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Synthetic and Mechanistic Aspects of the Immortal Ring‐Opening Polymerization of Lactide and Trimethylene Carbonate with New Homo‐ and Heteroleptic Tin(II)‐Phenolate Catalysts
Author(s) -
Poirier Valentin,
Roisnel Thierry,
Sinbandhit Sourisak,
Bochmann Manfred,
Carpentier JeanFrançois,
Sarazin Yann
Publication year - 2012
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201102261
Subject(s) - homoleptic , chemistry , cationic polymerization , tin , catalysis , polymerization , ring opening polymerization , lactide , alkoxy group , polymer chemistry , ether , solvent , medicinal chemistry , ring (chemistry) , yield (engineering) , mössbauer spectroscopy , stereochemistry , crystallography , organic chemistry , polymer , materials science , metal , alkyl , metallurgy
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.