Stable free radical polymerization analysis

Finally, the use of stable free radical polymerization techniques in supercritical C02 represents an exciting new topic of research. Work in this area by Odell and Hamer involves the use of reversibly terminating stable free radicals generated by systems such as benzoyl peroxide or AIBN and 2,2,6,6-tetramethyl-l-piperidinyloxy free radical (TEMPO) [94], In these experiments, styrene was polymerized at a temperature of 125 °C and a pressure of 240-275 bar C02. When the concentration of monomer was low (10% by volume) the low conversion of PS which was produced had a Mn of about 3000 g/mol and a narrow MWD (PDI < 1.3). NMR analysis showed that the precipitated PS chains are primarily TEMPO capped, and the polymer could be isolated and then subsequently extended by the addition of more styrene under an inert argon blanket. The authors also demonstrated that the chains could be extended... 

It should be noted that, whereas the preceding discussion has been cast in terms of free-radical polymerizations, the thermodynamic argument is independent of the nature of the active species. Consequently, the analysis is equally valid for ionic polymerizations. A further point to note is that for the concept to apply, an active species capable of propagation and depropagation must be present. Thus, inactive polymer can be stable above the ceiling temperamre for that monomer, but the polymer will degrade rapidly by a depolymerization reaction if main chain scission is stimulated above T.. 

Jaisinghani and Ray (40) also predicted the existence of three steady states for the free-radical polymerization of methyl methacrylate under autothermal operation. As their analysis could only locate unstable limit cycles, they concluded that stable oscillations for this system were unlikely. However, they speculated that other monomer-initiator combinations could exhibit more interesting dynamic phenomena. Since at that time there had been no evidence of experimental work for this class of problems, their theoretical analysis provided the foundation for future experimental work aimed at validating the predicted phenomena. Later studies include the investigations of Balaraman et al. (43) for the continuous bulk copolymerization of styrene and acrylonitrile, and Kuchanov et al. (44) who demonstrated the existence of sustained oscillations for bulk copolymerization under non-isothermal conditions. Hamer, Akramov and Ray (45) were first to predict stable limit cycles for non-isothermal solution homopolymerization and copolymerization in a CSTR. Parameter space plots and dynamic simulations were presented for methyl methacrylate and vinyl acetate homopolymerization, as well as for their copolymerization. The copolymerization system exhibited a new bifurcation diagram observed for the first time where three Hopf bifurcations were located, leading to stable and unstable periodic branches over a small parameter range. Schmidt, Clinch and Ray (46) provided the first experimental evidence of multiple steady states for non-isothermal solution polymerization. Their... 

A parallel study has reported the synthesis of crosslinked polymer microspheres in supercritical carbon dioxide [54]. Heterogeneous free-radical polymerization of divinyl benzene and ethyl benzene were carried out at 65 C and 310 bar using AIBN initiator to form the crosslinked polymer. It is shown that in the absence of surfactants as stabilizers, polymerization of the mixture containing 80 % divinyl benzene + 20 % ethyl benzene leads to poly(divinylbenzene) microspheres of about 2.4 micron diameter [Figure 14]. In the presence of a carbon dioxide-soluble diblock copolymer as a stabilizer, polymerization of the mixture with the same monomer ratio proceeds as an emulsion and lead to smaller crosslinked particles (ca. 0.3 micron). Thermal analysis shows that the crosslinked polymer that is formed from these polymerizations is stable up to 400 C. 

Mara. Kamei and Osada [109 described a detailed study of the thermal decomposition of TNT. They examined the decomposition by differential thermal analysis, thermogravimetry, infra-red spectroscopy. HSR and mass spectrometry. One of their most important findings was that TNT produced free radicals already in the vicinity of the melting point, that is SO C. The substances which promote the decomposition of TNT are free radicals which are stable at room temperature. They are insoluble in benzene or chloroform and are partly oxidized polymeric substances. 

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