Sterling et al. suggest delocalisation effects onto the three-membered bridges relaxing Pauli-repulsion and thus stabilising the propellane core. Reactivity Propellanes, especially the synthetically studied [1.1.1]Propellane, is known to possess omniphilic reactivity. Anions and radicals add towards the interbridgehead bond resulting in bicyclo[1.1.1]pentyl-units. In contrary, cations and metals decompose the tricyclic core towards monocyclic systems by opening of the bridged bonds forming exo-methylene cyclobutanes. For [3.1.1]propellane only radical addition is reported. The reactivity of other propellanes is far less explored and their reactivity profile is less clear. Polymerization In principle, any propellane can be polymerized by breaking the axial C–C bond to yield a radical with two active centers, and then joining these radicals in a linear chain. For the propellanes with small cycles (such as [1.1.1], [3.2.1], or 1,3-dihydroadamantane), this process is easily achieved, yielding either simple polymers or alternating copolymers. For example, [1.1.1]propellane yields spontaneously an interesting rigid polymer called staffane; and [3.2.1]propellane combines spontaneously with oxygen at room temperature to give a copolymer where the bridge-opened propellane units [–C8H12–] alternate with [–O–O–] groups. Synthesis The smaller-cycle propellanes are difficult to synthesize because of their strain. Larger members are more easily obtained. Weber and Cook described in 1978 a general method which should yield [n.3.3]propellanes for any n ≥ 3. Members True propellanes [1.1.1]Propellane, C5H6, CAS number (K. Wiberg and F. Walker, 1982). It is a highly strained molecule: the two central carbons have an inverted tetrahedron geometry, and each of the three cycles is the notoriously strained cyclopropane ring. The length of the central bond is only 160 pm. It is an unstable product that undergoes thermal isomerization to 3-methylenecyclobutene at 114 °C,and spontaneously reacts with acetic acid to form a methylenecyclobutane ester. Several synthetic procedures are established making it accessible on scales useful for synthesis to derive bicyclo[1.1.1]pentane which are used a bioisosteres for para-substituted arene systems. [2.1.1]Propellane, C6H8, CAS number (K. Wiberg, F. Walker, W. Pratt, and J. Michl). This compound was detected by infrared spectroscopy at 30 K but has not been isolated as a stable molecule at room temperature (as of 2003). It is believed to polymerize above 50 K. The bonds of the shared carbons have an inverted tetrahedral geometry; the compound's strain energy was estimated as 106 kcal/mol. [2.2.1]Propellane, C7H10, CAS number (F. Walker, K. Wiberg, and J. Michl, 1982). Obtained gas-phase dehalogenation with alkali metal atoms. Stable only in frozen gas