||Rh complexes such as [Rh(COD)Cl]2 can polymerize phenylacetylene (PA) and p-methylphenylacetylene (p-MePA) in aqueous medium to give highly stereoregular polymers. Additives such as NaCl reduce the stereoregularity of the polymers. As high yield has been reported for the polymerization of PA in triethylamine, the catalytic properties of Rh complexes with nitrogen containing ligands were studied. Our result shows that though the yields were improved, the stereoregularity of these polymers were reduced. Also, we have developed two water-soluble Rh-based catalyst systems capable of producing highly stereoregular (100% cis-content) polymers in high yields. Moreover, a homogeneous and free-standing poly(phenylacetylene) membrane can be prepared directly by dropping the monomer at the surface of the catalyst solution within several seconds. New monomers of 3-bromo-4-ethynylthiophene and 3-ethynyl-4-(trimethylsilyl)thiophene were successfully polymerized using Mo- and W- based catalyst systems. Effects of catalysts, cocatalysts and temperature on their polymerizations were studied and the polymerization conditions for producing poly(thienylacetylenes) with minimum thiophene polymerization were optimized. In our studies on optical properties of substituted polyacetylenes, we found that transmission spectra of polyacetylenes shift horizontaIly to longer wavelengths when the concentration of the polymer solutions (c) increase. The transmission spectra at all the investigated concentrations are structureless, much like transmission curves of cut-off filters. The cut-off wavelength (λc) is defined as the wavelength at which light transmittance is 0.1%. Interestingly enough, by using different substituted polyacetylenes, λc can move over the whole visible spectral range. Besides, the experimental data for all the investigated polyacetylenes suggest an optical transmission relationship of λc=αlog10(bc)+k where α and k are constants and b is pathlength. The novel optical property of the substituted polyacetylenes may find usage in various aspects such as light filtration and colour imaging. Furthermore, as some of the polymerization systems employ water rather than organic solvents as polymerization medium, the process is environment friendly and the production cost is low. Hence, they have high potential in industrial applications.