Kinetic Analysis of the Immortal Ring-Opening Polymerization of Cyclic Esters: A Case Study with Tin(II) Catalysts

A kinetic analysis of the metal-catalyzed immortal ring-opening polymerization (ROP) of cyclic esters is presented, based on a first-principles approach without making assumptions regarding the active species. The kinetics of all immortal ROP reactions performed with a metal catalyst and an exogenous chain transfer agent are characterized by the initiation, propagation and exchange rate constants (ki, kp, and ke, respectively). Curve fitting to this kinetic scheme in the initial stage of the polymerization allows the extraction of ki and kp from a single experiment. This has been illustrated for the ROP of l-lactide using tin(II) complexes of the type {LOi}Sn(X) ({LOi} = aminophenolate ancillary ligand, X = N(SiMe3)2 or methyl (S,S)-lactate), Sn(OiPr)2 or Sn(N(SiMe3)2)2 as precatalysts paired with excess iPrOH as a coactivator. Nonlinear regressions (R2 > 0.999) illustrate the three possible scenarios, ki kp. The kinetic model can be extended to any metal (pre)catalyst for the immo...