= Solution
\b[The ideal number-average molar mass is approximately 25.11 kg mol$^{-1}$.] Quantitative uptake of 100 lactide and 100 styrene molecules per bifunctional initiator gives
$$
\boxed{M_n=M_B+100M_{\rm lactide}+100M_{\rm styrene}\simeq277.41+100(144.13)+100(104.15)=25105.4\ \mathrm{g\,mol^{-1}}.}
$$
B has formula $\mathrm{C_{17}H_{27}NO_2}$. Lactide's entire cyclic diester mass is incorporated; multiplying its molar mass by two again would double-count its two lactyl units. The estimate assumes quantitative initiation, conversion, nitroxide recapping and no chain transfer or loss of chain-end fragments.
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