Cross-linked polymeric gels are widely used in applications ranging from biomaterial scaffolds to additives in enhanced oil recovery. Despite this, fundamental understanding of the effect of polymer concentration and reaction mechanism on the scaffold structure is lacking. We measure scaffold properties and structure during gelation using multiple particle tracking microrheology. To determine the effect of concentration, we measure gelation as polymer interactions are increased in the backbone precursor solution: below, at and above the overlap concentration, c*. To determine structural changes due to the gelation mechanism, we measure gelation between the same polymers undergoing both step- and chain-growth reactions using self-assembling maleimide:thiol and photo-initiated acrylate:thiol chemistries, respectively. We determine the critical relaxation exponent, n, a measure of structure. n decreases with increasing concentration, indicating a change from a percolated (c < c*) to a tightly cross-linked network (c* < c). The gelation mechanism does not have a measurable effect on the scaffold structure. (C) 2018 American Institute of Chemical Engineers