Poly(phenyleneethynylene)s (PPEs) capable of reacting with thiol-contg. mols. have been designed and synthesized with no.-av. mol. wts. ranging from 8000 to 11,000. The PPEs contain pendent 3a,4,7,7a-tetrahydro-4,7-epoxy-1H-isoindole-1,3(2H)-dione groups available to participate in conjugate addn. and Diels-Alder chem. after being thermally activated. If the maleimide group is left unmasked during palladium-catalyzed cross-coupling polymn., it leads to side reactions and the prodn. only of short chain oligomers (Mn < 3000, DP ∼ 4). Reversible Diels-Alder reactions between the maleimides and furan were detd. to be a very effective method to produce a masked Michael acceptor that can be unveiled after polymn. under relatively mild thermal conditions. Cycloreversion to the maleimide has been monitored by thermogravimetric anal. (TGA), TGA-MS, IR, and NMR. A PPE contg. the masked maleimide unit has been modified with a thiolated carboxy-X-rhodamine (ROX) dye, and the resulting absorbance and fluorescence spectra as well as gel permeation chromatograms (GPC) are presented. [on SciFinder(R)]