Mechanistic Studies on Ni(dppe)Cl₂-Catalyzed Chain-Growth Polymerizations: Evidence for Rate-Determining Reductive Elimination

Abstract

The mechanisms for Ni(dppe)Cl(2)-catalyzed chain-growth polymerization of 4-bromo-2,5-bis(hexyloxy)phenylmagnesium chloride and 5-bromo-4-hexylthiophen-2-ylmagnesium chloride were investigated. Rate studies utilizing IR spectroscopy and gas chromatography revealed that both polymerizations exhibit a first-order dependence on the catalyst concentration but a zeroth-order dependence on the monomer concentration. (31)P NMR spectroscopic studies of the reactive organometallic intermediates suggest that the resting states are unsymmetrical Ni(II)-biaryl and Ni(II)-bithiophene complexes. In combination, the data implicate reductive elimination as the rate-determining step for both monomers. Additionally, LiCl was found to have no effect on the rate-determining step or molecular weight distribution in the arene polymerization.

Kumada Catalyst‐Transfer Polycondensation: Mechanism, Opportunities, and Challenges

“…In contrast, for dppe-supported polymerizations, Ni II -biaryl complexes preceding the RE step (8, Scheme 4) were observed as the catalyst resting states, which suggests RE as the ratedetermining step. [63] This result is in accordance with kinetic investigations that reveal a zeroth-order dependence of the initial polymerization rate on monomer concentration, and a first-order dependence on the catalyst concentration. It is also noteworthy that the rate of polymerization of dppe-based polymerizations increases stronger with temperature than dppp-supported polymerizations.…”

“…Thus, KCTP not only follows the chain-growth mechanism but even displays the characteristics of a ''living'' polymerization process. [62] While we and several other research groups have confirmed the chain-growth mechanism for the KCTP of 1 [22,24,63,64] and other monomers, [28][29][30][31][65][66][67] Achord and Rawlins [68] reported data that contradict the observations of the livingness of KCTP. [24,69] General Mechanism…”

“…Obviously, the opposite coupling mode when monomers attack the sterically non-hindered Ni by their sterically hindered magnesium ends is less favored (Scheme 8). This difference in steric hindrance occurring at the TM step is less important for the dppebased catalyst, as in this case TM is not the rate-determining step, [63] which explains the similar reaction rates for this ligand. Finally it needs to be mentioned that the unsubstituted monomer 4 also homopolymerizes and copolymerizes with 2-bromo-5-chloromagnesio-3-dodecylthiophene by various initiators in a controlled way.…”

“…Polymerizations were conducted for both Ni(dppe)Cl 2 and Ni(dppp)Cl 2 , for which a difference in the rate-limiting steps was found. [63,78] For the dppp-supported KCTP, Ni II -monoaryl intermediates that precede the TM step were observed as the catalyst resting states (7, Scheme 4). [78] This result is in accordance with kinetic studies of McCullough and coworkers [22] and Lamps and Catala et al [79] for Ni(dppp)Cl 2 -catalyzed KCTP, who found that the rate of polymerization is first-order on monomer concentration, suggesting TM as the rate-determining step.…”

“…For example, 1 polymerizes much faster at room temperature with dppe as the ligand than with dppp in a broad range of monomer concentrations (0.01-0.5 M), whereas the two rates are comparable at 0 8C. [63,78] Among the intermediates shown in Scheme 4, the Ni(0) complex 6 is only one that has not been observed spectroscopically, indicating its short-lived nature. Despite of this, we believe that Ni(0) complexes not only exist, but also play an important role in KCTP since they are the base of a fascinating catalyst ''ring-walking'' (RW) process which is discussed in the following.…”