Coupled equilibria in H-bond donating ring-opening polymerization: The effective catalyst-determined shift of a polymerization equilibrium

In the classic view of catalysis, a catalyst cannot alter the thermodynamically-determined endpoint of a reversible reaction. This conclusion is predicated on the assumption that the catalyst does not perturb the energy of product or reactant or does so to an equal extent. In the H-bond mediated ring-opening polymerization (ROP) of lactone monomers, the strength of the interactions of thiourea with product and reactant are not equal, and the magnitudes of these interactions are of similar energy to the free energy of reaction. The total monomer concentration at equilibrium in the thiourea/base cocatalyzed ROP of lactones is shown to be a function of the initial concentration of thiourea. Because the binding of thiourea to monomer and the polymerization reaction itself are both reversible, the application of varying amounts of thiourea catalyst directly alters the total amount of monomer in the reaction solution at equilibrium, which can be recovered at the end of the reaction