Halogens electropositive substituents

The presence of an electronegative substituent (halogen, nitrile or carboxyl group) or an electropositive (methyl or amine group) in the monomer molecule causes a polarization of double bonds, which manifests itself in the increase of the molecular dipole moment and the corresponding increase in the pol5mierization rate. From this rule, however, exceptions are because the butadiene molecules have no own dipole moment whereas they are easily polarized and thus readily polymerize. 

---