Synthesis of polymers in nanoreactors: A tool for manipulating polymer properties

The use of nanoreactors to confine monomers and synthesize polymers results in changes in the reaction kinetics and polymer properties making nanoconfinement a potential tool for manipulating and engineering polymer properties. In this perspective, we cover conventional nanoconfinement hosts, nanopore-confined free radical, step-growth, and ring-opening polymerizations, and changes in molecular weight, tacticity, glass transition temperature (T_g), thermal stability, and electrical properties. We use examples from research in our laboratory, as well as comparisons of the work in the literature, to illustrate the competing forces that drive these changes, namely molecular layering or orientation at the nanopore surface, decreased molecular and segmental diffusion, and catalytic or inhibitory effects caused by chemical moieties on the native or surface-functionalized nanopore surface. The majority of nanoconfined polymerizations are found to be accelerated, and in the case of free radical polymerizations to generally yield higher molecular weights and higher isotacticity. T_gs for the nanoconfined polymers tend to increase if strong interactions exist between the polymer and the confinement surface, but depressions are observed for confined polycyanurates; the importance of removing unreacted monomer and comparing results to the bulk material of same molecular weight and structure is emphasized. Examples are also provided of enhanced thermal stability and conductivity of polymers synthesized under nanoconfinement.