Bimolecular terminators

In homogeneous media acrylamide is terminated by bimolecular termination [51-53]. In this case the degree of polymerization (DP ), as defined by Chapiro [51] is ... 

The most important mechanism for the decay of propagating species in radical polymerization is radical-radical reaction by combination or disproportionation as shown in Scheme 5.1. This process is sometimes simply referred to as bimolecular termination. However, this term is misleading since most chain termination processes are bimolecular reactions. 

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. 

Of the six possible bimolecular termination reactions which HO2, SO4, and OH radicals might undergo only reactions (87), (88), (64) and (65) appear to be of importance under the experimental conditions covered... 

It should be clear from Section IV. B that a major difficulty involved in preparing monomeric iron-dioxygen adducts is the prevention of bimolecular termination reactions, leading via autoxidation to the formation of a ju-oxo dimer, thus... 

We saw previously that a major factor in inhibiting the bimolecular termination reaction was the presence of sufficiently bulky ligands so that a monomeric dioxygen adduct could be isolated 135). A number of synthetic metal porphyrins 239) have been prepared recently which satisfy the above requirement, and bind molecular oxygen we shall now proceed to discuss these. 

Where B = pyridine, piperidine or 1-methylimidazole, in methylene chloride solution, but under normal conditions rapid irreversible autoxidation takes place 232) leading to the formation of the well characterised 247, 248) fi-oxo product, (TPP)Fe(IlI)—0—Fe(III) (TPP) and since the rate of oxidation decreases 249, 250) with increasing excess of axial base, B, it follows 232, 251) that a five co-ordinate species, Fe(II) (Base)TPP, is probably involved as an intermediate which can then undergo a bimolecular termination reaction with Fe(II) (Base)02TPP, followed by autoxidation. Firstly 251),... 

Like all controlled radical polymerization processes, ATRP relies on a rapid equilibration between a very small concentration of active radical sites and a much larger concentration of dormant species, in order to reduce the potential for bimolecular termination (Scheme 3). The radicals are generated via a reversible process catalyzed by a transition metal complex with a suitable redox manifold. An organic initiator (many initiators have been used but halides are the most common), homolytically transfers its halogen atom to the metal center, thereby raising its oxidation state. The radical species thus formed may then undergo addition to one or more vinyl monomer units before the halide is transferred back from the metal. The reader is directed to several comprehensive reviews of this field for more detailed information. 

If we use initiators R-R which have very high reactivities for the chain transfer reaction to the initiator and/or primary radical termination, i.e., ordinary bimolecular termination is neglected, it is expected that a polymer will be obtained with two initiator fragments at the chain ends (Eq. 7) ... 

Because the polymerization with the thermal iniferters previously described was performed at a high temperature, some side reactions might be unavoidable, e.g., ordinary bimolecular termination between polymer radicals, disproportionation between a polymer radical and a small radical leading to deactivation of the iniferter site, initiation by the radical generated from the iniferter sites, rearrangements of the structure of the iniferter sites, and spontaneous initiation of polymerization. 

These are two possibilities for the polymerization of MA deviated from the ideal living radical polymerization (i) the chain end of poly(MA) formed primary radical termination with a DC radical does not dissociate or dissociates at an unfavorable position like 41 (ii) bimolecular termination leading to the deactivation of the iniferter sites occurs preferentially to the primary radical termination with the DC radical which reproduces the iniferter site. 

The polymerization of MA with 7 was carried out in the presence of 13, i.e., 7 and 13 were used as two-component iniferters [175]. When an identical amount of 13 to 7 was added to the system, the polymerization proceeded according to a mechanism close to the ideal living radical polymerization mechanism. Similar results were also obtained for the polymerization of VAc. These results indicate that the chain end of the polymer was formed by the competition of primary radical termination and/or chain transfer to bimolecular termination, and that it could be controlled by the addition of 13. 

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. 

Here,i j is the initiation rate, is the rate constant for the bimolecular termination, and K is the equilibrium constant From Eq. (64), the polymerization rate Rp is represented as... 

