Substituent Effects on Ring Opening Allene Metathesis: Polymerization Rate Enhancement and Regioregularity

Polyallenamers represent an emerging class of polymeric materials with highly tailorable and adaptive properties. While their generation has been achieved (via cross-coupling, postpolymerization modifications, and ring opening allene metathesis polymerization (ROAlMP)), molecular weight limitations and the inability to obtain precise microstructures have prevented the ability to utilize allenes to their full potential. Herein, we demonstrate the effects that allylic derivatization of cyclic allenes (4-R-1,2-CNDs) have on the polymerization rate and regioselectivity of ROAlMP. Bulky substituents were shown to favor the formation of distal products, leading to perfectly regioregular polymerizations (>97% head-to-tail). Kinetic investigations indicated that bulky substituents also led to large polymerization rate enhancements (>20×), allowing for the generation of high molecular weight material (M_n > 120 kDa) and effective chain extension. Density functional theory calculations indicate that bulky substituents destabilize the metallacyclobutane intermediate to a greater extent than the transition state (cycloreversion), leading to unprecedented rate enhancements.