Guest-Controlled Topochemical Polymerization and Dimerization of Macrocyclic Pillar[5]arene

Abstract Topochemical polymerization enables precise control over polymer structure by confining reactive sites within a preorganized crystal packing. However, accessing a different product generally requires redesign of the monomer. Here, we report a host–guest strategy that regulates the outcome of a photoinduced topochemical reaction without altering the monomer structure. We synthesized a macrocyclic pillar[5]arene monomer bearing polymerizable 1,3-diene groups. In the crystalline state, this monomer formed one-dimensional columns, and its packing mode was found to vary depending on the length of the encapsulated guest molecules. When 1,2-dichloroethane was encapsulated in the monomer cavity, its short length allowed the reactive 1,3-diene groups to align closely within the column. As a result, topochemical polymerization proceeded through a single-crystal-to-single-crystal transformation, affording a covalently linked one-dimensional polymer. By contrast, when the monomer cavity was occupied by the longer guest adiponitrile, the packing mode was altered, leading to preferential intercolumn dimerization. These results demonstrate that host–guest chemistry provides a distinct strategy for controlling the products of topochemical reactions.