Metal-insulator transition and emergence of spontaneous polarization in (La$_{1-x}$Sr$_x$MnO$_3$)$_m$/(LaNiO$_3$)$_n$ superlattices constructed from metallic building blocks*

La$_{0.7}$Sr$_{0.3}$MnO$_3$ and LaNiO$_3$ are metallic oxides. However, short-period superlattices of the form (La$_{0.7}$Sr$_{0.3}$MnO$_3$)$_m$ /(LaNiO$_3$)$_n$ show insulating behavior depending on $m$ and $n$. In particular, the insulating property is robust when $m=n=2$ (SL 2/2). Here, using first-principles density functional theory (DFT) and DFT+U (static $d-d$ Coulomb interaction), we explain the experimental observation and discuss a general mechanism that underlies such metal-insulator transition for different $m$ and $n$. The general mechanism is based on the finding that disproportionation on Ni sites is the key. Further, we predict that insulating SL 2/2 is ferroelectric with large spontaneous polarization. The ferroelectric distortion persists in the cases where the superlattices are metallic, which leads to the possibility of designing a new family of 'polar metals'. Finally, we discuss the origin of such polar distortion and its coupling with the magnetic properties (by considering spin-phonon coupling) of the material.