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Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...
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Homocouplings are significant side reactions in direct arylation polycondensation (DAP). This study identifies TBT and Cbz homocouplings as major defects, particularly under phosphine-free conditions, impacting polymer quality.

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Area of Science:

  • Organic Chemistry
  • Polymer Science
  • Materials Science

Background:

  • Direct arylation polycondensation (DAP) is a key method for synthesizing conjugated polymers.
  • Side reactions like homocoupling can introduce defects into polymer main chains, affecting material properties.
  • Understanding and controlling these side reactions is crucial for efficient polymer synthesis.

Purpose of the Study:

  • To identify and quantify homocoupling side reactions in the DAP of specific monomers.
  • To investigate the factors influencing the extent of homocoupling for different monomers.
  • To provide insights into minimizing main-chain defects in DAP.

Main Methods:

  • Size exclusion chromatography (SEC) was used to analyze polymer molecular weight and polydispersity.
  • NMR spectroscopy was employed to identify and quantify homocoupled species.
  • Reaction conditions, including the presence of phosphine ligands and temperature, were varied.

Main Results:

  • Homocoupling of both 4,7-bis(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (TBT) and 2,7-dibromo-9-(1-octylnonyl)-9H-carbazole (CbzBr2) monomers was observed to a considerable extent.
  • TBT homocoupling was more prevalent under phosphine-free conditions and could be suppressed by adding phosphine ligands.
  • Cbz homocoupling exhibited a strong temperature dependence, increasing at higher temperatures.

Conclusions:

  • Homocoupling represents a significant source of main-chain defects in DAP, particularly for the studied monomer combination.
  • Reaction conditions, such as the absence of phosphine ligands and elevated temperatures, exacerbate homocoupling.
  • Careful control of reaction parameters is essential to mitigate homocoupling and improve polymer quality in DAP.