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The spin-exchange effect in spin-polarized electron collisions withunpolarized open-shell molecules, O$_2$, B$_2$, S$_2$ and Si$_2$, has beenstudied by the R-matrix method with the fixed-bond approximation. All of thesemolecules have ${}^3 \Sigma_{g}^{-}$ symmetry in their ground states. Usualintegrated cross sections with unpolarized electrons has also been studied. Weused the complete active space self consistent field orbitals and put more than10 target electronic states in the R-matrix models. In electron O$_2$ elasticcollisions, calculated polarization fractions agree well with the experimentalresults, especially around the ${}^4 \Sigma_u^-$ resonance. In e-B$_2$, S$_2$and Si$_2$ elastic collisions, larger spin-exchange effect is observed comparedto the e-O$_2$ elastic collisions. In all four cases, spin-exchange effectbecomes prominent near resonances. This association of resonance and magnitudeof the spin-exchange effect was studied by explicitly removing the resonanceconfigurations from the R-matrix calculations. In general, spin-exchange effectis larger in e-B$_2$ collisions than in e-S$_2$ and Si$_2$ collisions, and issmallest in e-O$_2$ collisions.

Exchange effects in elastic collisions of spin-polarized electrons with open-shell molecules with $^3\Sigma_g^-$ symmetry