We studied the mechanism of the chain-growth polymerization of 2-bromo-5-chloromagnesio-3-hexylthiophene (1) with Ni(dppp)Cl2 [dppp = 1,3-bis(diphenylphosphino)propane], in which head-to-tail poly(3-hexylthiophene) (HT-P3HT) with a low polydispersity is obtained and the M(n) is controlled by the feed ratio of the monomer to the Ni catalyst. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra showed that the HT-P3HT uniformly had a hydrogen atom at one end of each molecule and a bromine atom at the other. The reaction of the polymer with aryl Grignard reagent gave HT-P3HT with aryl groups at both ends, which indicates that the H-end was derived from the propagating Ni complex. The degree of polymerization and the absolute molecular weight of the polymer could be evaluated from the 1H NMR spectra of the Ar/Ar-ended HT-P3HT, and it was found that one Ni catalyst molecule forms one polymer chain. Furthermore, by reaction of 1 with 50 mol % Ni(dppp)Cl2, the chain initiator was found to be a bithiophene-Ni complex, formed by a coupling reaction of 1 followed by insertion of the Ni(0) catalyst into the C-Br bond of the dimer. On the basis of these results, we propose that this chain-growth polymerization involves the coupling reaction of 1 with the polymer via the Ni catalyst, which is transferred intramolecularly to the terminal C-Br bond of the elongated molecule. We call this mechanism "catalyst-transfer polycondensation".
To examine whether catalyst-transfer polycondensation, which affords well-defined polythiophenes, has generality for other conjugated polymers, the synthesis of poly(p-phenylene) (PPP) with various Ni catalysts was investigated. Monomer 1, 1-bromo-4-chloromagnesio-2,5-dihexyloxybenzene, was polymerized with Ni(dppe)Cl2 in the presence of equimolar LiCl to give PPP with a narrow polydispersity. The number-average molecular weight (Mn) of PPP thus obtained increased in proportion to the conversion of 1, indicating that this polymerization also proceeded in a chain-growth polymerization manner. Furthermore, the molecular weight of PPP was controlled by the feed ratio of 1 to the Ni catalyst up to at least Mn = 30000.
Summary: The polymerization of 2‐bromo‐3‐hexyl‐5‐iodothiophene (1) with isopropylmagnesium chloride and Ni(dppp)Cl2 was quenched with 5 M hydrochloric acid instead of water to yield head‐to‐tail poly(3‐hexylthiophene) (HT‐P3HT) with a very low polydispersity. The $\overline M _{\rm n}$ of the polymer was controlled by the feed ratio of 1 to Ni(dppp)Cl2. Quenching with 5 M hydrochloric acid seemed to promote protonolysis of HT‐P3HT‐Ni complexes before the coupling reaction between HT‐P3HT.GPC profiles of HT‐P3HT obtained after quenching with water and 5 M hydrochloric acid, respectively.imageGPC profiles of HT‐P3HT obtained after quenching with water and 5 M hydrochloric acid, respectively.
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