We describe a facile route to structurally diverse guanidinium organic catalysts by reaction of carbodiimides with secondary amines. The efficacy of these catalysts for the living ring-opening polymerization (ROP) of lactide was demonstrated including predictable molecular weights and end-group fidelity.Theoretical studies indicate that the acyclic guanidines are less basic than 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), but as for the more active TBD, they catalyze ring opening by activation of the alcohol and by stabilizing the resultant tetrahedral intermediates through hydrogen bonding. These results demonstrate that weak secondary interactions are an important concept for controlled polymerization.Ring-opening polymerization (ROP) of cyclic esters, such as lactide and lactones, is a powerful method for generating polyesters with controlled molecular weight and end-group functionality; several reviews of the elegant organometallic catalytic routes to such materials have appeared. 1 In addition to polymerization techniques that give predictable molecular weights, narrow molecular weight distributions, and end-group fidelity, many advanced applications require the removal of undesired contaminants such as heavy metal ions from catalysts. Toward this goal, new methods based on organocatalytic ring-opening polymerization have been developed. 2-9 Examples of successful organocatalysts for the ROP of cyclic esters, carbonates, and siloxanes include 4-dimethylaminopyridine, 3 phosphines, 4 N-heterocyclic carbenes (NHC), 5 acids, 6 bifunctional aminothioureas, 7 phosphazene bases, 8 amidines, and guanidines. 9,10 The bicyclic guanidine moiety is a substructure common to many biologically active natural products and the catalytic role in natural products has received considerable attention. 11 Guanidines and chiral guanidines have been used in several asymmetric transformations, 12 but the synthesis of the catalysts, particularly the chiral bicyclic guanidine's is very demanding. 13,14 Along these lines, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBDH þ , pK a = 26 in acetonitrile) 15 drew our attention as a ROP catalyst for cyclic esters since it is commercially available and easily handled with reported use as a transesterification catalyst. 16 For example, in a ROP of lactide in the presence of TBD in a 1000:1:10 monomer/catalyst/initiator monomer conversion was complete in 2 min, rivaling the NHC catalysts in turnover frequency. 17 As TBD is a strong base, our initial hypothesis was that it deprotonates the alcohol and propagates by an anionic mechanism. However, solvent effects on the polymerization with TBD were inconsistent with a purely anionic mechanism. For the TBD-catalyzed ROP of lactide, the reaction was fastest in CH 2 Cl 2 (>90% conversion in 2 min), slightly slower in THF (60% conversion in 2 min), and much slower in DMF (0% conversion in 2 min), the opposite of what would be expected for a conventional anionic polymerization.These observations and the fact that the much faster rate of TBD relative to oth...