Herein, we report for the first time the underlying mechanism of realizing a bipolar rectifier from a donor-σ-acceptor system (11- mercaptoundeca-2,4,8,10-tetraenenitrile) in a configuration where the spins of the magnetic electrodes are kept parallel. To find out the role of the σ bond on the unique spin polarized quantum transport properties, we have critically examined how a shift in the position of the σ bond in the same molecular skeleton can lead to the transformation of a bipolar rectifier to a unipolar one. It has also been noticed that the addition of another σ bond in the said molecular skeleton can change the transport properties further. In this case, the new device shows bias specific rectification in both the spin channels and in particular, the rectification ratio (42) associated with the down spin channel is appreciably high. Our analyses reveal that both the position and number of the σ bond can modify the relative abundance of the 2p-density of states of the individual carbon atoms near the Fermi level of the left and right side Fe (100) electrodes which actually is accountable for the realization of such interesting device characteristics. The projected device density of states, transmission spectra and transmission eigenstates also corroborate nicely with the present in-silico observation. All the calculations are performed with density functional theory based nonequilibrium Green functions technique. © 2017 American Chemical Society.