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

New Journal of Chemistry Pub Date: 2003-11-21 DOI: 10.1039/B306700A

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) ?. Compound 2 contains six-coordinate tin in a pseudo-octahedral geometry. The OCMe2C(O)NMe2 groups form cis-chelates with short, ca. 2.03 ?, and long, ca. 2.26 ?, 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 Ph3SnOBut 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.

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