Many industrially important polymers are prepared by radical polymerisation of water-soluble monomers. Despite the industrial importance of these polymers, knowledge of parameters required for effective control of the structure and hence properties thereof remains inadequate. These monomers often show a very strong but counter-intuitive dependence of propagation rate, kₚ, on concentration in water, with rates at low monomer concentration as much as an order of magnitude faster than at high concentration.
The aim of the research is to establish why the propagation rate coefficient for a large number of moderately-hydrophilic monomers is higher at low concentrations than in aqueous solution, but approaches the bulk value at higher concentrations. The hypothesis is that partitioning effects – caused by the robust short-range structure of water molecules – causes this anomalous behaviour in the propagation rate coefficient, restricting the monomer to regions of disordered water at low concentrations.
Molecular dynamics (MD), small-angle neutron scattering (SANS) and quasi-elastic neutron scattering (QENS), were utilised to investigate the structure and dynamics of two such monomer, N-vinyl pyrrolidone and N,N-dimethylacrylamide, at different concentrations in order to test the hypothesis. Experiments were conducted at 25°C and concentration ratios of 1-10, 1-50, 1-200 monomer to water molecule number ratios as well as bulk monomer, and an additional concentration ratio of 1-100 was also investigated using only MD simulations.
Results indicate an increase in the size of monomer clusters with increase in concentration, but that there is little disruption of the short-range order of the water molecules, suggesting that monomer is excluded from ordered regions. The rate of long-range diffusion was found to increase with decreasing monomer concentration, of both monomer and water molecules. However, this change in rate of dynamics alone is not sufficient to explain the magnitude of the observed kₚ behaviour. Radial distribution functions calculated from MD simulations indicate that monomer-water hydrogen bond-ing is stronger than water-water hydrogen bonding, and previous studies have shown that the nearby C=C group’s reactivity is affected by neighbouring hydrogen bonds. As concentration increases, the monomer cluster sizes increases and the amount of hydrogen bonding decreases. As the reactivity of the C=C group affects the propagation rate, the anomalous propagation rates observed are likely due to partitioning affects, with a combination of micro-domains, reduced monomer-water hydrogen bonding and slower long-range dynamics.