The various diacrylates having different alkyl side-chain are systematically synthesized to investigate the relation between solubility in diacrylates for liquid crystals and phase separation process. In order to analyze the kinetics in photo-polymerization induced phase separation for a homogeneous mixture consisted of liquid crystals and monomers, time resolved confocal laser scanning microscopy with a high resolution is used. The difference between binodal decomposition and spinodal decomposition can be observed for the first time in the system of LC/monomer mixture. The occurring either spinodal or binodal decomposition in the phase separation process depends on the number of methylen in side-chains in diacrylates.