Synthetic and Mechanistic Aspects of the Immortal Ring-Opening Polymerization of Lactide and Trimethylene Carbonate with New Homo- and Heteroleptic Tin(II)-Phenolate Catalysts

Several new heteroleptic SnII complexes supported by amino-ether phenolate ligands [Sn{LOn}(Nu)] (LO1=2-[(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)methyl]-4,6-di-tert-butylphenolate, Nu=NMe2 (1), N(SiMe3)2 (3), OSiPh3 (6); LO2=2,4-di-tert-butyl-6-(morpholinomethyl)phenolate, Nu=N(SiMe3)2 (7), OSiPh3 (8)) and the homoleptic Sn{LO1}2 (2) have been synthesized. The alkoxy derivatives [Sn{LO1}(OR)] (OR=OiPr (4), (S)-OCH(CH3)CO2iPr (5)), which were generated by alcoholysis of the parent amido precursor, were stable in solution but could not be isolated. [Sn{LO1}]+[H2N{B(C6F5)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 119Sn Mossbauer spectroscopy. In the presence of iPrOH, compounds 15, 7, and 9 catalyzed the well-controlled, immortal ring-opening polymerization (iROP) of L-lactide (L-LA) with high activities (ca. 150550 molL-LAmolSn-1 h-1) for tin(II) complexes. The cationic compound 9 required a higher temperature (100 degrees C) than the neutral species (60 degrees 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 119Sn{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 {LO1}H in a dynamic temperature-dependent and concentration-dependent equilibrium. Upon further addition of L-LA, two active species were detected, [Sn{LO1}(OPLLA)] (12) and [Sn(OPLLA)2] (14), which were also in fast equilibrium. Based on assignment of the 119Sn{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 15 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 SnII species.