Past exam of the chemistry B course of the University of Cambridge / 2023 / Paper 2 / B9

The unequal P–N distances reflect unequal local bonding interactions in the alternating-chain conformation, rather than an extended conducting P=N pi system. nitrogen is much more electronegative than phosphorus, so the backbone sigma bonds are strongly polar. nitrogen lone-pair density can also donate into appropriately oriented antibonding P–substituent orbitals: negative hyperconjugation, schematicallyThe strengths of these interactions depend on the orientation of the donor and acceptor orbitals. In a planar cis–trans backbone, the P–N links are not all geometrically equivalent. Different overlap and charge redistribution give alternating greater and smaller effective P–N bond orders, hence shorter and longer bonds. A formal alternating P=N/P–N drawing is a useful shorthand for this inequality, but does not imply that isolated textbook single and double bonds are present with no electronic communication.

The historical island picture captures the limitation of pi interaction to local P–N–P regions rather than a continuous band along the whole chain. A modern description need not invoke strong phosphorus $3d$–nitrogen $2p$ bonding; polar bonding and donation into sigma-star orbitals provide the useful explanation. Calculations on oligomers found bond alternation for planar cis–trans conformations but not for optimized helices. Bond alternation is therefore conformation dependent, not a universal consequence of the repeat formula alone.^[ref]

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.

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.

polysulfur has a chain of S–S sigma bonds with filled lone-pair orbitals. Its occupied states do not supply a partly filled delocalized band, so the chain is an electrical insulator.

In polythiazyl, an approximately planar S–N chain permits overlap of orbitals perpendicular to the backbone. In an elementary pi bond electron picture, nitrogen supplies one electron and sulfur two to this manifold: three pi electrons per SN unit, rather than a completely filled, isolated two-centre bonding pair. The extended states are therefore not all fully occupied. More realistic descriptions include the crystallographic repeat, interchain interactions and overlapping bands crossing the Fermi level; the odd electron count of a chosen chemical repeat is suggestive, not by itself a proof of metallicity.

The essential distinction is occupied, localized bonding/lone-pair states in sulfur chains versus available extended states at the Fermi level in crystalline sulfur nitride. The measured high conductivity along the polymer fibres and its metallic temperature dependence provide the experimental basis, alongside the structural evidence for the chain. Conductivity depends on order and defects as well as chemical composition.^[ref]

First hydrolyse both Si–Cl groups to give a silanol of monomer functionality two, then condense Si–OH groups to form the Si–O–Si backbone:monomer functionality two favours chains, with cyclic oligomers also possible. Removal of condensation products or equilibration under appropriate conditions favours high molar mass. The hydrolysis and condensation can overlap in a one-pot process; the equations distinguish their chemical functions rather than requiring isolation of every silanediol.

R${}_3$SiCl acts as a monofunctional chain termination reagent. hydrolysis gives R${}_3$SiOH, which forms a terminal R${}_3$Si–O unit but cannot continue growth through a second silanol group. Increasing its proportion therefore increases the number of capped ends and lowers the attainable average chain length. The resulting material generally has lower viscosity and less chain entanglement, rather than acquiring a cross-linked network merely from end-capping.

RSiCl${}_3$ introduces branching and, at sufficient conversion and loading, cross-links. hydrolysis gives RSi(OH)${}_3$, with three possible siloxane connections per silicon. Incorporation links chains into a network, suppressing flow and usually increasing stiffness and dimensional retention. A sufficiently connected permanent network behaves as a thermoset or cross-linked elastomer rather than an ordinary flowing linear polymer. A small concentration need not immediately produce a macroscopic gel; conversion and network connectivity matter.

Prepare a chlorophosphazene ring from phosphorus pentachloride and ammonium chloride, isolate the cyclic trimer, open the rings to make the linear chloropolymer, and finally replace P–Cl by P–OEt:The first stage can produce a mixture of cyclic oligomers, so isolation of hexachlorocyclotriphosphazene is important. Controlled thermal ring-opening polymerization, conventionally around $250{}^{\circ }\mathrm{C}$ for the classical route, gives soluble linear polydichlorophosphazene before excessive cross-linking. Moisture exclusion avoids uncontrolled hydrolysis of the reactive P–Cl precursor.

The target is poly[bis(ethoxy)phosphazene]. The final step is nucleophilic substitution with an ethoxide salt in a compatible dry medium, replacing two chlorides per phosphorus. Polymerize the chlorinated ring first, then substitute the linear polymer: complete ethoxylation of the isolated ring does not by itself produce the requested high polymer. The primary macromolecular-substitution work explicitly includes the ethoxy derivative.^[ref]

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.