Addition polymerizations are usually carried out bulk and solution polymerizations. Condensation polymerizations are carried out mostly without solvents. Bulk addition polymerization is a homogeneous process which uses an organic initiator. The above probably implies that polyvinylidine chloride precipitates out of the solution of vinylidine chloride monomer when it reaches a certain molecular weight, but that polystyrene is soluble in styrene to infinite molecular weight. The higher the temperature, the lower the molecular weight of the polymer produced.
Bulk polymerization has a built in hazard.
The thermal conductivity of monomers and polymers is low, and as the viscosity builds up, the ability for heat transfer via convection is substantially diminished. If the heat energy cannot be dissipated, temperature rises, and at higher temperatures the reaction is going to go faster, so this is a positive feedback loop with disastrous consequences. For bulk polymerization , removal of unreacted monomer can be a problem. This is a large concern if your safe polymer was prepared from monomers which are toxic. Dimerization termination is more likely, and the rate of chain transfer is faster.