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

International audienceA 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 immortal ROP of any cyclic ester, as exemplified using trimethylene carbonate as a monomer or employing a germylene precatalyst. A kinetic treatment for the late phase of immortal ROP reactions is introduced, which also gives direct access to kp. In agreement with the ROP kinetic data for {LOi}Sn(N(SiMe3)2), Sn(OiPr)2, Sn(N(SiMe3)2)2, and the new Sn(OiPr)(N(SiMe3)2 recorded in the presence of various quantities of iPrOH, synthetic and 119Sn{1H} NMR data provide evidence for reversible production of tin(II) bis(alkoxide) when a small excess (1–3 equiv) of alcohol is used with tin(II) precatalysts. It is also shown that, regardless of the identity of the precatalyst, Sn(OiPr)2 and Sn(O-polymeryl)2 are, respectively, the actual initiating and propagating species when immortal ROP reactions are performed in the presence of a larger excess of alcohol (7 equiv or more vs Sn)