The theory describing in molecular terms radical photopolymerization due to which the refractive index modulation distribution producing a hologram is formed. The theory extends the usual only-diffusion concept to early stages of polymerization, where the opposite diffusion of short polymers (oligomers) is possible which is reduced with increasing polymerization degree. The results of the theory are compared with our experimental results. The idea that, inversely, the experimentally detected curves describing the hologram growth may be used for the study of polymerization kinetics is put forward.