Samples of a disubstituted polyacetylene, poly(1-trimethylsilyl-1-propyne), containing on the average 60% of cis-C=C bonds and prepared with NbCl ₅ as catalyst were studied by means of Kerr effect measurements in solution and molecular hydrodynamics methods. The resulting data were correlated with the properties of the sample prepared with another catalytic system, TaCl₅/BuLi. Samples prepared under different catalytic conditions were found to have much different electrooptical properties. It was concluded that the TaCl₅/BuLi catalyst allows preparation of polymers with longer continuous sequences of monomeric units of the same isomeric structure as compared with those obtained with NbCl₅.