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 CIMg-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)Cl2 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 copolyrnerizes 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 thiophcne is a suitable building block for designing of new conjugated polymers via chain-growth Kumada catalyst-transfer polycondensation.