We present the use of three polysiloxane based copolymers as a source for carbon doping and nanosize inclusions for superconducting MgB2 composites. The polysiloxanes were prepared by atom transfer radical polymerization and the composites were fabricated by the short time spark plasma sintering method. Superconducting critical temperatures were higher than expected from the carbon content found from X-ray diffraction analysis of the (110) peak of MgB2. A core-shell model for grain doping is proposed. Both the upper critical fields and the critical current densities are higher in preceramic-doped samples than in pure MgB2 in agreement with the carbon doping level. However, despite the highest upper critical field, the critical current density is lower in the case when ferrocene grafted polysiloxanes are used. We attribute this fact to the formation of iron-based nanostructures as a result of polymer pyrolisis which have a pair breaking effect.