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

The preparation and characterization (IR, H-1, C-13{H-1}, Sn-119 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) Angstrom. Compound 2 contains six-coordinate tin in a pseudo-octahedral geometry. The OCMe2C(O)NMe2 groups form cis-chelates with short, ca. 2.03 Angstrom, and long, ca. 2.26 Angstrom, 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 -50degreesC in toluene-d(8), the six-coordinate isomer is favored and the NMR data are consistent with the structure observed in the solid state. At +50degreesC, the NMR data are consistent with a five-coordinate species in which reversible chelation of eta(2)- and eta(1)-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 Ph3SnOCH MeC(O)OEt, have been determined from NMR studies in benzene-d(6) where X=NMe2 or OPri. Similarly, the reaction between Ph3SnOBut and (p-tolyl)(3)SnOPri 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.