Multifunctional polymeric photosensitizers containing both pendant nitroaryl groups as a photosensitizer moiety and pendant benzyltributylphosphonium chloride as a substrate-attracting group were synthesized by substitution reactions of poly((chloromethyl)styrene) (PCMS) with potassium salts of photosensitizing compounds by using phase-transfer catalysis followed by addition reactions of the resulting polymers with tributylphosphine (TBP) in N#V-dimethylformamide (DMF) under mild reaction conditions and by the addition reaction of PCMS with TBP followed by the substitution reaction of the potassium salt of the photosensitizing compound under the same reaction conditions. These reactions proceeded very smoothly to give the corresponding new multifunctional polymeric photosensitizers. One of the same polymeric photosensitizers was also prepared in 51% yield by radical copolymerization of the corresponding monomers by using azobis(isobutyronitrile) in DMF. These polymers were soluble in water and methanol, and the polymeric photosensitizers containing pendant benzyltributylphosphonium chloride as a substrate-attracting group and pendant 4-nitrophenoxy (PTBP-NP) or 4-(4-nitrophenoxy)butylate moieties as a chromophore showed higher photosensization efficiency than the corresponding low molecular weight photosensitizer for trans-cis photoisomerization of potassium cinnamate in water. Furthermore, it was found that although the multifunctional polymeric photosensitizer PTBT-NP has excellent efficiency, the control of content of pendant photosensitizer moiety in the polymer and of concentration of the photosensitizer in the reaction system is very important to get high efficiency.