= Solution
\b[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.
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