Termination in heterogeneous polymerization

The kinetics of termination in suspension polymerization is generally considered to be the same as for solution or bulk polymerization under similar conditions and will not be discussed further. A detailed discussion on the kinetics of termination in emulsion polymerization appears in recent texts by Gilbert and Lovell and El-Aasser and readers should consult these for a more comprehensive treatment. 

The steps involved in entry of a radical into the particle phase from an aqueous phase initiator have been summarized in Section 3.1.11. Aqueous phase termination prior to particle entry should be described by conventional dilute solution kinetics (Section 5.2.1.4.1). Note that chain lengths of the aqueous soluble species are short (typically 10 units). 

Even though the chemical reactions are the same i.e. combination, disproportionation), the eflecls of compartmentalization are such that, in emulsion polymerization, particle phase termination rates can be substantially different to those observed in corresponding solution or bulk polymerizations. A critical parameter is h, the average number of propagating species per particle. The value of h depends on the particle size and the rates of entry and exit. 

Many emulsion polymerizations can be described by so-called zero-one kinetics. These systems are characterized by particle sizes that are sufficiently small iliat entry of a radical into a particle already containing a propagating radical always causes instantaneous termination. Thus, a particle may contain either zero or one propagating radical. The value of will usually be less than 0.4. in these systems, radical-radical termination is by definition not rate determining. Rates of polymerization are determined by the rates of particle entry and exit rather than by rates of initiation and termination. The main mechanism for exit is thought to be chain transfer to monomer. It follows that radical-radical leiTnination, when it occurs in the particle phase, will usually be between a short species (one that has just entered) and a long species. 

Treatments (Smith-Ewart, pseudo-bulk ) have been devised which allow for the possibility of greater than one radical per particle and for the effects of chain length dependent termination. Further discussion on these is provided in the references mentioned above.  

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Termination in heterogeneous polymerization is discussed in Section 5.2.1,5 and the more controversial subject of termination during living radical polymerization is described in Section 5.2.1.6. Termination in copolymerization is addressed in Section 7.3. 

Heterogeneous polymerization processes (emulsion, miniemulsion, non-aqueous dispersion) offer another possibility for reducing the rate of termination through what are known as compartmcntalization effects. In emulsion polymerization, it is believed that the mechanism for chain stoppage within the particles is not radical-radical termination but transfer to monomer (Section 5.2.1.5). These possibilities have provided impetus for the development ofliving heterogeneous polymerization (Sections 9.3.6.6, 9.4.3.2, 9.5.3.6). 

Polyfvinyl chloride) (PVC) is produced by mass, suspension, and emulsion processes. Mass polymerization is an exatiiple of a heterogeneous bulk system. PVC is virtually insoluble in vinyl chloride because the polymer is about 35% more dense than the monomer under normal polymerization conditions. Vinyl chloride, however, is quite soluble in polymer. The two phases in PVC polymerizations are pure monomer and monomer-swollen polymer. Polymerization proceeds in both phases, but it is very much faster in the polymer-rich phase because the mobility of macro radicals and mutual termination reactions are. severely restricted (cf. Section 6.13.2). 

Ionic polymerizations are not as well understood as radical polymerizations because ionic polymerizations are characterized by a wide variety of modes of initiation and termination. The nature of the reaction media in ionic polymerizations is often not clear since heterogeneous inorganic initiators are often involved. Further, it is extremely difficult in most instances to obtain reproducible kinetic data because ionic polymerizations proceed at very high rates and are extremely sensitive to the presence of small concentrations of impurities and other adventitious substances. 

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