Which monomer is best suited to anionic polymerisation initiated by $\mathrm{NaNH}{}_2$ in liquid ammonia?

Anionic chain carriers need a substituent that stabilises the carbanion, such as $-\mathrm{CN}$ (electron withdrawing). The alkyl- and alkoxy-substituted alkenes are electron rich and favour cationic or Ziegler-Natta routes instead.