Polymerization of 5-Alkylidene-2-Norbornenes with Highly Active Pd–N-Heterocyclic Carbene Complex Catalysts: Catalyst Structure–Activity Relationships

We studied the addition polymerization of readily available bifunctional norbornene derivatives, 5-alkylidene-2-norbornenes, in the presence of modified Pd complexes with N-heterocyclic carbene ligands activated by borates. The Pd–N-heterocyclic carbene complex/phosphine/borate systems selectively catalyze the addition polymerization of 5-methylene-2-norbornene and 5-ethylidene-2-norbornene with the participation of the endocyclic norbornene double bond, whereas the exocyclic double bond remained intact. The catalysts exhibited extremely high activity and durability: the activity was higher than 1∙108 gpolymer/(molPd∙h) and some catalysts were active at a very high monomer/Pd molar ratio of 2∙107 or at a Pd complex concentration of 5∙10-6 mol %. The structure–catalytic activity relationships were established for Pd–N-heterocyclic carbene complexes: the Pd complexes bearing five-membered heterocyclic rings and/or less sterically hindered aryl groups at nitrogen atoms in carbene ligands exhibited the highes...