and gas-separation [9,10] membranes. Linear PPs with phenyl substituents can also serve as precursors for graphene nanoribbons (GNRs), nano meter-wide strips of graphene. The underlying concept is to appropriately design branched PP structures that can be fully planarized by CC bond formation between the benzene rings, typically through oxidative cyclodehydrogenation. [1,2,11-13] GNRs synthesized by such bottom-up methods have not only contributed to fundamental studies in condensed matter physics, but they also show potential for future applications in nanoelectronics, photonics, and quantum computing. [14-16] One of the most efficient methods to synthesize PPs is through the use of Diels-Alder cycloadditions between cyclopentadienone (CP) derivatives, usually 2,3,4,5-tetraarylcyclopentadienones, and alkynes leading to both linear and dendritic PPs. [1,2] In comparison with other polymerization methods, typically involving transition metal catalyzed or mediated reactions, Diels-Alder reactions, which proceed without metal catalysts or other reagents, also exhibit high efficiency. [12,17] A 2 B 2-type Diels-Alder polymerizations between an A 2 monomer having two CP moieties and a bisalkyne serving as a B 2 monomer have been known for more than 70 years [18,19] and have provided numerous linear PPs with a backbone of poly[(p-phenylene)-ran-(m-phenylene)]. [2] On the other hand, CP derivatives carrying an alkyne functional group can serve as bifunctional monomers that undergo AB-type Diels-Alder polymerization to yield linear PPs with surprisingly high persistence length. [17,20] Moreover, PPs synthesized by this procedure exhibit higher degrees of polymerizations (DP) than those made by A 2 B 2-type polymerization. [17,21,22] In 2014, we have reported an AB-type Diels-Alder polymerization of tetraphenyl-CP 1a leading to linear PP-Ia with the poly[(p-phenylene)-ran-(m-phenylene)] backbone (Figure 1a). The weight-average molecular weight (M w) of PP-Ia exceeded 600 000 g mol −1 , which was much larger than values obtained with previous PPs prepared by A 2 B 2-type Diels-Alder polymerization. The structure of PP-Ia can be projected into a plane without spatial overlap of benzene rings. "Planarization" of PP-Ia through oxidative cyclodehydrogenation could thus provide structurally defined GNRs. [17] Moreover, we have demonstrated the synthesis of laterally extended linear PP-Ib using CP-based monomer 1b with Polyphenylenes (PPs) are unique polymers showing high mechanical strength and chemical stability, and having potential applications, for example, in proton transfer and gas-separation membranes. Moreover, phenyl-substituted linear PPs can serve as precursors for bottom-up syntheses of graphene nanoribbons (GNRs), a new class of nanoscale carbon materials that appear promising for nanoelectronics. Notably, lateral extensions of linear PPs with appropriate "branched" phenyl substituents, that is, avoiding spatial overlap of benzene rings in their projections into a plane, can lead to wider GNRs with modulated elect...