Spontaneous polymerization in the emulsion polymerization of styrene and chlorobutadiene

Spontaneous initiation in emulsion polymerization may be more important: th an in the corresponding bulk free-radical polymerization. A methodology is derived for finding the locus of spontaneous polymerization in emulsion pol ymerization, through use of spin traps and catalysts which can accelerate d ecomposition of peroxides. Applying this to both styrene and chloroprene (c hlorobutadiene), it is found that this generation occurs to some degree wit hin all phases present. The rate of spontaneous initiation is relatively sm all in styrene emulsion polymerization but large in chloroprene. A means of including this effect in modeling rates and molecular weight distributions is derived, which also shows how rate parameters for the process may be ob tained from experimental molecular weight distributions. This methodology i s applied to these two monomers, with a series of seeded emulsion polymeriz ations using polystyrene host seeds for both. For styrene polymerization, t he spontaneous initiation rate is low and varies with latex preparation; co nsistent values for this rate coefficient for a given latex are obtained by independent measurements involving two different techniques, thereby verif ying the methodology. Applying this methodology to chloroprene, it is found that the effect of spontaneous initiation is much larger and probably aris es from peroxides formed by exposure to oxygen. For chloroprene, spontaneou s radical generation occurs both within the particles and in any monomer dr oplets present, with different chain-stopping mechanisms occurring in these two phases. It is a major influence on rates and molecular weight distribu tions, even in the presence of large amounts of added initiator; chain stop page in droplets is largely by transfer to monomer, whereas chain stoppage within particles is by termination with short radicals formed by spontaneou s initiation. Arrhenius parameters for the rate coefficient for transfer to monomer are obtained from the molecular weight distributions for the chlor oprene system: k(tr)/dm(3) mol(-1) s(-1) = 10(4.3) exp(-30.9 kJ mol(-1)/RT) .