Disclosed are polymerization initiators as may be utilized for addition of polymers to a substrate surface. The initiators are azo-based initiators that include multi-functionality through addition of multiple anchoring agents to an inner azo group. Disclosed polymerization initiators can be utilized to form high density and high molecular weight polymers on a surface such as a particulate surface. Formed materials can be beneficial in one embodiment in fracking applications, providing composite proppant/polymer materials that can prevent leakage of polymers from a subterranean geologic formation.