Well-Defined Star-Shaped Poly(p-benzamide) via Chain-Growth Condensation Polymerization: Use of Tetra-Functional Porphyrin Initiator to Optimize Star Polymer Formation

Most star polymers with controlled molecular weight and low polydispersity are synthesized by living polymerization, in which only initiation and propagation reactions, but not chain transfer reaction, take place. Accordingly, the obtained star polymers are generally not contaminated with linear polymers. Recently, we have developed chain-growth condensation polymerization, a kind of living polymerization yielding condensation polymers with controlled molecular weight and narrow molecular weight distribution. However, synthesis of star-shaped poly(p-benzamide)s through this method, using multifunctional initiators, afforded not only the desired star-shaped poly(p-benzamide) via chain-growth polymerization, but also a linear poly(pbenzamide) via self-polycondensation of the monomer when the amount of the initiator was decreased. This contamination with linear polymer was more liable to occur as compared to the case of chain-growth condensation polymerization with a monofunctional initiator under the same conditions. In addition, separation of the products by means of high-performance liquid chromatography (HPLC) was necessary to confirm which product was the star polymer and which the linear polymer. However, by using an initiator containing porphyrin, the absorption maximum of which is around 430 nm and quite different from that of poly(p-benzamide), we could easily differentiate the star polymer bearing the porphyrin initiator unit from the linear polymer without this initiator unit by means of GPC with a variable-wavelength UV detector, and thus we could optimize the polymerization conditions for selective synthesis of star aromatic polyamides. Furthermore, there are rather few reports on the synthesis of porphyrin-cored star polymers with rigid arms, as compared with star polymers with flexible-coil polymer arms. For example, porphyrin-cored star polyfluorene was synthesized by Suzuki coupling polycondensation of bromofluorene boronic ester in the presence of a porphyrin having four iodofluorene moieties. In this polymerization, the length of the arms was not controlled and the molecular weight distribution was broad. Therefore, not only the synthesis, but also the properties of porphyrin-cored star polymers with well-defined rigid or semirigid arms remain to be fully established. In this article, we investigated the chain-growth condensation polymerization of 4-(octylamino)benzoic acid esters 1 with porphyrin-cored tetra-functional initiator 3 under various conditions and optimized the conditions for selective synthesis of star-shaped poly(p-benzamide)s with suppression of the formation of linear selfcondensed polyamides (Scheme 1).