(51c) Kinetics and Thermodynamics in Nanoconfined Polymerizations
The behavior of materials confined at the nanoscale has been of considerable interest over the past several decades. We have recently investigated the effects of nanoconfinement on polymerization kinetics and thermodynamics for various classes of polymerization, including trimerization of cyanate esters, free radical reaction of methacrylates, and ring-opening polymerization of dicyclopentadiene. Changes in reaction rates under confinement can generally be explained by changes in local packing, diffusivity, and surface effects, whereas changes in monomer/polymer equilibria reflect entropic effects. One goal of the work is to test reports in the literature for how diffusivity and chain entropy scale with nanoconfinement size and molecular size. For example, the early onset of autoacceleration in nanoconfined methyl methacrylate polymerization is well described by a model assuming that the diffusion coefficient scales with molecular size to the -3 power and with nanopore diameter to the 1.3 power. On the other hand, modeling the literature data from Yannopoulos and coworkers for the equilibrium polymerization of sulfur, we find that confinement entropy scales with molecular size to the second power and with nanopore diameter to either the -3.0 or -3.8 power, the former of which fits slightly better. In addition, polymer properties can be influenced by synthesis under nanoconfinement, including significant increases in isotacticity and molecular weight for poly(methyl methacrylate). Implications will be discussed.