Living polymer radical

In the case of PMMA dissolved in acetone, the change of LSI could be correlated both with the separation of fragments and, to some extent, with the lifetime of the intermediates which contribute to the main-chain scission [71]. The LSI decreased in two modes, probably due to the two pathways for the main-chain scission. The fast mode with a lifetime of about 20 ps was influenced neither in its extent nor in its rate by the addition of 02 or mercaptane. Therefore the first mode was ascribed to the diffusional separation of fragments which are generated by the main-chain scission through the direct decomposition of electronically excited or ionic intermediates. The slow mode with a lifetime of 6 ms was suppressed, to an extent, depending on the ( -concentration it was attributed to long-lived polymer radicals. The added 02 reacts with lateral polymer radicals to prevent their decomposition. 

The object was to determine the fraction of live polymer radical relative to all polymer species, X, noting that the C-O concentration in polystyrene is negligible (Fig. 2.3.14). Thus, if chain transfer to solvent was neglected, i.e., [C]=[D]=[ 1=0. Alternately,... 

A look into values of Ch from previously presented experimental data for the FRRPP of styrene in ether was made. If the polymerization rate is solely based on propagation reaction between the monomer and the live polymer radicals, then from Table 2.4.3, the effective activation energy for k in Eq. (2.2.3) is the same as the propagation activation energy of 7,051 cal/mol K. With a reactor operating temperature of 80°C, this results in a dimensionless activation energy of y = 10. Table 2.5.1 shows that this value of y provides results within the cutoff of Ch < -1,000 for FRRPP to occur. 

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. 

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. 

Up until this point, we have determined that the photoinitiated polymerization of the FIO monomer in the smectic phase is slower than in the isotropic phase when the continuoiis ouq>ut of a mercury lamp is us as the initiating light source. The consequences and origin of this rate phenomenon will be further explored with respect to dark polymerization in the smectic phase after the initiating light source is suddenly terminated. The results will provide direct evidence via exotherm and ESR analysis of the long-lived polymer radical chains in the smectic phase, as well as an estimate of the termination/propagation rate constant ratio in the smectic phase. 

Grafting onto Termination of growing polymer radical, cation, and anion, formed during the polymerization of various monomers initiated by conventional initiator in the presence of nanoparticles and the deactivation of living polymer radical, cation, and anion with functional groups on nanoparticle surface... 

Actually, chain-growth polymerization involves other reactions. The reaction shown above is a propagation reaction, as defined in Chapter 5. This reaction is accompanied by an initiation reaction, which provides a source of free radicals, and by a termination reaction, which consumes free radicals. For polystyrene polymerization, the termination reaction is the combination of two live polymer radicals, such as those shown above, to form a molecule of dead polymer. 

The possibility of obtaining a long-lived polymer radical was first reported by Melville [36] when either methyl methacrylate or chloroprene was polymerized photochemically in the gas phase. The resulting polymeric products deposited on the walls of the reaction vessel as fine particles was found to contain trapped free radicals which could be used to initiate subsequent polymerization of a second monomer in the absence of any further illumination. 

The existence of living polymer radicals in emulsion polymerization of styrene [37] in the presence of oxidized polypropylene and triethylene tetraamine was ascertained by preparing a MMA—St block copolymer. Polymerization of styrene proceeded after removal of the initiator fi-om the... 

In the above, M is the monomer, I is the initiator, R is the primary radical, P is the live polymer radical with n monomer repeat units, M is the dead polymer with the n monomer repeat units, and X is the solvent, impurity, or chain transfer agent. At high monomer conversion, the polymer s mobility decreases and termination reactions become diffusion controlled ( gel effect ). As a result, the polymerization rate increases rapidly and the polymer s molecular-weight distribution becomes broad. 

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