Propagation polymer radical

In these equations I is the initiator and I- is the radical intermediate, M is a vinyl monomer, I—M- is an initial monomer radical, I—M M- is a propagating polymer radical, and and are polymer end groups that result from termination by disproportionation. Common vinyl monomers that can be homo-or copolymeri2ed by radical initiation include ethylene, butadiene, styrene, vinyl chloride, vinyl acetate, acrylic and methacrylic acid esters, acrylonitrile, A/-vinylirnida2ole, A/-vinyl-2-pyrrohdinone, and others (2). 

I- is the initiating radical, P is the chain-propagating polymer radical that subsequendy abstracts a hydrogen atom from another polymer molecule,... 

As the polymerization reaction proceeds, scosity of the system increases, retarding the translational and/ or segmental diffusion of propagating polymer radicals. Bimolecular termination reactions subsequently become diffusion controlled. A reduction in termination results in an increase in free radical population, thus providing more sites for monomer incorporation. The gel effect is assumed not to affect the propagation rate constant since a macroradical can continue to react with the smaller, more mobile monomer molecule. Thus, an increase in the overall rate of polymerization and average degree of polymerization results. 

When the polymerization of St was carried out with 51 under conditions identical to those in Fig. 3, i.e., [7]/4=[8]/2=51=2X 10-3 mol/1, the formation of benzene-insoluble polymers was observed from the initial stage of the polymerization. Although 7 and 8 induced living radical mono and diradical polymerization similar to that previously mentioned, benzene-insoluble polymers were formed in the polymerization with 51, and the molecular weight of the soluble polymers separated decreased with the reaction time. This suggests that a part of the propagating polymer radicals underwent ordinary bimolecular termination by recombination, leading to the formation of the cross-linked polymer, which was prevented by the addition of 13. 

The use of this phenomenon to control carbon-carbon bond-forming reactions relies on R being rapidly converted into another transient radical which, in the case of a polymerisation, occurs by repetitive addition to a monomer double bond to give the propagating polymer radical, P Thus, the PRE prevents dead polymer (P—P ) formation and the dormant concentration of P—T remains effectively constant. It follows that the excess of T ensures that reversible termination and addition of P to monomer are dominant reactions allowing all polymer chains to grow practically simultaneously (Section 10.5.4). 

E.s.r. evidence for a living (propagating) polymer radical in grafting reactions of vinyl monomers with irradiated cellulose has also been reported (26). 

These lead to a simple relationship between the stationary concentrations of semi-isnacol radicals and propagating polymer radicals, Le. 

Branching occurs in some. free-radical polymerization of monomers like ethylene, vinyl chloride, and vinyl acetate in which the propagating polymer radical is very reactive and can lead to branching by chain transfer to the backbone chain of another polymer molecule or onto its own chain (see Chapter 6). 

The difference between inhibitors and retarders is simply one of degree and not kind. Both are either chain transfer agents or act by addition processes to provide an alternative reaction path to propagating polymer radicals ... 

The reactivities of the propagating polymer radicals, however, exert greater influence on the rates of propagation than do the reactivities of the monomers. Resonance stabilization of the polymer... 

The reactivities of the propagating polymer-radicals, however, exert greater influence on the rates of propagation than do the reactivities of the monomers. Resonance stabilization of the polymer-radicals is a predominant factor. This fairly common view comes from observations that a methyl radical reacts at a temperature such as 60°C approximately 25 times faster with styrene than it does with vinyl acetate [72]. In homopolymerizations of the two monomers, however, the rates of propagation fall in an opposite order. Also, poly(vinyl acetate)-radicals react 46 times faster with n-butyl mercaptan in hydrogen abstraction reactions than do the polystyrene-radicals [71]. The conclusion is that the polystyrene radicals are much more resonance stabilized than are the poly (vinyl acetate)-radicals. Several structures of the polystyrene-radicals are possible due to the conjugation of the unpaired electrons on the terminal carbmis with the adjacent unsaturated groups. These are resonance hybrids that can be illustrated as follows ... 

Formation of Living Propagating Polymer Radicals in Microsphoes.44... 

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. 

Formation of Lmng Propagating Polymer Radicals in Microspheres... 

As it can be seen from Fig. 3, the poly(NMMAm) microscpheres of 1000-3000 A diameter are dispersed in the poly(p-MeSt) matrix. Thus, on polymerization, the amide monomers are converted to polymer microspheres which are dispersed in the polymerization system These microspheres are assumed to contain many living propagating polymer radicals. 

Figure 15a shows an ESR spectrum observed in the reaction of poly(NMAAm) radicals with isopropenyl methyl ketone (IPMK). This spectrum is similar to that of the poly(MMA) radical and has therefore been assigned to the propagating polymer radical (X) of IPMK. 

On the other hand, as shown in Figs, lb and 2b, the propagating polymer radicals of NMMAm and MAm show five line spectra, suggesting that these polymer radicals exist mainly in the deformed conformation. 

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. 

Figure 24 shows the results observed for the reaction of the poly(NMMAm) radical with an a-MeSt/maleic anhydride(MAn) mixture, also a typical alternating cbpolymerization system. The propagating polymer radical (XIX) of MAn was prepared by the reaction of the poly(NMMAm) radical with MAn and its spectrum is shown in Fig. 24a. The triplet is considered to be due to the a- and P-hydrogens in radical XIX.

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