Increasing the Initiation Efficiency of Ruthenium-Based Ring-Opening Metathesis Initiators: Effect of Excess Phosphine

Ring-opening metathesis polymerization (ROMP) has evolved into a valuable tool for the polymer chemist. The polymerization is generally mild and well controlled, and a large pool of readily available cyclic olefins (monomers) can be polymerized to nearly any size or shape. Employing the Ru-based initiator 1 (or its more active derivative 2) permits incorporation of high degrees of functionality and affords polymers with novel mechanical, electronic, and more recently biological properties. However, the polydispersity indices (PDIs) of the polymers obtained from initiator 1 are generally broad (between 1.3 and 1.5), which arises from an unfavorable rate of initiation (k_i) relative to propagation (k_p) as well as considerable secondary metathesis (“backbiting”). This creates difficulties when attempting to accurately predict polymer molecular weight a priori or when preparing well-defined block copolymers (where complete initiation is necessary). A recent disclosure from Gibson and co-workers revealed that the initiation efficiency of 1 was enhanced when the PCy_3 ligands were substituted with Cy_2PCH_2Si(CH_3)_3. When the resulting complex (3) was used to initiate the ROMP of norbornene derivatives (similar to 4), the k_i/k_p was found to be 4.35 (up from 0.06 when initiated with 1), and the resulting polymers were nearly monodispersed (PDIs ∼ 1.1). The enhanced initiation was attributed to a combination of the lower basicity and smaller size of Cy_2PCH_2Si(CH_3)_3 (relative to PCy_3) which respectively helped facilitate phosphine dissociation (a key step in Ru-based ROMP, see below) and increase monomer accessibility