The intrinsic soft lattice nature of organometal halide perovskites (OHPs) makes them very tolerant to defects and ideal candidates for solution-processed optoelectronic devices. However, the soft lattice results in low stability towards external stresses such as heating and humidity, and induces high density of phonons, causing strong electron-phonon coupling. Here, we report solid-state polymerization of OHPs using unsaturated 4-vinylbenzylammonium as organoammonium cations without damaging perovskite structure and its tolerance to defects. The polymerized perovskites show enhanced stability and flexibility. Furthermore, the polymerized 4-vinylbenzylammonium group improves perovskite lattice rigidity substantially, resulting in reduced electron-phonon coupling and non-radiative recombination rate, and enhanced carrier mobility because of suppressed phonon scattering. We finally demonstrate efficient polymerized perovskite based light-emitting diodes with an external quantum efficiency of 23.2% and enhanced operation stability.