Anionic Polymerization of o-Phthalaldehyde and rac-Lactide by Discrete, Binucleated Dilithium mu-Amide Catalysts

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. Reaction of binucleating bis(pyrazolyl)alkane ligands PD(R)H with two equivalents of LiHMDS (HMDS– = –N(SiMe3)2) gives complexes PD(R)Li2(HMDS) with a symmetric mu-amide structure, analogous to simple alkali amides. These complexes polymerize rac-lactide by a mechanism analogous to LiHMDS. Notably, PD(4-Me)Li2(HMDS) gives lower dispersity and better molecular weight control than LiHMDS, consistent with a more well-defined catalyst. In the polymerization of o-phthalaldehyde, PD(H)Li2(HMDS) and PD(4-Me)Li2(HMDS) give higher activity and higher selectivity than LiHMDS or butyllithium, commonly used initiators for this reaction, and also gives modest stereoselectivity. Our detailed computational analysis of o-phthalaldehyde insertion resulted in a highly cooperative mechanism, with mu-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.