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

• Published in: Chemistry : a European journal 2012-03, Vol.18 (10), p.2998-3013
• Main Authors: Poirier, Valentin, Roisnel, Thierry, Sinbandhit, Sourisak, Bochmann, Manfred, Carpentier, Jean-François, Sarazin, Yann
• Format: Article
• Language: English
• Subjects: Carbonates; Catalysis; Catalysts; Chemical Sciences; Chemistry; Derivatives; Dynamics; lactide; Ligands; NMR spectroscopy; phenolate ligands; Polymerization; ring-opening polymerization; Tin
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201102261

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 Mössbauer 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 molL−LA molSn−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 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 1–5 could not operate according to an activated‐monomer mechanism. These mechanistic studies removed a numbe