We have prepared a series of soluble polyalkylthiophenes in which it is possible to modulate continuously the electronic properties such as the conjugation length and the energy-gap. The copolymerization of 3-butylthiophene and 3,4-dibutylthiophene units in well defined ratios leads to materials with optical properties depending on the composition of the chain. Using size exclusion chromatography (s.e.c.) and n.m.r. techniques we have found that the copolymer average composition is in good agreement with the composition of the co-monomer feeds at the polymerization start. The optical characterization was performed with u.v.-visible absorption and Raman spectroscopy and this allowed us to monitor the conjugation length properties of the copolymers.