Structural Effect of Organic Catalytic Pairs Based on Chiral Amino(thio)ureas and Phosphazene Bases for the Isoselective Ring-Opening Polymerization of Racemic Lactide

Polymer materials featuring stereocontrolled monomer units often exhibit drastically different properties than their stereorandom counterparts. Control over the polymer tacticity thus represents a powerful means to access functional polymers of modular thermomechanical properties. In the present work, a series of chiral amino(thio)ureas (U1, TU1–TU5) are used in duos with phosphazene organic bases for the stereoselective ring-opening polymerization (ROP) of racemic lactide (rac-LA). These chiral binary organocatalytic pairs allow for relatively fast, highly chemo- and isoselective ROP of rac-LA at room temperature in toluene, yielding semicrystalline and metal-free stereoblock-like materials based on polylactide (PLA), with a melting temperature in the range 138–176 °C. The chiral dimethylaminourea, denoted as U1, shows faster ROP reactions relative to its dimethylaminothiourea TU1 analogue, when combined with any of the phosphazene bases, consistent with the formation of less stable intermediates from the urea relative to the more acidic thiourea-containing counterpart. On the other hand, the tethered dialkylamino moiety of TU1–TU3 is shown to have a non-innocent role both in the catalytic activity and the isoselectivity of the ROP process, whereby decreasing the basicity of this group leads to a decrease in the reaction rates. The active mechanism also proves to be dependent on the identity of the amino(thio)urea and the phosphazene base, and a mechanistic rationale for the experimental results is presented. Thus, the strongest organic base used in conjunction with U1 and TU1–TU5, leading to a high polymerization rate but lesser stereocontrol, induces an ionic-like mechanism. In contrast, organocatalytic pairs based on the least basic phosphazene favor a more associative mechanism involving hydrogen bond interactions, providing slower ROP reactions, but producing highly isotactic PLAs.