Synthetic and mechanistic aspects of the immortal ring-opening polymerization of lactide and trimethylene carbonate with new homo- and heteroleptic tin(II)-phenolate catalysts

摘要

Several new heteroleptic Sn ~(II) complexes supported by amino-ether phenolate ligands [Sn{LO ~n}(Nu)] (LO ~1=2-[(1,4,7,10- tetraoxa-13-azacyclopentadecan-13-yl)methyl]-4,6-di-tert-butylphenolate, Nu=NMe _2 (1), N(SiMe _3) _2 (3), OSiPh _3 (6); LO ~2=2,4-di-tert-butyl-6-(morpholinomethyl)phenolate, Nu=N(SiMe _3) _2 (7), OSiPh _3 (8)) and the homoleptic Sn{LO ~1} _2 (2) have been synthesized. The alkoxy derivatives [Sn{LO ~1}(OR)] (OR=OiPr (4), (S)-OCH(CH _3)CO _2iPr (5)), which were generated by alcoholysis of the parent amido precursor, were stable in solution but could not be isolated. [Sn{LO ~1}] ~+[H _2N{B(C _6F _5) _3} _2] ~- (9), a rare well-defined, solvent-free tin cation, was prepared in high yield. The X-ray crystal structures of compounds 3, 6, and 8 were elucidated, and compounds 3, 6, 8, and 9 were further characterized by ~(119)Sn M?ssbauer spectroscopy. In the presence of iPrOH, compounds 1-5, 7, and 9 catalyzed the well-controlled, immortal ring-opening polymerization (iROP) of L-lactide (L-LA) with high activities (ca. 150-550 mol L-LA mol Sn -1 h -1) for tin(II) complexes. The cationic compound 9 required a higher temperature (100 °C) than the neutral species (60 °C); monodisperse poly(L-LA)s were obtained in all cases. The activities of the heteroleptic pre-catalysts 1, 3, and 7 were virtually independent of the nature of the ancillary ligand, and, most strikingly, the homoleptic complex 2 was equally competent as a pre-catalyst. Polymerization of trimethylene carbonate (TMC) occurs much more slowly, and not at all in the presence of LA; therefore, the generation of PLA-PTMC copolymers is only possible if TMC is polymerized first. Mechanistic studies based on ~1H and ~(119)Sn{1H} NMR spectroscopy showed that the addition of an excess of iPrOH to compound 3 yielded a mixture of compound 4, compound [Sn(OiPr) _2] _n 10, and free {LO ~1}H in a dynamic temperature-dependent and concentration-dependent equilibrium. Upon further addition of L-LA, two active species were detected, [Sn{LO ~1}(OPLLA)] (12) and [Sn(OPLLA) _2] (14), which were also in fast equilibrium. Based on assignment of the ~(119)Sn{~1H} NMR spectrum, all of the species present in the ROP reaction were identified; starting from either the heteroleptic (1, 3, 7) or homoleptic (2) pre-catalysts, both types of pre-catalysts yielded the same active species. The catalytic inactivity of the siloxy derivative 6 confirmed that ROP catalysts of the type 1-5 could not operate according to an activated-monomer mechanism. These mechanistic studies removed a number of ambiguities regarding the mechanism of the (i)ROPs of L-LA and TMC promoted by industrially relevant homoleptic or heteroleptic Sn ~(II) species. The tale of Sn and the immortal ring-opening: In the presence of an alcohol, homoleptic bis(phenolate)-, heteroleptic phenolate-alkoxide-, and homoleptic bis(alkoxide)-tin(II) are in dynamic equilibrium; the latter two are active in the controlled immortal ROP of lactide.