= Solution
\b[A is poly(norbornene); B is the corresponding sequential <block copolymer> with a methyl-ester-substituted second block.] The process is <ring-opening metathesis polymerization>. Each strained bicyclic <alkene> becomes a cyclopentane unit whose two backbone attachment positions are joined to neighbouring units through $\mathrm{-CH=CH-}$ links. The second block retains its $\mathrm{CO_2Me}$ substituent on the cyclopentane framework. Thus a useful connectivity representation is $\mathrm{end-[CH=CH-C_5H_8]_{n}-[CH=CH-C_5H_7(CO_2Me)]_{m}-end}$, with the cyclopentane attachment positions at 1 and 3. This representation specifies connectivity rather than a single backbone <alkene> geometry.
The ruthenium <alkylidene> reacts with a <monomer> <alkene> through a <metallacyclobutane>, then fragments with the other pair of bonds broken. A new ruthenium <alkylidene> remains at the growing end, allowing propagation and then addition of the second <monomer>. Relief of the bicyclic <monomer>'s <ring strain> supplies the principal enthalpic driving force; the loss of <monomer> translational <entropy> opposes <polymerization>. Sequential addition yields blocks only if the first population of active ends survives until the second addition.
\Image[/past-exam-of-the-chemistry-course-of-the-year-4-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-2-polymer-connectivity.png]
{title=Original polymer connectivity fragments and the reductive ozonolysis product}
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