The effect of structural changes in regiorandom poly(3-dodecylthiophene) (PDDT) on the interactions of polarons was studied. It was found that changes in side-chains and in the entire polymeric system, expressed by the glass transition temperature T g (~230K), affected the system's magnetic properties. Structural changes observed in the range of T~100-300K are dominated by processes dissipating magnetic energy. At low temperatures (T<100K) energy dissipating processes were not observed, rather magnetic properties were determined by transitions of polarons to bipolarons. Overall the material behaves as a paramagnet, with the curve (susceptibility versus temperature) characteristic for antiferromagnetics.