Living propagating radical

Sato, T. and Otsu, T. Formation of Living Propagating Radicals in Microspheres and Their Use in the Synthesis of Block Copolymers. Vol. 71, pp. 41 —78. 

In the paper published in 1900, he reported that hexaphenylethane (2) existed in an equilibrium mixture with 1. In 1968, the structure of the dimer of 1 was corrected to be l-diphenylmethylene-4-triphenylmethyl-2,5-cyclohexadiene 3, not 2 [38]. Since Gomberg s discovery, a number of stable radicals have been synthesized and characterized, e.g., triarylmethyls, phenoxyls, diphenylpicryl-hydrazyl and its analogs, and nitroxides [39-43]. The radical 1 is stable, if oxygen, iodine, and other materials which react easily with it are absent. Such stable radicals scarcely initiate vinyl polymerization, but they easily combine with reactive (short-lived) propagating radicals to form non-paramagnetic compounds. Thus, these stable radicals have been used as radical scavengers or polymerization inhibitors in radical polymerization. 

Determination of the Formation Rate of Living Propagating Radicals by... 

In the usual radical polymerization, the propagating polymer radicals are generally highly reactive, and hence short-lived. However, some polymerization systems have been reported to involve long-lived (living) propagating radicals. 

Recently, we have found that acrylamide derivatives such as N-methylacrylamide-(NMAAm) and N-methylmethacrylamide (NMMAm) were polymerized by radical initiators in adequate solvents to form polymer microspheres, which contained the very stable propagating radicals of the amide monomers in high concentrations. Furthermore, the living polymer radicals were found to react readily with other vinyl monomers at room temperature, yielding block copolymers. We have also investigated these reactions by means of ESR. This article reviews our recent work on the formation of living propagating radicals, their reactions with vinyl monomers, and their use in block copolymer synthesis. 

Acrylamide (AAm) and methacrylamide (MAm) were likewise converted into the living propagating radicals V and VI by their photo-sensitized polymerization with DBPO in dioxane ESR spectra of polymer radicals V and VI are shown as (a) and... 

As mentioned above, some amide monomers are easily converted into their living propagating radicals when they undergo photo-sensitized polymerization with DBPO in benzene or dioxane at room temperature. Therefore, it is of interest to examine the post-effect in the photo-sensitized polymerization of the amide monomers. Fig. 4 shows the results obtained in the post-polymerizations of AAm in dioxane and NMMAm in benzene at room temperature . As it can be seen from the figure, a small post-effect was observed in the AAm polymerization. On the other hand, no... 

Kinetic Study on the Polymerization of NMMAm with Formation of Living Propagating Radicals in Microspheres... 

The NMMAm monomer is,for the most part, converted into living propagating radicals apparently a unimolecular termination by polymer radical occlusion occurs. Presumably, only a small portion of the living polymer radicals function as active centers for the polymerization, while the others are dormant in the microspheres. Otherwise, MW and hence ] of the poly(NMMAm) would increase with the conversion. 

NMMAm is polymerized with AIBN in benzene to give living propagating radicals according to the following scheme ... 

As shown in Figs. 6 and 10b, short induction periods were observed at 50 °C, both in the polymerization and the formation of living propagating radicals. We suggest that in the early stages of the polymerization several oligomer radicals of NMMAm associate to form a nucleus, where a relatively high concentration of radicals accelerates the reaction between these radicals, and therefore retards the polymerization. This is considered to be responsible for the induction periods. 

It is very difficult to obtain ESR spectra of the propagating polymer radicals of 1,1-diphenylethylene (DPE) and a-methylstyrene (a-MeSt) in the usual radical polymerization, because these monomers have little homopolymerizability. However, the present method can yield easily the living propagating radicals of such monomers at room temperature. 

As described above, when NMAAm or NMMAm is photo-polymerized in benzene at room temperature, the amide monomer is converted to polymer microspheres containing its living propagating radicals in high concentrations. These living radicals react easily with other vinyl monomers. These reactions can be applied to the synthesis of block copolymers. 

Table 5 shows the effects of temperature and time on the reaction of the poly-(NMAAm) radicals with MA. Although the M A conversion increased with increasing reaction time at 50 °C, this system tended toward dead-end polymerization as shown in Fig. 26. Furthermore, the conversion of MA at 40 °C (run MA-9) was higher than that at SO C. These results show that living propagating radicals were deactivated, probably by chain transfer and bimolecular termination, at the higher temperature. Block copolymer constituted 75-83 % of the total resulting polymer at 40-50 °C. 

Thus, similarly to NMAAm and NMMAm, NPMAm was readily converted to its living propagating radical (XX) in the heterogeneous polymerization in benzene... 

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