This invention relates to non-uniformly substituted ("blocky") hydroxyethylcelluloses (HECs) and derivatives thereof that exhibit associative behavior in both neat solutions and in filled systems. The HECs and derivatives thereof exhibit unique and highly desirable rheology and are more efficient in thickening aqueous systems than prior art HEC products. These blocky HECs can be distinguished from prior art and commercial HEC products by having an unsubstituted anhydroglucose trimer ratio (U3R) greater than 0.21 and the hydroxethyl molar substitution greater than about 1.3 and less than about 5. This invention also relates to processes for making blocky HEC and uses thereof in functional systems.