Heterogeneous polymerization termination

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). 

The final step is termination of chain growth mostly by radical transfer reaction to monomer [306], whereas combination or disproportionation are observed only to a small extent. The monomer radical is able to start a new chain. The most widely used procedures for preparation of commercially PVC resins are, in order of their importance, suspension, emulsion, bulk, and solution polymerization. A common feature of the first three methods is that PVC precipitates in liquid VC at conversions below 1%. The free polymerization of VC in a precipitating medium exhibits an accelerating rate from the beginning of reaction up to high conversion [307]. This behavior is called autoacceleration and is typical for heterogeneous polymerization of halogenated vinyls and acrylonitrile [308]. 

The secondary termination may be operative, however, in the heterogeneous polymerization where monomer-rich microdomains diaracterized by low viscosity contain a large amount of initiator. Here, the microdomains are separated from each other by the polymer shell (wrapping effect) and, therefore, radical deactivation takes place by participation of mobile radicals. 

When monomer and polymer are mutually insoluble, the loss of radical activity may proceed in several ways 1) Burnett and Melville [59] attributed the abnormal behavior of heterogeneous polymerization of vinyl monomer to the enhanced termination of macroradicals by coagulation of colloidal particles containing growing radicals. Thomas and Pellon [60] have found that the rate... 

Peggion et al. [76] have reported that the rate of emulsion polymerization of vinyl chloride is proportional to the 0.5th power of the initiator concentration (with anionic emulsifier (SDS) above its CMC). The reaction order thus differs slightly from that proposed for the conventional emulsion polymerization of vinyl monomers. These data were discussed in terms of the bimolecular termination of growing radicals. The number of particles was found to be independent of the initiator concentration. The data were supposed to be affected by the restricted penetration of radicals and monomers into the polymer phase and separation of the growing radicals from the monomer phase and chain transfer events. The trapping of radicals increases with conversion and so the reaction order with respect to [initiator] increases [77]. The authors [76] suggested that the emulsion polymerization of VC is similar to a heterogeneous polymerization [56] in which the formation of occluded radicals increases the order with respect to [initiator]. 

Heterogeneous polymerization processes (emulsion, miniemulsion, nonaqueous dispersion) offer another possibility for reducing the rate of termination through what are known as compartmentalization effects. In emulsion polymerization, it is... 

The methodology of living polymerization is ideally suited for the preparation of well-defined, star-branched polymers and copol)miers with low degrees of compositional heterogeneity. Because termination and chain transfer reactions are absent and the chain ends may be stable for sufficient time periods (the laboratory time scale), these pol)mierizations have the following useful synthetic attributes for star polymer synthesis ... 

Ring-opening metathesis polymerization (ROMP) of 1,4-cyelooctadiene was used to prepare poly(l,4-B) terminated with halo end groups.647 This was then used as a macroinitiator of ATRP with heterogeneous Cu bpy catalysts to form PS- >/ti /r-poly(l,4-B)-WoeA -PS and PMMA-Moc.T-poly(l,4-B)-Wof A-PMMA. 

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