Molecular properties of poly(dialkoxyphosphazene) (PDAOP) series with the aliphatic side-chains contained 3, 4, 6, and 7 carbon atoms were investigated in dilute benzene solutions by techniques of electro-optical Kerr effect and dynamo-optical Maxwell effect, and by dielectric spectroscopy. The sign inversion of Kerr effect and Maxwell effect in PDAOP solutions from positive to negative was detected at the side-chain length range of 5-6 carbon atoms. These phenomena were explained by the strong dependence of anisotropy of optical polarizability of PDAOP monomeric unit on chemical structure and change of its sign at the indicated side-chain lengths. It was demonstrated that PDAOPs possess a critical side-chain-length interval at which their physical properties, depending on the intramolecular ordering, display a singularity. It was also shown that the studied PDAOP samples are not flexible-chain polymers. They are characterized by Kuhn segment 47-65 Å, the value of which increases with the elongation of aliphatic side-chain. Dielectric properties of PDAOP were correlated to the properties of the macromolecules with manifestation of kinetic rigidity under a treatment of electric field.