  We propose a general technique to produce cold spin-polarized molecules, in
which rotationally excited states are first populated by coherent microwave
excitation, and then allowed to spin-flip and relax via collisional quenching,
which populates a single final spin state. We illustrate the high selectivity
of the technique for $^{13}$C$^{16}$O molecules immersed in a cold buffer gas
of helium atoms, achieving a high degree ($\geq$95\%) of nuclear spin
polarization at 1 K.
