A swollen bulk polymerization approach for the preparation of a preform for manufacturing graded-index plastic optical fibers was examined. The preform prepared by the proposed approach was transparent and bubble-free and had a parabolic profile of the refractive index, with the highest refractive index at the center of the rod. The influences of the polymerization temperature and the type of dopant on the properties of the prepared preform were discussed. A higher temperature resulted in a steeper parabolic profile of the refractive index for the formed preform, and a lower temperature resulted in a higher refractive index near the core-cladding region of the preform. The refractive-index profiles of the preform were affected by the addition of the different types of dopants used. The concentration distribution of the dopant in the preform correlated to the refractive-index distribution. In a comparison of bromobenzene (BB) and 1,2,4-trichlorobenzene (TB) as dopants, it was observed that BB distributions along the profile were more uniform than TB distributions, and a dopant with a larger molecular volume was more concentrated in the core center. It was concluded that the refractive-index distribution of the graded-index preform could be controlled by the both temperature and the addition of different types of dopants during the swollen bulk polymerization process. (C) 2003 Wiley Periodicals, Inc.