Ligand-Based Steric Effects in Ni-Catalyzed Chain-Growth Polymerizations Using Bis(dialkylphosphino)ethanes

Abstract

The role of ligand-based steric effects was investigated in the polymerization of 4-bromo-2,5-bis(hexyloxy)phenylmagnesium chloride. Three different Ni(L-L)Cl2 catalysts were synthesized using commercially available bis(dialkylphosphino)ethane ligands with varying steric properties. One of these catalysts (Ni(depe)Cl2) outperformed the others for this polymerization. The polymer characterization data were consistent with a chain-growth mechanism. Rate and spectroscopic studies revealed a rate-limiting reductive elimination for both initiation and propagation with Ni(depe)Cl2. In contrast, less hindered Ni(dmpe)Cl2 and more hindered Ni(dcpe)Cl2 were ineffective polymerization catalysts; NMR spectroscopic studies indicated that competing decomposition and uncontrolled pathways intervene. For other monomers, Ni(depe)Cl2 performed similar to the conventional catalysts. Copolymerization studies revealed that block copolymers could be effectively prepared. Overall, these studies indicate that altering the ligand-based steric properties can have a significant impact on the chain-growth polymerization.

Evidence for Catalyst Association in the Catalyst Transfer Polymerization of Thieno[3,2-b]thiophene

“…This has also been shown by Lanni et al in the case of a depe ligand with phenylene monomers. 26 In case C, the isolated Ni 0 (dppp)-molecule would also yield one singlet peak; however, due to the instability of a free Ni 0 -particle, further reaction of this entity with a second Ni 0 (dppp)-particle can occur, forming Ni and Ni(dppp) 2 which shows a singlet at 12.8 ppm relative to H 3 PO 4 . 54−56 The expected 31 P NMR spectrum in case B is less predictable, as this implies the isolation of the catalyst-associated intermediate, which is not common during a CTP reaction.…”

“…In order to exclude that the association could be too weakalthough a stronger association is expectedwe investigated a catalyst system which associates stronger with the conjugated polymer, i.e., Ni(depe)Cl 2 . 16,26 This catalyst showed no improvement and did not lead to polymer formation. A last Ni-catalyst system that was selected was a Ni II −N-heterocyclic carbene (NHC) complex catalyst, used by the group of Mori in the polymerization of poly(3-hexylthiophene).…”

Investigation of Bimetallic Nickel Catalysts in Catalyst‐Transfer Polymerization of π‐Conjugated Polymers

“…From the crystal structure, we conclude that the metal π‐aryl complex for the benzene diamine catalysts is weaker compared to Ni(dppp)Cl 2 due to the increased cone angle of the ligand. Ligand steric properties in CTP exhibit a “goldilocks” effect where a too large or too small cone angle is detrimental to CTP performance leading to unproductive pathways (i.e., chain termination) . The structure of Ni(dppp)Cl 2 can be viewed as the optimized geometry for the controlled synthesis of P3HT.…”

“…To further advance the field of conjugated polymers using CTP, a better understanding of monomer‐catalyst interactions is essential . Recent studies to improve the methodology of CTP primarily focused on the reactive ligands, ancillary ligands, and the transition metal . Baker et al designed a catalyst that incorporated an unsymmetrical propyl‐bridged diphosphine ligand as a new approach to tune catalyst reactivity .…”

Atom transfer versus catalyst transfer: Deviations from ideal Poisson behavior in controlled polymerizations

“…In studies by McNeil and coworkers, dependence on monomer was observed to switch from first to zeroth order when the ligand was changed from dppp to dppe due to differences in the extent of steric contribution from the ligand. 25,35 Our model indicates that catalysts with greater affinity for the growing chain and (using the catalyst chain walking Rates of oxidative addition and reductive elimination. Throughout our computational scheme, we have, for simplicity, assumed the rate of oxidative addition to be equal to that of reductive elimination.…”