A mass spectrometry analysis has been performed on complex architecture polymeric material produced during reversible addition fragmentation chain transfer (RAFT) polymerizations yielding star polymers. Para-acetoxystyrene (AcOSty) has been polymerized at 60 degrees C, using azobisisobutyronitrile (AIBN) as the thermally decomposing initiator, in the presence of the R-group approach tetrafanctional RAFT agent (1,2,4,5-tetrakis-(2-phenyl-thioacetyl-sulfanylmethyl)-benzene). In addition to ideal star material, a variety of products unique to this mode of polymerization have been identified. These include star-star couples, stars terminated with initiator fragments, star-star couples terminated with initiator fragments and linear polymers, supporting the notion that these species are responsible for the structured molecular-weight distributions measured for these systems when analyzed via gel permeation chromatography. The analysis begins with a study of AcOSty polymerizing (i) in the absence of any mediating agent and (ii) in the presence of a monofunctional RAFT agent, revealing the mode of termination of propagating poly(AcOSty) radicals as combination and that some ionization biases exist among variants of poly (AcOSty). The interpretation of the mass spectrometry data has been aided by a novel kinetic model of star polymerizations, allowing the rationalization of experimental observations with theoretical expectations. (c) 2008 Wiley Periodicals, Inc.