Thermal initiation and ordinary bimolecular termination also occur during polymerization in addition to initiation by the dissociation of the adduct or the active polymer chain-end dissociation and reversible temination (formation of the dormant species). Therefore, the degree of the control of the molecular weight and the molecular weight distribution is determined by the ratio of the polymer chains produced under control and uncontrol. If the contribution of the thermal initiation and bimolecular termination is very small, the molecular weight distribution is close to the Poisson distribution, i.e., Mw/Mn=1 + 1/Pn, where Pn is the degree of polymerization. It was shown that when the number of... 

As the initiator, a common radical initiator and arenesulfonyl chloride are also used [286,287]. As shown in Table 6, this polymerization has a significantly large polymerization rate, and it is hardly disturbed by impurities such as alcohol and water [288]. ATRP with Cu complex was also applied to the polymerization of acrylates [289,290], methacrylates [290-297], and AN [298] as well as St [288, 297, 299]. Because of the suppressed bimolecular termination, hyperbranched polymers are readily prepared [292], being similar to the polymerization with TEMPO previously described. 

It can be observed that the initial rate of polymerization decreases and the autoacceleration peak is suppressed as the TED concentration is increased. The TED molecules generate dithiocarbamyl (DTC) radicals upon initiation. As a result, termination may occur by carbon-carbon combination which leads to a dead polymer and by carbon-DTC radical reaction which produces a reinitiatable ( living ) polymer. The cross-termination of carbon-DTC radicals occurs early in the reaction (with the carbon-carbon radical termination), and this feature is observed by the suppression of the initial rate of polymerization. As the conversion increases, the viscosity of the system poses mass transfer limitations to the bimolecular termination of carbon radicals. As has been observed in Figure 3, this effect results in a decrease in the ktCC. However, as the DTC radicals are small and mobile, the crosstermination does not become diffusion limited, i.e., the kinetic constant for termination of carbon-DTC radicals, ktCS, does not decrease. Therefore, the crosstermination becomes the dominant reaction pathway. This leads to a suppression of the autoacceleration peak as the carbon-DTC radical termination limits the carbon radical concentration to a low value, thus limiting the rate of polymerization. This observation is in accordance with results of previous studies (10) with XDT and TED, where it was found that when there was an excess of DTC radicals, the carbon radical concentration was lower and the cross-termination reaction was the dominant termination pathway. 

The polymerization reaction was found to develop both faster and more extensively as IQ was increased, up to a certain value above which identical RTIR curves were recorded. Consequently, the (Rp)max value reaches an upper limit, as shown in Figure 5 where (Rp)max was plotted versus Iq on a logarithmic scale. The slope of the straight line obtained at low light intensities, 0.55, is close to the 0.5 value expected for a photoinitiated radical polymerization involving bimolecular termination reactions. 

It is not possible to give here a detailed account of, nor to take issue with, every aspect of the interpretation which the authors give to their results. Their main conclusion is that the inverse correlation between DP and conductivity proves that the principal chain breaking reaction must be a bimolecular termination between free cations at the growing end of the chain and free anions in the solution. However, the arguments which lead to... 

We assume that only free ions propagate the reaction and take part in the transfer and the bimolecular termination reactions, we neglect the unimolecular termination, characterised by kt, which can only occur in an ion-pair, since the very small value of kf kfl hardly exceeds the experimental uncertainty. 

Unlike ionic polymerizations, the termination of the growing free radical chains usually occurs by coupling of two macroradicals. Thus, the kinetic chain length (v) is equal to DP/2. The chemical and kinetic equations for bimolecular termination are shown below (Equations 6.17 and 6.18). 

Alternately, the termination mode may change from the normal bimolecular termination between propagating radicals to primary termination, which involves propagating radicals reacting with primary radicals [Berger et al., 1977 David et al., 2001 Ito, 1980] ... 

For the case where retardation is strong kz/kp 3> 1), normal bimolecular termination is negligible. Under these conditions the first term in Eq. 3-136 is negligible and one has... 

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