Polymerization of 5-Ethylidene-2-norbornene in the Presence of Pd–N-Heterocyclic Carbene Complexes with Phosphine and Pyridine Ligands

The polymerization of commercial strained cycloalkene, 5-ethylidene-2-norbornene, in the presence of Pd–N-heterocyclic carbene complexes (two of which are synthesized for the first time) containing phosphine or pyridine ligands is studied. Upon activation with borate Na+[B(3,5-(CF3)2C6H3)4)]–, the studied Pd complexes catalyze the polymerization of 5-ethylidene-2-norbornene according to the addition scheme. Polymerization proceeds selectively and involves only the endocyclic (norbornene) double bond of the monomer, while the exocyclic (ethylidene) double bond remains unaffected. Pd–N-heterocyclic carbene complexes with phosphine ligands are inactive in polymerization, while similar complexes with 3-chloropyridine ligands show a higher activity. Two new Pd complexes are synthesized.