Although the self-condensing atom transfer radical polymerization (SCATRP) of inimers with typical comonomers has been extensively performed, there have been few reports to correlate the reactivity ratio with the growth of the molecular weights (MWs) and the development of branched structures. Thus, the SCATRP of inimers of different reactivity ratios, namely, 4-chloromethylstyrene (CMS) and maleimide (MI) inimers, with a large excess of styrene (St) were carried out, respectively, to examine the effect. The conversion and MW were monitored by gas chromatography, gel permeation chromatography, and multiangle laser light scattering. The results suggested that CMS merely functioned as an initiator for St at the early stage; this led to linear macroinimers, which underwent SCATRP and gave rise to randomly branched polystyrene (PS) only at high conversion. The MI inimers formed charge-transfer complexes with St and underwent the SCATRP to result in hyperbranched copolymers at first; this initiated the atom transfer radical polymerization of St and led to star-shaped PS. With the objective of improving the processability and melt fluidity, the physical properties of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) blends with linear, randomly branched, and star-shaped PS were compared. In comparison with those with linear PS, the PPO/branched PS blends exhibited a higher glass-transition temperature, a higher melt flow index, and a comparable thermal stability because of the spherical architecture of the branched PS. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121:2957-2968, 2011