A compound (monomer) comprising i) one or more arylethynyl groups (A-functional groups), ii) one or more ring structures comprising two conjugated carbon-to-carbon double bonds and a leaving group L (B-functional groups), and iii) one or more ethynyl groups (C'-functional groups), characterized in that said A- and C'-functional groups are capable of reaction under cycloaddition reaction conditions with said B-functional groups to thereby form a cross-linked, polyphenylene polymer.