γ-Agostic Species as Key Intermediates in the Vinyl Addition Polymerization of Norbornene with Cationic (allyl)Pd Catalysts: Synthesis and Mechanistic Insights

Abstract

Several cationic (allyl)Pd(II) complexes were synthesized and shown to be highly active for (2,3)-vinyl addition polymerization of norbornene (NB) to yield polymers with low molecular weight distributions (MWDs) ranging from 1.2-1.4. Despite the low MWDs, slow initiation was followed by rapid propagation preventing molecular weight control of the poly(norbornene). Several intermediates in these polymerizations initiated with [(2-R-allyl)Pd(mesitylene)](+) complexes were fully characterized (NMR and X-ray diffraction). Consistent with previous observations the allyl and NB units couple in cis-exo fashion to yield a sigma,pi-complex capped by mesitylene. Mesitylene is readily displaced by NB to form an agostic intermediate in which NB acts as a bidentate ligand and binds to the cationic Pd center via the pi-system and a gamma-agostic interaction with the syn hydrogen at C7. The identity of this complex was established by NMR spectroscopy and single-crystal X-ray diffraction. It is significant since it suggests bidentate binding of NB in the propagating species, which cannot be observed by NMR spectroscopy. The NMR studies suggest that the second insertion, i.e., insertion of NB in the agostic intermediate, is the slow initiation step and the subsequent insertions are extremely fast. Therefore, slow chelate opening is the major limitation preventing a living polymerization. This hypothesis was explored using a series of cationic substituted pi-allyl complexes; significantly increased reactivity was observed when electron-withdrawing groups were introduced into the allyl moiety. However, despite these modifications initiation remained slow relative to chain propagation.