ABSTRACT: The heterogeneous atom transfer radical polymerization (ATRP) of tert-butyl methacrylate (tBMA) using CuX/N,N,N',N",N"- pentamethyldiethylenetriamine (PMDETA) catalytic system with various initiators R-X (X) Br, Cl) was investigated. The importance of the structure of the initiator on the polymerization of tBMA was briefly examined. Polymerization of tBMA with ethyl 2-bromoisobutyrate resulted in uncontrolled polymerization due to slow initiation because of the low back strain effect and relatively high propagation rate constant of tBMA. Well-controlled polymers were obtained with p-toluenesulfonyl chloride and 2,2,2-trichloroethanol initiators. This can be explained by the fast initiation coupled with the rapid establishment of the equilibrium between the active and dormant species in the polymerization. Poly(methyl methacrylate) macroinitiators were used to synthesize poly(tBMA-b-MMA-b-tBMA) triblock copolymers in a range of tBMA compositions. The use of a mixed halogen system in the block copolymer synthesis leads to fast initiation and fast deactivation, resulting in controlled molecular weights and a low polydispersity index. Gradient polymer elution chromatography was used to confirm the block copolymer structure. The retention time of the block copolymers lie in between the retention times of the two homopolymers, indicating the existence of a (block) copolymer structure. This technique was successfully used to identify the influence of the ATRP reaction conditions on the final block copolymer structure.