PPV and its alkoxy-substituted derivatives are a well-established class of materials for polymeric light emitting diode application. MEH-PPV (poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene]s) as a typical example of the solution processable conjugated polymer for PLED device fabrication, possesses good solubility in common organic solvents. However, gelation or micro-gelation always occurs during the polymerization process, especially when the polymer is prepared from the bis-bromomethyl monomer through the Gilch route. Herein, we present the synthesis of fully soluble MEH-PPV by the addition of molecular weight modifiers (chain stoppers or Gee radical scavengers) in the polymerization system. The polymers synthesized in this work were characterized by FTIR, NMR, W-vis spectroscopy and GPC. The results indicate that the molecular weights of the polymers can be effectively controlled by the addition of chain stopper (benzyl bromide) or chain blocking antioxidant (2,6-di-tert-butyl-4-methylphenol). The mechanism of polymerization in the presence of these additives is also discussed.