Polymers, with differently sized crown ethers as alkali cation binding sites and photodimerizable cinnamoyl moieties in their side-chains, were prepared by the cationic copolymerization of the corresponding monomers. The form of the crown-cation complexes (l : l or 2 : l) was studied by measuring quantum yields for the photodimerization of the crown-connected cinnamoyl units in the presence of alkali metal chlorides and by measuring the shift of )~ax of alkali metal picrates in THF on addition of the crown polymers. The alkali metal cation binding ability and selectivity of the photodimerizable polymers with different crown ethers (studied by a method of picrate salts extraction) were compared with those of blended samples of the photodimerizable polymers with one kind of crown ether unit. On irradiation with u.v. light, the crown-connected cinnamoyl moieties of the polymers dimerized even in dilute solutions. Studies were made of the effects on the alkali cation binding ability and selectivity of: the photo-The effects on the alkali cation binding ability and selectivity of: the photodimerization of the cinnamoyl moieties of the polymers; the degree of photodimerization of the polymers; and the kind of alkali cations used as templates during the photodimerization of the polymers; were studied.