a gradient refractive - index plastic rod is prepared by prepolymerizing a mixture containing a first monomer and at least a component selected from a second monomer and a non - reactive dopant sealed in a tube , while rotating the tube horizontally around its longitudinal axis ; and polymerizing the resulting prepolymer or prepolymers while keeping the tube vertical . in the vertical polymerization stage , the polymerization undergoes in the direction from the periphery to the center of the tube due to an increase of polymerization rate caused by gel effect of the prepolymers which has been piled up on the tube wall during the centrifugal stage , so that the monomers contained in the central part of the tube diffuse outwardly , and thus a plastic rod having a larger region of gradient refractive - index is obtained .