J. Am. Chem. Soc. http://doi.org/b69r (2017)
Because they lack end groups, cyclic polymers have intrinsically different physico-chemical properties than their linear counterparts. But achieving cyclic polymers usually requires a major synthetic effort; as a result, only a limited number have been reported. Now, Aoki et al. have devised a route to synthesize cyclic polymers with quantitative cyclization yield.
There are two main strategies to make cyclic polymers: cyclization of a linear polymer or ring-expansion of a cyclic molecule. The first strategy is marred with low selectivity during the end-to-end coupling, whereas the second usually requires multiple synthetic and purification steps. Aoki et al. propose to start with a short linear oligomer containing an ammonium group and ending with a crown ether macrocycle. Two of such molecules self-assemble quantitatively to form a [c2] daisy chain, where the oligomer threads inside the crown ether thanks to the interaction between the crown ether and the ammonium group. Then, a living polymerization reaction elongates the initial oligomer to a desired extent. Finally, the reaction is quenched by the introduction of a bulky end group with a urethane moiety. At this point, the ammonium group is oxidized to release the crown ether, sliding it to the urethane groups, yielding the final cyclic polymer. Aoki et al. demonstrate gram-scale synthesis of these polymers with 72% yield.