Influence of Alkyl Substitution Pattern on Reactivity of Thiophene-Based Monomers in Kumada Catalyst-Transfer Polycondensation

Transmetalation (TM) was found to be the key step defining the ability of alkyl-substituted thiophene-based ClMg-Th(R)-Br monomers to be polymerized under Ni-catalyzed Grignard metathesis/Kumada catalyst transfer polycondensation conditions. For both dppe- and dppp-supported catalysts (dppe = 1,2-bis(diphenylphosphino)ethane, dppp = 1,2-bis(diphenylphosphino)propane), chain propagation is prohibited if the TM step would lead to sterically hindered head-to-head (HH) Th(R)-Ni-(R)Th intermediates in which alkyl substituents at both thienyl rings are in the “ ortho ” position relative to the Ni center. However, polymerization proceeds if no alkyl substituents are present at least at one “ ortho ” position relative to the Ni center. This rule is fulfilled for “regular” 2-bromo-5-chloromagnesio-3-hexylthiophene ( 2 ) which easily polymerize in the presence of various Ni initiators. However, “reversed” 5-bromo-2-chloromagnesio-3-hexylthiophene ( 1 ) has sterically hindered Grignard function and therefore the initiation step is hampered with Ni(dppe)Cl 2 or Ni(dppp)Cl 2 initiator precursors, but it proceeds with sterically nonhindered Ph-Ni(dppe)-Br and Ph-Ni(dppp)-Br initiators. “Disubstituted” 2-bromo-5-chloromagnesio-3,4-dihexylthiophene ( 3 ) does not polymerize by any initiators. “Unsubstituted” monomer 2-bromo-5-chloromagnesiothiophene ( 4 ) smoothly homopolymerizes and copolymerizes with 2-bromo-5-chloromagnesio-3-dodecylthiophene ( 2a ) by various unsupported and surface-immobilized initiators via the chain-growth mechanism. This reflects the unimportance of the “ ortho ” stabilization of the chain-propagating Ni species for achieving a good polymerization control, if Ni catalysts are ligated by bidentante dppp and dppe ligands. The obtained results demonstrate that unsubstituted thiophene is a suitable building block for designing of new conjugated polymers via chain-growth Kumada catalyst-transfer polycondensation.