Evidence of Ni(0) Complex Diffusion during Grignard Metathesis Polymerization of 2,5-Dibromo-3-hexylthiophene

Abstract

It has been previously reported that the Grignard Metathesis polymerization of 3-alkyl thiophenes proceeds by a quasi-“living” chain growth or “catalyst transfer polycondensation” mechanism. Several publications present data in support of the transition metal species acting as an initiator and specifically maintaining Ni(0) association with a single growing polymer chain. However, our study of 2,5-dibromo-3-hexylthiophene polymerization via GPC, MALDI-TOF, and GC-MS verifies independently and in conjunction with one another that the transition metal is involved in transfer/diffusion from chain to chain during polymerization and presents the possibility of additional mechanistic controlling factors. Kinetic monitoring and characterization studies support transfer of the transition metal through diffusion from chain to chain during polymerization.

The Synthesis of Conjugated Polythiophenes by K umada Cross‐Coupling

“…These findings suggested that the catalyst did not remain associated with a single chain, but rather diffused during the polymerization, though such diffusion did not lead to a lack of control. Whilst the influence of monomer concentration was not reported, these results appeared similar to those reported by Rawlins [80], who showed the molecular weight to be controlled by the monomer concentration (as discussed above). Previous studies performed by McCullough and Williams, using the McCullough method to selectively generate the thienyl Grignard, had identified high-regioregular polymers of high molecular weight (M n 35 kDa, PDI = 2), using Ni(dppp)Cl 2 as the catalyst [8].…”

“…Although the majority of studies has been supportive of the proposed chain growth mechanism, recent investigations by Achord and Rawlins using the mixed thienyl Grignard (1 and 2) with Ni(dppp)Cl 2 (generating initiator 5 in situ) point to a different conclusion [80]. By using corrected gel-permeation chromatography (GPC)-derived molecular weights, in combination with MALDI, they report evidence that the molecular weight of the polymer could be controlled by the concentration of the monomer solution, rather than by the monomer : initiator ratio.…”

“…By using corrected gel-permeation chromatography (GPC)-derived molecular weights, in combination with MALDI, they report evidence that the molecular weight of the polymer could be controlled by the concentration of the monomer solution, rather than by the monomer : initiator ratio. It was also found that the Ni(0) catalyst does not remain associated with one polymer chain, but instead is involved in transfer/diffusion from chain to chain during the polymerization [80]. Despite this, the polymerization remains relatively controlled with narrow polydispersities obtained.…”

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

“…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…”

“…In addition, these results disprove the proposed Ni(0) transfer/diffusion during KCTP. [68] Alternatively, the observed selectivity of the intramolecular transfer may be explained by the fact that the C-Br bond at the chain-end is located much nearer to the eliminated Ni(0) species than other C-Br bonds (present in 1 or other aryl halides), so that effectively the intramolecular oxidative addition (which should be a fast, non ratedetermining step) is simply the more probable pathway. The observed selectivity could also be due to a strong propensity of Ni (0) 31 P NMR measurements of KCTP using thiophene-and phenylene-based monomers at 0 8C.…”