Diffusion coefficients of the monomer and oligomers in hydroxyethyl methacrylate

The diffusion coefficients are reported of rubbery ternary systems consisting of the polymer, its monomer analogue (i.e., the saturated equivalent of the monomer), and trace quantities of oligomers (dimer, trimer, tetramer and hexamer) for 2-hydroxyethyl methacrylate (HEMA). These have been obtained with pulsed-field-gradient NMR spectroscopy with a polymer weight fraction (fp) of 0 ≤ fp ≤ 0.4. The oligomers are macromonomers synthesized with a cobalt catalytic chain-transfer agent. The diffusion coefficients are about an order of magnitude smaller than those for monomers such as methyl methacrylate; this effect is ascribed to hydrogen bonding in HEMA. The diffusion coefficient Di of an i-meric oligomer has been fitted with moderate accuracy by an empirical universal scaling relation, Di(fp)/D1(fp) ≈ i-(0.66+2fp), previously found to provide an adequate fit to corresponding data for styrene and for methyl and butyl methacrylates. The approximate empirical scaling relation seems to hold for a remarkably wide range of types of monomer/polymer systems. These results are of use in modeling rates and molecular weight distributions in free-radical polymerization, particularly for termination (which is chain-length-dependent and is controlled by the diffusion coefficient of chains of the low degrees of polymerization studied here).