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

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.