Solutions of a binaphthoxy phosphazene copolymer (containing chiral 2,2'-dioxy-1,1'-binaphthyl units with 50% R and S configurations distributed along the chains) in N-methyl pyrrolidone were studied by means of continuous flow experiments and small amplitude oscillatory flow tests. A sudden viscosity decrease was observed in the polymer concentration range (3940 wt %), evidencing a liquid-crystalline polymer behavior. This has been confirmed by other rheological methods which have demonstrated that, for a sufficiently high concentration, the solutions of the binaphthoxy phosphazene copolymer give rise to a lyotropic system with formation of rigid rods (axial ratio of 10) stacked parallel to each other. The lyotropic properties of our binaphthoxy phosphazene copolymer are compatible with a regular helical structure, similar to that found for a homoleptic binaphthoxy phosphazene, which contains only S configuration. This suggests that the chains of 50% R/S binaphthoxy phosphazene copolymer are, in average, close to the strictly alternating R-S copolymeric structure of the syndiotactic isomer. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2215-2221, 2010