Synthetic, Structural, Mechanistic, and Computational Studies on Single-Site β-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.

An Overview of the Synthesis and Synthetic Mechanism of Poly (Lactic acid)

“…Many single site catalysts composed of various multivalent ligands were synthesized, and synthetic methods are known to synthesize PLA matters stereocontrolled from DL-lactide [52] (Figure 6). …”

“…In Figure 6, the monomer combines weakly in the direction of trans of nitrogen donor of BDI ligand. The monomer and alkoxide ligand first exchange their sites by rotation, and then progress to TS1, the first transition state [52].…”

Review – recent development of ring-opening polymerization of cyclic esters using aluminum complexes

“…It is generally believed that ROP of cyclic esters operated by 'coordination-insertion' mechanism ( Figure 1). [49][50][51] Tolman group studied the kinetics of ROP of ɛ-caprolactone (CL) initiated by aluminum complexes, they suggested that ROP involved reversible binding of a cyclic ester to the metal (k 1 /k À1 ), attaching alkoxide to the activated carbonyl (k 2 /k À2 ) and generating a new propagating alkoxide (k 3 /k À3 ), The latter two steps were conceivably reversible. Although, the barrier for the ring-opening (k 3 ) would be lower than that of the ringclosing (k À3 ) and alkoxide deinsertion (k À2 ), k 3 was probably post-rate determining, so they simplified the mechanistic description to include only k 1 , k À1 , and k 2 .…”

Bipyrrolidine salan alkoxide complexes of lanthanides: synthesis, characterisation, activity in the polymerisation of lactide and mechanistic investigation by DOSY NMR

“…2,7,[9][10][11] Indeed, many studies have shown that the PLA microstructure can be controlled by the judicious choice of metal centre and ligand(s). However, despite much effort, including some computational studies, [12][13][14][15] challenges still remain to fully understand the interplay between metal, ligand, monomer and the growing polymer chain. There remains a high degree of serendipity in the stereochemical outcome of the polymerisation, with unpredictable tacticities achieved from metal-ligand combinations.…”