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 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=O i Pr ( 4 ), ( S )‐OCH(CH 3 )CO 2 i Pr ( 5 )), which were generated by alcoholysis of the parent amido precursor, were stable in solution but could not be isolated. [Sn{LO 1 }] + [H 2 N{B(C 6 F 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 Mössbauer spectroscopy. In the presence of i PrOH, compounds 1 – 5 , 7 , and 9 catalyzed the well‐controlled, immortal ring‐opening polymerization ( i ROP) 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 1 H and 119 Sn{ 1 H} NMR spectroscopy showed that the addition of an excess of i PrOH to compound 3 yielded a mixture of compound 4 , compound [Sn(O i Pr) 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{ 1 H} 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.