We present an NMR study of structural defects in poly(3-decylthiophene)s synthesized by different methods. The conjugation length dependent properties of the polymers are monitored through optical absorption and Raman scattering. The polymers obtained by FeCl3 oxidation and by electropolymerization show mainly correlated defects in the enchainment. A lower number of defects, which are uncorrelated, is introduced with the Grignard reaction yielding a polymer with the highest mean conjugation length. The extension of the defect free polymer segments is, however, not very different for the different polymers studied. For the polymer synthesized with FeCl3 the temperature dependence of the NMR line width is discussed in terms of partial doping and spin dynamics.