Nitrophenyl 4-((2-((2-methyl-1-oxo-2-propenyl)oxy)ethyl)oxy)benzoate (BEM) was synthe- sized and copolymerized with 4'-(((2-((2-methyl-1-oxo-2-propenyl)oxy)ethyl)ethyl)amino)-4-nitroazobenzene (DR1M) in dioxane at 60 °C. BEM and DR1M copolymerize in a random fashion with reactivity ratios rDR1M ) 0.82 and rBEM ) 1.03. The copolymers are amorphous. Interactions between the azo and the BEM side groups produce a shift of absorption maxima in the electronic spectra. Birefringence can be photoinduced and photoerased on films of all copolymers. The photoinduced birefringence per azo chromophore is very high in copolymers with low azo contents, about 4 times that of poly(DR1M). A cooperative motion of BEM groups with the azo groups is postulated as the reason for the additional birefringence. To our knowledge this is the first example of cooperative motion in amorphous polymers. The growth and relaxation of the birefringence are fitted by biexponential equations and "writing" rate constants are calculated for two processes, one is assigned to the fast motion of side groups and the other to the slow motion of the main chain. The fast "writing" rate increases with the DR1M content while the slow "writing" rate is almost constant. From the relaxation of the birefringence, about 85% of it is maintained in copolymers with high BEM contents. The maximum "writing" efficiency obtained for poly- (DR1M-co-BEM) is 5 10-6 cm3/J (at about 50 mol % DR1M) which is higher than that of poly(DR1M).