Modeling the catalyst resting state in aryl tin(iv) polymerizations of lactide and estimating the relative rates of transamidation, transesterification and chain transfer

Abstract

The preparation and characterization (IR, ¹H, ¹³C{¹H}, ¹¹⁹Sn NMR spectroscopy, elemental analysis and single crystal X-ray structure determination) are reported for Ph₃SnOCMe₂C(O)OEt (1) and Ph₂Sn[OCMe₂C(O)NMe₂]₂ (2). In the solid state, compound 1 contains four-coordinate tin with evidence for incipient bond formation to the ester oxygen: Sn⋯O = 2.648(2) Å. Compound 2 contains six-coordinate tin in a pseudo-octahedral geometry. The OCMe₂C(O)NMe₂ groups form cis-chelates with short, ca. 2.03 Å, and long, ca. 2.26 Å, Sn–O bonds to alkoxide and amide oxygen atoms, respectively. In solution, compound 1 remains four-coordinate but compound 2 exists as an equilibrium mixture of six-coordinate and five-coordinate species as judged by NMR spectroscopy. At −50 °C in toluene-d₈, the six-coordinate isomer is favored and the NMR data are consistent with the structure observed in the solid state. At +50 °C, the NMR data are consistent with a five-coordinate species in which reversible chelation of η²- and η¹-OCMe₂C(O)NMe₂ is fast on the NMR time scale. The molecular structure of 2 and its dynamic solution behavior is proposed to resemble that of Ph₂Sn[OCHMeC(O)NMe₂]₂ formed in the polymerization of L-lactide by Ph₂Sn(NMe₂)₂. The high formation tendency of this compound is proposed to be responsible for the preferential formation of cyclic lactide oligomers (LA/₂)_n by intrachain transesterification, in contrast to polymerizations employing Ph₂Sn(OPrⁱ)₂, which produce long chains of H–(LA/₂)_n–OPrⁱ where LA = [OCHMeC(O)OCHMeC(O)]. The kinetics of the reactions between Ph₃SnX and each of Me₂CHC(O)OMe, Me(MeO)CHC(O)OEt and Ph₃SnOCHMeC(O)OEt, have been determined from NMR studies in benzene-d₆ where X = NMe₂ or OPrⁱ. Similarly, the reaction between Ph₃SnOBu^t and (p-tolyl)₃SnOPrⁱ has been followed. The former reactions represent transamidation and transesterification, and the latter models chain transfer. These findings, when compared to the earlier studies of the ring-opening of lactide and its subsequent ring-opening polymerization, indicate that the rate follows the order: chain transfer > ring-opening > ring-opening polymerization > transesterification, although the latter is influenced by the ester end-group.