Here we report the kinetics of the surface-initiated atom transfer radical polymerization (ATRP) of styrene from the surface of functionalized montmorillonite clay as a function of graft density. Compared with analogous ATRP reactions with free initiator, we observe a seven-fold increase in the polymerization rate at the highest graft density, 1 chain/nm2, whereas bulk kinetics are recovered as the graft density is reduced. We hypothesize that this phenomenon is a consequence of local concentration heterogeneities that shift the ATRP equilibrium in favor of the active state and present a simple phenomenological kinetic model that accounts for our data. These findings present an important consideration relevant to the design of precisely defined molecular architectures from surfaces via surface-initiated ATRP. Disciplines Polymer Science CommentsReprinted with permission from Macromolecules 42 (2009) ReceiVed NoVember 6, 2008; ReVised Manuscript ReceiVed February 3, 2009 ABSTRACT: Here we report the kinetics of the surface-initiated atom transfer radical polymerization (ATRP) of styrene from the surface of functionalized montmorillonite clay as a function of graft density. Compared with analogous ATRP reactions with free initiator, we observe a seven-fold increase in the polymerization rate at the highest graft density, ∼1 chain/nm 2 , whereas bulk kinetics are recovered as the graft density is reduced. We hypothesize that this phenomenon is a consequence of local concentration heterogeneities that shift the ATRP equilibrium in favor of the active state and present a simple phenomenological kinetic model that accounts for our data. These findings present an important consideration relevant to the design of precisely defined molecular architectures from surfaces via surface-initiated ATRP.