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Well-Defined Alkyl Heteroscorpionate Magnesium Complexes as Excellent Initiators for the ROP of Cyclic Esters

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posted on 02.04.2020, 16:54 by Luis F. Sánchez-Barba, Andrés Garcés, Mariano Fajardo, Carlos Alonso-Moreno, Juan Fernández-Baeza, Antonio Otero, Antonio Antiñolo, Juan Tejeda, Agustín Lara-Sánchez, María I. López-Solera
The reaction of the heteroscorpionate lithium salts [Li(tbpamd)(THF)] [tbpamd = N-ethyl-N‘-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] and [Li(pbpamd)(THF)] [pbpamd = N,N‘-diisopropylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] with 1 equiv of RMgCl proceeds to give very high yields of the neutral heteroscorpionate alkyl magnesium complexes [Mg(R)(NNN)] (NNN = tbpamd, R = C3H5 1, tBu 2, CH2SiMe3 3; NNN = pbpamd, R = C3H5 4, tBu 5, CH2SiMe3 6). On heating toluene solutions of complexes 13, 5, and 6, a ligand redistribution process occurs to give the corresponding 6-coordinated sandwich complexes [Mg(tbpamd)2] (7) and [Mg(pbpamd)2] (8). Interestingly, the allyl derivative 4 can be easily transformed to 8 at room temperature. In addition, the cationic sandwich complex [Mg(tbpamdH)2]Cl2 (9) [tbpamdH = N-ethyl-N‘-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidine] was obtained from 7 by means of a protonation process. Finally, alkyl-containing complexes 16 can act as highly effective single-component living initiators for the ring-opening polymerization of ε-caprolactone and lactides over a wide range of temperatures. ε-Caprolactone is polymerized within seconds to give high molecular weight polymers with narrow polydispersities. Lactide afforded PLA materials with medium molecular weights and polydispersities as narrow as Mw/Mn = 1.05. Additionally, polymerization of l-lactide occurred without racemization in the propagation process and offered highly crystalline, isotactic poly(l-lactides) with high melting temperatures (Tm = 160 °C). Polymer end-group analysis shows that the polymerization process is initiated by alkyl transfer to the monomer.