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Anionic polymerization with alkali metals represents one of the most commercially significant and synthetically versatile polymerization methods. However, structural analysis of active alkali metal polymerization catalysts is complicated by aggregation and multicenter reactivity. This work describes discrete dilithium complexes for the anionic polymerization of rac-lactide and o-phthalaldehyde. The reaction of binucleating bis(pyrazolyl)alkane ligands PDRH with two equivalents of LiHMDS (HMDS− = −N(SiMe3)2) gives complexes PDRLi2(HMDS) with a symmetric μ-amide structure, analogous to simple alkali amides. These complexes polymerize rac-lactide by a mechanism analogous to LiHMDS. Notably, PD4‐MeLi2(HMDS) gives lower dispersity and better molecular weight control than LiHMDS, consistent with a more well-defined catalyst. In the polymerization of o-phthalaldehyde, PDHLi2(HMDS) and PD4‐MeLi2(HMDS) give higher activity and higher selectivity than LiHMDS or butyllithium, commonly used initiators for this reaction, and also give modest stereoselectivity. Our detailed computational analysis of o-phthalaldehyde insertion resulted in a highly cooperative mechanism, with μ-alkoxide bridging of the two metals in the resting state and with back-coordination of the furanyl oxygen. Aldehyde insertion occurs through synchronous nucleophilic addition and migration of the chain end alkoxide, not by aldehyde coordination and insertion. This detailed picture of polymerization by a lithium catalyst will inform mechanistic analysis and catalyst design in anionic polymerization. 

Poly(lactic acid) is the most successful biodegradable synthetic polymer. Yet, its physical properties need to be improved to replace petrochemical polymers or to serve emerging applications. Sequence-selective copolymerization of lactide with lactones can provide more versatile polyesters, but ideal alternating selectivity remains elusive. We report binucleated dizinc catalysts that show exceptional alternating selectivity in this reaction. Metalation of binucleating bis(pyrazolyl)alkanes (PDRH) gives neutral complexes PDRZn2X3 (X− = Cl−, Br−, I−) with two distinct zinc sites. Anion metathesis gives μ-phenolate cationic complexes [PDRZn2X2]+. The neutral iodide complexes PDRZn2I3 polymerize lactide with moderate dispersity (Đ = 1.18−1.38). Monozinc analogs give much lower activity, implicating a cooperative polymerization. End-group analysis and active site interrogation are consistent with coordination insertion polymerization by a dialkoxide complex. Based on model dizinc alkoxides, in situ NMR analysis, and DFT modeling, we provide a plausible active site structure and mechanistic rationale for cooperativity. We obtained copolymers of lactide and ε-caprolactone showing the highest degree of alternation obtained with a main group catalyst. Mechanistic analysis implicates transesterification with unusual alternating selectivity. We also obtained copolymers with γ-butyrolactone, δ-valerolactone, and glycolide. This report also represents a rare use of bimetallic catalysts to obtain new selectivity in lactide polymerization. 

We report the first binucleating aniline ligand (1), and compare its dizinc complexes to analogous phenolate complexes both structurally and in ring-opening polymerizaion catalysis.