Chain transfer agents
In Vinylic polymerisation several other molecules react along with main growing chain to interrupt the additional growth of the original chain. This takes lowering of average atomic mass of the polymer such types of reagents are known chain transfer agents. The general example CCl4, CBr4 etc
For instance in the existence of CCl4 styrene polymerizes to make a polymer of lower average molecular mass that as well consists of some chlorine.