title: Synthetic, structural, mechanistic, and computational studies on single-site beta-diketiminate tin(II) initiators for the polymerization of rac-lactide.
abstract: A family of tin(II) complexes supported by beta-diketiminate ligands has been investigated as initiators for the polymerization of rac-lactide. Kinetic studies reveal a first-order dependence on [lactide], but with a significant induction period. Linear plots of M(n) versus conversion and [M](o)/[I](o) versus conversion, along with narrow molecular weight distributions (typically 1.07-1.10), are indicative of well-controlled, "living" polymerizations. Less sterically hindered derivatives promote faster propagation than their bulky analogues, in accord with a more accessible active site. Enhanced rates of polymerization are observed for ligands bearing halogenated N-aryl substituents, a consequence of the more Lewis acidic nature of the Sn(II) centers. All of the initiators exhibit a similar bias toward heterotactic polylactide, which is attributed to a chain-end control mechanism influenced predominantly by the presence of the Sn 5s(2) lone pair of electrons rather than the steric or electronic properties of the beta-diketiminate ligand. The tin(II) isopropyl-(S)-lactate complex, ((Me)BDI(DIPP))SnOCH(Me)COO(i)Pr (14), has been synthesized as a model compound for the propagating species by treatment of ((Me)BDI(DIPP))Sn(NMe(2)) with isopropyl-(S)-lactate. An X-ray structure determination showed that the lactate ligand forms a five-membered chelate ring with a weak donor bond from the carbonyl oxygen atom to the tin center. A B3LYP density functional computational study indicates that insertion of the first lactide monomer into the tin(II) alkoxide bond is facile, with the induction period arising from a slower insertion of the second (and possibly third and fourth) monomer units.
authors.affiliation: Department of Chemistry, Imperial College London, UK.
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Dove
authors.forename: Vernon C
authors.initials: VC
authors.lastname: Gibson
authors.forename: Edward L
authors.initials: EL
authors.lastname: Marshall
authors.forename: Henry S
authors.initials: HS
authors.lastname: Rzepa
authors.forename: Andrew J P
authors.initials: AJ
authors.lastname: White
authors.forename: David J
authors.initials: DJ
authors.lastname: Williams
mesh_terms: D018360:Crystallography, X-Ray; D008958:Models, Molecular; D015394:Molecular Structure; D011091:Polyesters / Q000737:chemistry*; D014001:Tin / Q000737:chemistry*
chemical_list: D011091:Polyesters; C033616:poly(lactide); D014001:Tin