Mechanistic aspects of the Suzuki polycondensation of thiophenebisboronic derivatives and diiodobenzenes analyzed by MALDI-TOF mass spectrometry

Thiophenebisboronic derivatives (acids and esters) have been successfully utilized for the first time in palladium-catalyzed Suzuki polycondensations to prepare well-defined alternating thiophene-phenylene copolymers. 2,5-Thiophenebis(boronic acid) and its corresponding 1,3-propanediol and pinacol diesters have been synthesized and polymerized with 2,5-dialkoxy-1,4-diiodobenzenes in the presence of Pd(OAc)2 or Pd(PPh3)4 catalysts. SEC analysis showed that the polymers have moderate molecular weight with a polydispersity of 1.2 to 2.4. The role of the boronic derivatives and the catalyst on the yield and molecular weight of the resulting polymers have been investigated in detail. MALDI-TOF mass spectrometry has been used to elucidate the limiting steps in the polymerization and to assess the end groups. The results suggest that hydrolytic deboronation limits the formation of high molecular weight polymers. The polymers prepared using Pd(OAc)2 possess various end groups and contain macrocycles, whereas Pd(PPh3)4 produces much cleaner polymers. However, the use of Pd(PPh3)4 introduces phenyl end groups via aryl-aryl exchange between the catalytic palladium intermediate and the triphenylphosphine ligand. We found that this aryl-aryl exchange can be suppressed by the introduction of side chain branching in the 2,5-dialkoxy-1,4-diiodobenzene, and as result defect-free, perfectly alternating, chains with mainly thiophene end groups have been obtained in monomodal molecular weight distribution.