Zinc chloride effectively catalyzed the reaction of allyl-substituted episulfide and cyclic trithiocarbonate to obtain the corresponding bisallyl monomer containing tetrathiaspiro ring in high yield. The homogeneous mixture of the bisallyl monomer, multifunctional thiols, and a catalytic amount of photo-radical initiator undergoes a photoinduced thiol-ene reaction to result in cross-linked films. Thus the films obtained in this way, although they are aliphatic polymers containing neither aromatic ring nor halogen atom, have a high refractive index (n(D)(25)) of around 1.63.