Substituted poly(para)phenylenes (PPPs) are conjugated polymers with an attractive application potential in various fields of materials science. They are synthesized nearly exclusively using catalytic cross-coupling polymerization reactions based on Pd- or Ni-catalysts. Among these synthetic approaches to access alkoxy-substituted PPPs, Kumada catalyst transfer polymerization (KCTP or GRIM polymerization) would offer certain economic advantages over Suzuki-type polymerization as it relies on the utilization of a non-precious metal for catalysis. It also results in less total costs of the utilized reagents, avoiding additional preparative steps such as synthesis, isolation, and purification of boronic acid derivatives necessary for the Suzuki reaction. In fact, KCTP is nowadays the state-of-the-art method for the synthesis of polythiophenes. However, the application of KCTP for the synthesis of alkoxy-substituted PPPs leads to polymers with low molecular weights, limiting their practical applicability. Here, we developed a synthesis protocol that resulted in MEH-PPP with a molecular weight of M n = 133 kg/mol and BHex-PPP with M n = 153 kg/mol relative to polystyrene, outperforming the previous state of the art by a factor more than 5. Also, a tetra(ethylene glycol)-substituted PPP has been prepared by this procedure, with a molecular weight exceeding the previously reported results for analogous structures. Such molecular weights can be obtained in a reasonable reaction time (5 days) using low concentrations of an N-heterocyclic carbene-coordinated Ni complex. The polymerization kinetics suggested a chain-growth mechanism with a chain transfer step. The latter is caused most likely by a bimolecular interaction of the Ni-species at the polymer chain ends.
During 1990s, poly(para)phenylenes (PPPs) are one of the most prominent and hyped classes of conjugated polymers. Even though they have been heavily investigated for different applications, they are now eking out a rather niche existence. It is believed that this decline of interest partly has come from the early obstacle of synthesizing high-molecular weight, processable, and defect-free PPPs. Early examples of PPPs are not only rather oligomers than polymers but also contain many regiochemical and structural defects. Furthermore, early unsubstituted materials are infusible and insoluble, which have made their practical application almost impossible. Another reason for the decline of research interest in PPPs may be their underperformance in early applications, particularly in organic light-emitting diodes (OLEDs), which ultimately lead to a lack of follow-up publications. However, over the last two decades not only more precise and advanced synthesis methods have arisen but also a more profound understanding of those applications has been achieved within which new technological approaches have emerged. It is believed that PPPs would benefit from this development. Accordingly, in this perspective, the synthesis, structures, properties, and applications of PPPs reported so far as well as their potential in future technologies are discussed.
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