There are two kinds of conjugated polymers

the degenerate-ground-state (DGS) polymers and the non-degenerate (NDGS) ones．For the latter

the confinement effect induces self-trapped excitons (STE) and biexciton (STB)S

and produces photoinduced polarization reversion (PPR)

when the polymer is photoexcited．Some new kinds of polymers such as PDPA have been synthesized recently．This kind of polymers has degenerate-ground-states and does not possess confinement effect and self-trapped exciton．However

the length of the real polymer chain is finite

the soliton and antisoliton in the polymer chain cannot be separated far away．It is proved that such soliton-antisoliton pair in DGS polymers can also produce negative polarization and photoinduced polarization reversion．