Differently, the discovered polymers from Dataset D utilize a rigid ladder-type backbone with a spirobifluorene (SBF) unit, like many other ladder-type PIMs . The fused benzene rings in the SBF unit reduce the flexibility of the backbone around the spirocenter, and the two-bond ladder connections restrict the rotation ability of the backbone. The reduced chain flexibility may also prohibit chain motion to help resist physical aging . To further increase permeability, P-DNN-D1 and P-DNN-D2 attach a fused tetramethyltetrahydronaphthalene (TMN) to the SBF unit, which incorporates additional aliphatic rings and methyl substituents .