 Solution

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 $-\mathrm{CH}=\mathrm{CH}-$ links. The second block retains its $\mathrm{C}{\mathrm{O}}_2\mathrm{Me}$ substituent on the cyclopentane framework. Thus a useful connectivity representation is $\mathrm{end}-[\mathrm{CH}=\mathrm{CH}-{\mathrm{C}}_5{\mathrm{H}}_8{]}_{\mathrm{n}}-[\mathrm{CH}=\mathrm{CH}-{\mathrm{C}}_5{\mathrm{H}}_7(\mathrm{C}{\mathrm{O}}_2\mathrm{Me}){]}_{\mathrm{m}}-\mathrm{end}$, 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.