Monoradical termination

The molecular weights of polyacrylamide produced in reversed micelles have been found to be Independent of the rates of polymerisation and initiation due to monoradical termination Involving degradative chain transferees, on the otherhand the photopolymerisation of... 

Dilatometry and rotating sector techniques were combined to follow the photopolymerization of acrylamide in AOT reverse micelles with AIBN as the initiator and toluene as the organic phase [51,52]. Very high polymerization rates were observed with total convCTsion to polym achieved in a few minutes and good heat transfer. A monoradical termination reaction was found (R oc [I] [M]° ), caused by a degradative chain transfer to toluene. The reaction occurs at the water-oil interface by transfer of the growing polymer radical to toluene, followed by the exo-diffusion of tiie new ben lic radical, the latter bdng too stable to reinitiate polymerization. 

If reactions other than those considered so far did not take place to an appreciable extent, the average degree of polymerization would be directly related to the kinetic chain length v. Assuming termination by combination of radicals, as is indicated by experiments previously cited, each polymer molecule formed in a monoradical-initiated polymerization would consist of two kinetic chains grown from two other-... 

Actually, the diradical and monoradical species in Reactions 2 and 3, respectively, are the same since both involve the participation of the complex as a diradical species. The termination and propagation steps in these reactions are related in that the interaction between two radicals... 

In addition to the practical interest, the process presents challenges encouraging further fundamental exploration. A thorough study not reported here, has been performed on the mechanism and kinetics of the polymerization of acrylamide in AOT/water/toluene microemulsions (Carver, M.T.r Dreyer, U. Knoesel, R. Candau, F. Fitch, R.M. J. Polym. Sci. Polym. Chem. Ed., in press. Carver, M.T. Candau, F. Fitch, R.M. J. Polym. Sci. Polym. Chem. Ed., in press). The termination reaction of the polymerization was found to be first order in radical concentration, i.e. a monoradical reaction instead of the classical biradical reaction. Another major conclusion was that the nucleation of particles is continuous all throughout the polymerization in contrast to conventional emulsion polymerization where particle nucleation only occurs in the very early stages of polymerization. These studies deserve further investigations and should be extended to other systems in order to confirm the unique character of the process. 

A growing chain is deactivated when it reacts with another chain to form a dead macromolecule. The recombination of two growing monoradicals is an example of this. Termination reactions destroy active centers both the rate of polymerization and the degree of polymerization are lowered. The deactivation through reaction of two free radicals in one of the reasons why ionic polymerizations are faster than free radical polymerizations. The deactivation reaction between two free radicals has a small activation energy, and therefore occurs very rapidly. Thus, the concentration of growing free radicals is very low in the stationary state... 

Since we will not address branching here, the set has to be reduced to 2D, by summation over the branching index fe. The second dimension, the number of TDBs per chain, is concerned with the manner in which they are created. One of two possible mechanisms is transfer to monomer, producing a monoradical with a TDB according to the reaction equation shown in Table 9.4. Subsequent propagation of this monoradical leads to a chain with a TDB. The second mechanism is disproportionation, directly leading to chain with a TDB. Now, chains with more than one terminal double bond can be created in two ways insertion of chains with a TDB created by disproportionation termination, or by termination by recombination. If... 

The radical character of the arylacetylene curing reaction was already well established (7,8) and our results confirm it The initiation step could be seen as a monomolecular process (27,28) corresponding to the homolytical cleavage of the terminal acetylene C-H bond. However such a mechanism could not explain the formation of the 1,2,3-trisubstituted aromatic trimer , even by considering the possibility for the monoradicals to rearrange. It could not either explain the delay between the first 20% of i consumption and the appearance of the first formed product 6 and IQ (Figure 4). 

At the trimer diradical (tn) step, two reaction pathways are again possible termination by cyclization-aromatization giving trisubstituted benzene sf ies or propagation by addition to a monomer molecule to give tetramer diradicals In (scheme 3). TTie cyclization process will give directly the three aromatic trimers fi-JQ observed experimentally. This is an important point, because monoradical mechanism (27, 28) could not afford the formation of the 1,2,3-trisubstituted isomer type, such as compound Q. 

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