We have developed an approach at the Hartree-Fock level by which it is possible to calculate the electronic structures of large polymers with or without periodic sequences systematically. This elongation method is based on the concept of a cluster-series calculation which means the successive connection of cluster molecules at the molecular orbital level in approximating a large polymer as a cluster molecule. It has already been reported that we can extract the periodic condition of the electronic states within the series of extended clusters by using the cluster-series model. Recently, we tried to introduce the elongation method into the program package of semiempirical molecular orbital methods MOPAC. In the present paper, we report results of applications to the calculations of three polymer systems by using AM1 parameters, that is, the first system is the periodic polymer, the second is the interface between two blocks in a polymer chain, and the third is the local defect within a periodic polymer. In calculations of periodic polymers, clusters of polyethylene, polytetrafluoroethylene, polyacetylene, or polydifluoroacetylene were elongated in one direction, and the interfaces between the above polymer blocks with ethylene- or acetylene-type chain were dealt with by the two-directional elongation method. Also, the solitonic structures with one plus or minus charge within polyacetylene chain were created in elongation calculations of the bidirectional extension as models for the local defect in a periodic polymer. Moreover, we discussed periodic states of electronic structures in these systems from cluster-series calculations.