Allyl halides (allyl-X) and polyisobutylene with allyl halide end group (PIB-allyl-X where X = Cl, Br) were investigated as (macro)initiators in atom transfer radical polymerization (ATRP). Studies with low molecular weight allyl halide initiators that model the PIB-allyl-X macroinitiator were performed first. Polymerization of styrene (St) using allyl halides in the presence of CuX/4,4'-di (5-nonyl)-2,2'-bipyridine (dNbpy) catalyst was controlled, but polystyrene with lower polydispersity was obtained with the Br-based initiator-catalyst system. ATRP of methyl methacrylate (MMA) initiated by allyl halides was not as successful. Polymerization of MMA with allyl-Cl and CuCl/dNbpy catalyst resulted in polymer with molecular weight 4 times higher than the predicted value. The polymerization initiated by allyl-Br in the presence of CuCl/dNbpy catalyst was better controlled but still resulted in polymers with molecular weights 2 times higher than the theoretical values. The addition of 10 mol % St to MMA significantly improved the polymerization control. The values of the ATRP equilibrium constants (K-ATRP) of allyl halides were determined and were close to these of 2-haloisobutyrates. Therefore. the low initiation efficiency from allyl-Br during ATRP of NIMA is predominantly caused by the slow addition of allyl radical to monomer rather than low K-ATRP. Extension from PIB-allyl-Br was then conducted to obtain well-defined block copolymers. For example, starting from PIB-allyl-Br macroinitiator (M-n = 4600 g/mol. M-w/M-n = 1.12), a polyisobutylene-b-polystyrene (PIB-b-PSt, M-n = 21400 g/mol, M-w/M-n = 1.14), and polyisobutylene-b-poly (methyl methacrylate-co-styrene) (PIB-b-P(MMA-co-St), M-n = 26 000 g/mol, M-w/M-n = 1.32) were obtained. Extension of PIB-allyl-Br with MMA without styrene resulted in a block copolymer with a bimodal molecular weight distribution.