Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 2 / Section A / 18 / d / Solution

C has a saturated backbone and pendant $\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ groups in its second block. Hydrogenation converts every backbone $\mathrm{CH}=\mathrm{CH}$ to $\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2$; lithium aluminium hydride then reduces each $\mathrm{C}{\mathrm{O}}_2\mathrm{Me}$ to $\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$. The resulting connectivity is $[\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2-{\mathrm{C}}_5{\mathrm{H}}_8{]}_{50}-[\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2-{\mathrm{C}}_5{\mathrm{H}}_7(\mathrm{C}{\mathrm{H}}_2\mathrm{OH}){]}_{75}$.

Hydrogenation removes local double-bond rigidity and commonly lowers the segmental barrier to motion, favouring a lower glass transition temperature. Ester reduction changes polarity and side-group volume, while the new hydroxyl groups introduce hydrogen bonding, which can raise the hydroxyl-rich block's glass transition temperature and strengthen segregation. These competing effects preclude a numerical transition shift or an unconditional direction for both transitions from structure alone.

The repeat-unit formulae after both operations are ${\mathrm{C}}_7{\mathrm{H}}_{12}$ and ${\mathrm{C}}_8{\mathrm{H}}_{14}\mathrm{O}$. Taking standard atomic masses,This is approximately 14.27 kg mol${}^{-1}$ plus the net end-group mass. The termination chemistry is unspecified, so a more precise whole-chain mass is not determined; the methyl group lost during ester reduction must not remain in the second repeat's mass.