Alkali and earth-alkali salts of dicyclopentadiene dicarboxylic acid (DCP-DCA) were prepared and employed as monomers in the polyesterification with an alpha,omega-dihalide monomer, such as 1,4-dichlorobutane (DCB), 1,4-dibromobutane (DBB), alpha.alpha'-dichloro-p-xylene (DCX), and alpha,alpha'-dibromo-xylene (DBX). Novel linear polymers that possessed repeating moieties of dicyclopentadiene (-DCPD-) in the backbone were thus prepared. The IR and NMR spectra indicated that poly(tetramethylene dicyclopentadiene dicarboxylate) (PTMDD) with a number-average molecular weight (M-n) of about 1 x 10(4) and poly(p-xylene dicyclopentadiene dicarboxylate) (PXDD) with a M-n of 4-6 x 10(3) were obtained with an yield of about 80% via the polyesterification of the alkali salts with DBB and DCX, respectively. The reaction was carried out in the presence of a phase transfer catalyst, such as BzMe(3)NBr or poly(ethylene glycol), in DMF at 100 degrees C for 4 h. Oligomers with a lower M-n (1-2 x 10(3)) were obtained when the earth-alkali salts were employed as salt monomers. Compared to the irreversible linear polymers, poly(p-xylene terephthalate) (PXTP) and poly(p-xylene maleate) (PXM), prepared through the reaction between DCX and the potassium salts of terephthalic and maleic acid, respectively, the specific viscosities (eta(sp)) of the new linear polymers increased abnormally with the decrease of the temperature from 200 degrees C to 100 degrees C. This occurred due to the thermally reversible dedimerization/redimerization of-DCPD-moieties of the backbone of the polymers via the catalyst-free Diels-Alder/retro Diels-Alder cycloadditive reactions. The ratio of the eta(sp) at 100 degrees C and 200 degrees C of the reversible polymers was found to be much higher than that of PXTP and PXM, even when the heating/cooling cycle was carried out several times under a N-2 atmosphere. The obtained results indicated that thermally reversible covalently bonded linear polymer can be obtained by introducing the -DCPD- structure into the backbone of the polymer through the polymerization of a monomer containing the -DCPD- moiety. The reversible natures of the polymers and oligomers might be useful in preparing easily processable and recyclable polymers and thermosensor materials.