Stereoselective Production of Poly(<i>rac</i>-lactide) by ROP with Highly Efficient Bulky Heteroscorpionate Alkylmagnesium Initiators

Abstract

The reaction of the sterically hindered heteroscorpionate lithium acetamidinate [Li(κ3-pbptamd)(THF)] (pbptamd = N,N′-diisopropylbis(3,5-di-tert-butylpyrazol-1-yl)acetamidinate) and [Li(κ3-tbptamd)(THF)] (tbptamd = N–ethyl-N′-tert-butylbis(3,5-di-tert-butylpyrazol-1-yl)acetamidinate) with 1 equiv of RMgCl proceeds cleanly to give very high yields of the corresponding nonchiral monoalkyl magnesium complexes [Mg(R)(κ3-NNN)] (NNN = pbptamd, R = Me (1), Et (2), CH2SiMe3 (3), CH2Ph (4), C3H5 (5); NNN = tbptamd, R = Me (6), Et (7), CH2SiMe3 (8), CH2Ph (9), C3H5 (10)). Alternatively, these monoalkylmagnesium complexes can be obtained by protonolysis of the corresponding dialkylmagnesium reagents, R2Mg, with the proligands Hpbptamd and Htbptamd in very good yields. Interestingly, alkyls 6–10 are obtained as a mixture of structural isomers (a + b) in a ratio that depends on the level of steric demand of the alkyl group. The single-crystal X-ray structures of derivatives 4, 6a·Et2O, and 7a confirm a four-coordinate structure in which the metal center is in a distorted-tetrahedral geometry and the bulky heteroscorpionate ligands have a κ3 coordination mode. Interestingly, the alkyl-containing magnesium complexes 1–3 and 8a can act as highly efficient single-component initiators for the very rapid ring-opening polymerization of ε-caprolactone and lactides even at −10 °C. While ε-caprolactone was polymerized within seconds to give high-molecular-weight polymers with narrow polydispersities (Mn > 105, Mw/Mn = 1.17), lactide afforded PLA materials with medium molecular weights in only a few minutes. The polymerizations are living, as evidenced by the narrow polydispersities (Mw/Mn = 1.01) of the isolated polymers and the linear nature of the number average molecular weight versus conversion plot. Inspection of the kinetic parameters for rac-LA showed that propagations present the usual first-order dependence on monomer and catalyst concentration and that the polymerization rates observed compare favorably with the highest rates reported to date for the ROP of lactides. Microstructural analysis of poly(rac-lactide) by 1H NMR spectroscopy revealed that the substituents on the amidinate fragment have a significant influence on both the degree of stereoselectivity and the rate of polymerization. Thus, propagations occur with enhanced levels of heteroselectivity, producing at low temperatures enriched-heterotactic PLAs with a Pr value of up to 0.79. ROP experiments with [Mg(κ3-pbptamd)(CH2SiMe3)] (3) revealed that this complex is extremely fast for rac-lactide, with 420 equiv polymerized in 93% yield in 2 min at 20 °C.