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Joined 2026-09-21 Message
Incoming links: Polysiloxane
Past exam of the chemistry B course of the University of Cambridge 2023 Paper 2 B9 a iii Solution 2026-10-20
Neither backbone has a partially filled, extended electronic band available for easy charge transport. In polysiloxanes the bonding and nonbonding states are filled and separated from empty states by a large band gap. phosphazene local lone-pair interactions do not create the continuously delocalized, partially occupied pi band suggested by a naive P=N drawing. Their flexibility or formal multiple-bond notation does not imply electronic conductivity. This comparison concerns the undoped materials, not conductivity supplied by added mobile ions.
Past exam of the chemistry B course of the University of Cambridge 2023 Paper 2 B9 a ii Solution 2026-10-20
Successive Si–O links have the same local bonding role, unlike the unequal links selected by an alternating phosphazene conformation. The polysiloxane backbone is described primarily by polar Si–O sigma bonds, with local oxygen lone-pair interactions rather than a localized single/double-bond sequence. There is therefore no alternating Si–O bond-order pattern that requires two characteristic distances. Equal chemical environments give equal bond lengths in the ideal repeating structure; local strain or packing can perturb individual distances.
Past exam of the chemistry B course of the University of Cambridge 2023 Paper 2 B9 d Solution 2026-10-20
The chemical requirement for permanence is retention of the backbone and network in a wet environment. A suitably prepared polysiloxane has strong Si–O bonding, relatively slow backbone cleavage under mild aqueous conditions, and flexible chains that can be retained in a cross-linked elastic network. In the comparison intended here, hydrolysis of susceptible phosphazene substituents and P–N linkages leads to chain scission, reduced molar mass and loss of mechanical integrity, which conflicts with permanence.
This is a comparison of chemical durability, not a universal statement that every polyphosphazene degrades rapidly or that every polysiloxane is indefinitely inert. phosphazene side groups can strongly change access of water and hydrolysis rates; siloxanes can also be cleaved under sufficiently acidic or basic conditions. Those structural and kinetic qualifications matter even before any biological suitability is considered.