Diels-Alder polymerization: a versatile synthetic method toward functional polyphenylenes, ladder polymers and graphene nanoribbons

Diels-Alder reaction has been widely used in synthetic organic chemistry since its discovery in 1928. The catalyst-free nature, functional group tolerance and high efficiency of the Diels-Alder reaction also make it promising for the fabrication of functional polymeric materials. In particular, a large variety of functional polyphenylenes (polymer structures mainly consisting of phenylenes) and ladder polymers (double-stranded polymers with periodic linkages connecting the strands) have been obtained by this method, offering potential applications such as polymer electrolyte membranes and gas separation. More recently, tailormade polyphenylenes prepared by Diels-Alder polymerization have been used as precursors of structurally well-defined graphene nanoribbons (ribbon-shaped nanometer-wide graphene segments) with different widths, exhibiting long lengths (4600 nm) and tunable electronic bandgaps. This article provides a comprehensive review of the use of Diels-Alder polymerization to build functional polyphenylenes, ladder polymers and graphene nanoribbons.