.89(8) h-1 at 2 mM catalyst concentration. All complexes showed a preference for slight isotactic monomer enchainment with Pm = 0.60-0.68. Stereoerror analysis indicate that the chain-end determines stereocontrol. A dependance of stereocontrol on the steric bulk of the ligand, on the initial monomer concentration and on the symmetry of the catalytic site support that the chiral information on the chain-end is mediated via the catalytic site (catalytic-site-mediated chain-end control). [ABSTRACT FROM AUTHOR]