Prediction of Chain Propagation Rate Constants of Polymerization Reactions in Aqueous NIPAM/BIS and VCL/BIS Systems

Microgels
have a wide range of possible applications and are therefore
studied with increasing interest. Nonetheless, the microgel synthesis
process and some of the resulting properties of the microgels, such
as the cross-linker distribution within the microgels, are not yet
fully understood. An in-depth understanding of the synthesis process
is crucial for designing tailored microgels with desired properties.
In this work, rate constants and reaction enthalpies of chain propagation
reactions in aqueous N-isopropylacrylamide/N,N′-methylenebisacrylamide and
aqueous N-vinylcaprolactam/N,N′-methylenebisacrylamide systems are calculated
to identify the possible sources of an inhomogeneous cross-linker
distribution in the resulting microgels. Gas-phase reaction rate constants
are calculated from B2PLYPD3/aug-cc-pVTZ energies and B3LYPD3/tzvp
geometries and frequencies. Then, solvation effects based on COSMO-RS
are incorporated into the rate constants to obtain the desired liquid-phase
reaction rate constants. The rate constants agree with experiments
within a factor of 2–10, and the reaction enthalpies deviate
less than 5 kJ/mol. Further, the effect of rate constants on the microgel
growth process is analyzed, and it is shown that differences in the
magnitude of the reaction rate constants are a source of an inhomogeneous
cross-linker distribution within the resulting microgel.