Currently there is considerable interest in the controlled ringopening polymerization (ROP) of lactides (LAs) by welldefined metal initiators because of the biodegradable and biocompatible nature of polylactides (PLAs) and their potentially wide-ranging commercial applications. [1, 2] Particularly interesting is that some discrete metal complexes initiate the ROP of rac-LA or meso-LA in a stereoselective manner, [3][4][5][6][7][8][9] and thus PLAs with a variety of architectures ranging from isotactic, [3d-i] syndiotactic, [5] to heterotactic [6][7][8] can be obtained. Enantiomerically pure or racemic aluminum complexes with chiral salen-type ligands (salen = N,N'-bis-(salicylidene)ethylenediamine) were reported to polymerize rac-LA to form isotactic or stereoblock/stereogradient PLA, [3a-g] and to polymerize meso-LA to form syndiotactic PLA through enantiomorphic site control. [5a] Achiral aluminum-salen or salan complexes (salan = N,N'-bis(orthohydroxybenzyl)ethylenediamine) produce isotactic or heterotactic PLA from rac-LA through a chain-end control mechanism. [3h,i, 6] Magnesium, [7b,d] zinc, [7c,d] calcium, [7d,e] and yttrium [8] complexes are highly active for ROP of rac-LA, in some cases showing significant preference for heterotactic dyad enchainment. Despite considerable efforts devoted to initiator design, [3][4][5][6][7][8][9][10] factors governing stereocontrol during the ROP of lactides are still not well understood. Herein we report that a series of scandium complexes with 1,w-dithiaalkanediyl-bridged bisphenolato (OSSO)-type ligands (Table 1) show high heterotactic selectivity during the ROP of rac-LA. This selectivity involves a new type of dynamic monomer recognition based on the fluxionality of the ancillary ligand. Group 3 complexes with such ligands have previously been found to be active initiators for ROP of l-LA, [11a,b] and a slight heterotactic preference during the ROP