Pulsed field gradient, triple resonance, and three-dimensional (3D) NMR techniques at 750 MHz were used to study the initiation mechanism and to characterize the polymer chain end structures obtained by radical initiation of styrene polymerization with (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TPO). Additional polystyrene samples were prepared from alpha -d(1)-styrene and beta,beta -d(2)-styrene, and these samples were studied by 3D SQ-HCAP experiments. The results suggest that all the chain-end structures are formed by the addition of phosphinyl radical to the methylene carbon of styrene. SD-NMR permitted resolution of resonances of chain-end structures up to the tetrad level. With the help of molecular simulation and RIS calculations, the resonances of C-alpha on the chain-end structures were assigned up to the triad level. Poly(alpha,beta-C-13(2)-styrene) was prepared by TPO initiation, and its chain-end structures were characterized by the combined use of 3D SQ-HCAP and CT-HCCH experiments.