Suzuki Polycondensation toward High Molecular Weight Poly(m-phenylene)s: Mechanistic Insights and End-Functionalization

Synthesis of a poly(m-phenylene) by Suzuki polycondensation (SPC) using an AB-type m-phenylene monomer is reported (A and B refer to bromo and boron functional groups, respectively). Despite the attempted high molecular weight products, SPC under the conventional conditions using Pd[P(p-Tol)(3)](3) as catalyst gave rise to oligomeric products only. They comprise cycles and several series of open-chain poly(m-phenylene)s with various end group patterns. These patterns were caused by side reactions such as ligand scrambling. Buchwald's SPhos ligand was therefore alternatively employed for high turnover catalysis to accelerate SPC over such detrimental side reactions. This modification indeed led to the formation of high molecular weight products similar to 40 kDa (DP similar to 210), while oligomeric cyclic products still remained prominent. Cycle formation could, however, drastically be reduced by slow monomer addition. SPC was also performed in the presence of an excess monofunctional compound, R-A or R-B. The molecular weights of the products were found to be sensitive to the presence of R-B but not so much to R-A. This suggests a chain-growth-like mechanism as previously reported by Yokozawa et al. The modified SPC protocol was also used to demonstrate an efficient end-functionalization of the SPC products at both A and B termini with distinct chemical moieties. This revealed that the chain directionality expected from the AB monomer was largely retained when Pd(dba)(2) was used as palladium source for the SPC catalyst but not as for Pd(OAc)(2). This is presumably due to involvement of the homocoupling between phenyl boronates leading to reduction of Pd(II).