Expanding the Living Polymerization Toolkit: Mechanistic Insights and Optimization of Vinyl-Addition Polymerization of Norbornenes.

The living vinyl-addition polymerization of norbornenes is a robust method for accessing high-molecular-weight, solvent-processable materials with a chemically inert backbone and varied block copolymer architectures. However, the extent of its livingness and the underlying mechanisms of the loss of molecular weight control remain underexplored and unoptimized. In this work, we provide a functional guide to achieving improved molecular weight control through the systematic exploration of vinyl-addition polymerization by a tri-tert-butylphosphine-ligated cationic palladium initiator. We developed a time-delay chain extension experimental protocol to quantify the livingness of a given polymerization and enable identification of how the catalyst counteranion, trace impurities, and polymerization conditions impact the reactivity of the propagating chain end. Reagents that can both halt propagation and remove the polymer from the palladium chain end were identified through complementary studies of polymer quenching protocols. Ultimately, we establish a series of guidelines for achieving highly living vinyl-addition polymerizations of norbornene monomers and apply them to the synthesis of a nonablock copolymer on the benchtop, expanding the toolkit of living methodologies for the synthetic polymer chemist while providing an experimental framework for the optimization of living polymerizations.