Poly(phenyleneethynylene)s (PPEs) capable of reacting with thiol-containing molecules have been designed and synthesized with number-average molecular weights ranging from 8000 to 11000. The PPEs contain pendent masked maleimide units available to participate in conjugate additions and Diels-Alder chemistries after being thermally activated. If the maleimide group is left unmasked during palladium-catalyzed cross-coupling polymerization, it leads to side reactions and the production only of short chain oligomers (M-n < 3000 DP similar to 4). Reversible Diels-Alder reactions between the maleimides and furan were determined to be a very effective method to produce a masked Michael acceptor that call be unveiled after polymerization under relatively mild thermal conditions. Cycloreversion to the malcimide has been monitored by thermogravimetric analysis (TGA), TGA-MS, IR, and NMR. A PPE containing, the masked maleimide unit has been modified with a thiolated carboxy-X-rhodaimine (ROX) dye, and the resulting absorbance and fluorescence spectra as well as gel permeation chromatograms (GPC) are presented.