Cooperative Enantiomorphic-Site and Chain-End Chirality in Isoselective β-Lactone Polymerization

Polymer properties are profoundly impacted by tacticity, necessitating an understanding of the origins of stereocontrol to make high-performance materials. Recently, we reported that controlled incorporation of stereoerrors into trans-poly(3-hydroxy-2-methylbutyrate) (trans-PHMB) through the isoselective polymerization of rac-trans-3,4-dimethylpropiolactone (trans-DMPL) using racemic "sandwich" (BDI)Zn alkoxide complexes (where BDI = β-diketiminate) enables tuning of mechanical properties to match those of low-density polyethylene. However, the mechanism of stereocontrol of these catalysts was not explored. Herein, we report a thorough experimental and computational investigation uncovering a distinct manifestation of cooperative enantiomorphic-site and chain-end control, which we describe as site-mediated asymmetric chain-end control, where catalyst (Δ and Λ) and chain-end chiralities (R,R or S,S) work synergistically to influence the reaction's stereoselectivity and rate. Experimental and computational data converge remarkably well, revealing that a Δ-catalyst with an (R,R)-chain end (Δ-RR) is isoselective 90% of the time. In contrast, a Δ-catalyst with an (S,S)-chain end (Δ-SS) is unselective. Detailed Density Functional Theory (DFT) investigations suggest molecular-level rationale for this mechanism of stereocontrol, demonstrating that chain positioning, monomer distortion, and ligand interactions work in conjunction to foster isoselectivity. Finally, DFT and kinetic modeling reveal that when polymerizing rac-trans-DMPL, the populations of Δ-RR and Δ-SS are equal and the high overall isoselectivity (mm% = 71-73%; where mm% is the percent of triads with a meso configuration across both ester linkages) of the catalyst is attributed to Δ-RR being faster than Δ-SS.