The mechanism of the ring-opening polymerization (ROP) of lactide catalyzed by two partner hydrogen-bonding organocatalysts was explored. New amidoindoles 4 a,c, thioamidoindoles 4 b,d, amidobenzimidazoles 5 a,c, and thioamidobenzimidazoles 5 b,c were synthesized and used as activators of the monomer. In the solid state and in solution, compounds 4 and 5 showed a propensity for self-association, which was evaluated. (Thio)Amides 4 and 5 do catalyze the ROP of lactide in the presence of a cocatalyst, tertiary amine 3 a or 3 b, which activates the growing polymer chain through hydrogen-bonding. Reactions were conducted in 2-24 h at 20 degrees C; conversion yields ranged between 22 and 100 %. A detailed study of the intermolecular interactions undertaken between the participating species showed that, as expected, simultaneous weak hydrogen bonds do exist to activate the reagents. Moreover, interactions have been revealed between the partner catalysts 4/5+3. ROP catalyzed by these partner activators is thus governed by multiple dynamic equilibria. The latter should be judiciously adjusted to fine-tune the catalytic properties of (thio)amides and organocatalysts, more generally.

In this study, organocatalytic systems containing both basic and acidic sites, which can activate simultaneously the chain end and the monomer, were investigated in the ring-opening polymerization of L-lactide. To this end, equivalent amounts of (N,N-dimethylamino)pyridine (DMAP) and of its protonated form (DMAP 3 HX) were used as a dual catalytic system for L-lactide polymerization initiated by different alcohols. It is shown that the corresponding DMAP/DMAP 3 HX systems are significantly more active than DMAP alone, and yield well-controlled poly(L-lactide). Depending on the reaction conditions, the transesterification reaction can be prevented.

N-(3,5-Bis(trifluoromethyl)phenyl)-1H-indole-2-carboxamide 1e is an efficient hydrogen-bonding organocatalyst for the ring-opening polymerization of l-lactide. This new catalytic species does control the dispersity (1.08) and molecular weight (3460 g/mol vs 3064 in theory) of the poly(l-lactides) prepared in 2 h. (1)H NMR analysis showed that compound 1e complexes l-lactide in CDCl(3) through the two available NH groups (amide and indole). In particular, the catalytic species appeared to be mainly the H-bonding donor amide (1e in extended conformation, alone or dimer (1e)(2)) and, to a lesser extend, the dual H-bonding amido-indole (1e in its the pinched conformation). The first X-ray structure of the complex between a H-bonding organocatalyst and l-lactide also revealed a tight H-bonded network between the dimer (1e)(2) and l-lactide.