Thienyl MIDA Boronate Esters as Highly Effective Monomers for Suzuki–Miyaura Polymerization Reactions

The synthesis of highly regioregular
poly­(3-hexylthiophene-2,5-diyl), rr-P3HT, by Suzuki–Miyaura
polymerization is reported. The key N-methyliminodiacetic
acid (MIDA) boronate ester thienyl monomer was synthesized using a
one-pot multigram scale procedure, in high purity, and in good isolated
yield (80%) by direct electrophilic borylation. Conditions for the
hydrolysis of the MIDA protecting group and the polymerization reaction
were investigated. The optimal procedure gave rr-P3HT with >98%
HT couplings, excellent isolated yields (up to 94%), and polymer molecular
weights up to Mn = 18.7 kDa and Mw = 42.7 kDa. The performance of the MIDA containing
monomer was compared to that of the pinacol boronate ester under identical
polymerization conditions, with the latter producing lower molecular
weight polymers in reduced yield.