Unimolecular termination

Assuming that the number average degree of polymerization (DP ) is determined by chain transfer to monomer and assuming unimolecular termination relative to propagation (i.e., chain breaking due to solvent, polymer, impurities are absent), the simple Mayo equation55 ... 

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. 

The rate of chain-breaking is made up of the rates of unimolecular termination and monomer transfer, kt + kml = J0 say, and the rate of bimolecular chain-breaking by various reagents, / . Thus... 

In order to account for the close to first-order kinetic law observed, one has to consider the contribution of a unimolecular termination process that would involve only one reactive radical. Various reactions can be postulated ... 

Finally, it must be pointed out that the close to first-order kinetic law observed in this study is by no means specific to polymerizations induced by intense laser irradiation a similar kinetic law was obtained by exposing these multiacrylic photoresists to conventional UV light sources that were operated at much lower light-intensities (27,34). This indicates that the unimolecular termination process does not depend so much on the rate and type of initiation used but rather on the monomer functionality and on the cross-link density which appear as the decisive factors. 

Using the same toluene-benzoyl peroxide system Nakatsuka (105) measured polymerization rate and molecular weight as functions of temperature (40° and 58°) and of the concentration of.three retarders p-nitrophenol, 2,4-dinitrophenol and picric acid. Results were consistent with a kinetic scheme postulating (among other things) bimolecular initiation involving peroxide and monomer and spontaneous unimolecular termination of growing polymer chains. 

In general, in only one system studied so far, the a-methylstyrene system, has a 0.5 order dependence on dose rate been observed in the 103 to 106- rads/hour range (24). This failure of other systems to approach the predicted square-root behavior might be ascribed to a radio-lytically formed inhibitor—a unimolecular terminating agent. This eventuality is not incorporated into the scheme now under discussion. 

Termination has no effect on the final number-average molecular weights, because it does not change the total number of chains. However, termination may lead to incomplete polymerization if the initiator concentration is too low. In the case of unimolecular termination, the final monomer conversion is set by Eq. (1). 

Two mqor problems remain open concerning the nature of rome of the reactions involved in these systems, namely the mechanism of the unimolecular termination reaction detected by Subira et al. and the structure of the active ecies, i.e. their state of dissociation. An answer to the first of these problems might come from conadering the following reaction ... 

For fast initiatnn and unimolecular termination (first-order idative to active specfes) we... 

The kinetics of the polymerization of tri- and tetraphosphonitrilic chlorides in solution and in bulk have been studied by Patat and Kol-linsky (58) and Patat and Frombling (57). Hydrocarbons are unsuitable solvents, since they react to give hydrogen chloride successful results w ere obtained in carbon tetrachloride. The proposed mechanism involves unimolecular initiation, either by oxygen (in solution) or another phosphonitrilic molecule (in bulk). A bimolecular propagation step is followed by unimolecular termination. Traces of water were foimd by Renaud to have a significant effect on the polymerization process (62, 63). 

Among all the radical processes previously described, more specific techniques can lead to the synthesis of macromonomers. For instance, the use of borans in radical polymerization, or the radical polymerization based on unimolecular terminations, may allow macromonomers to be obtained. These specific techniques will be briefly summarized as only a few workers have investigated the use of such techniques, aiming at the synthesis of macromonomers. 

Some Japanese teams developed a novel technique, based on unimolecular termination, which allows separating both initiation and termination (or transfer) processes. After growing chains are obtained, the macroradical formed is able to react with another molecule (mainly unsaturated) to lead to a stable radical. This one may transfer to give another radical able to reinitiate a polymerization. This process was developed, aiming at synthesizing either telechelic oligomers [85] (Scheme 78) or macromonomers [86] (Scheme 79). 

Storey et al. [140] point out that in living polymerizations of monomers like isobutylene that are co-initiated by TiCLt temperatures as low as —80°C, the livingness is limited not by chain transfer to monomer but rather by a unimolecular termination process. Unimolecular terminations often involve p-proton expulsions to produce polymers with terminal unsaturation. They claim, however, that this does not happen here. Rather the normal ferf-chloride chain ends of polyisobutylene formed by this type of polymerization gradually become depleted. They propose, therefore, that an isomerization mechanism takes place instead in the presence of an active Lewis acid, tmder monomer starvation conditions. It can be illustrated as follows [140] ... 

In the initiation step, the initiator splits into two primary radicals R , each of which reacts with a functional group to form a chain initiating radical R ia. Because reactions by trapped radicals, as defined in this model by Wen and McCormick is very much slower than the reactions by active radicals, propagation and termination can be taken to involve only active radicals. Propagation consists of the growth of active radicals by the successive addition of functional groups. Because radical trapping is presumed to be permanent, it affects the rate in a manner similar to unimolecular termination. 

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. 

On the other hand, even a complete unimolecular termination by occlusion of polymer radicals may be expected to result in an apparent initiator order lower than unity, because the rate of occlusion depends on that of polymer formation, Rp. An increase of the initiator concentration causes an increase of R and consequently of the occlusion rate. That is, the rate of unimolecular termination due to polymer radical occlusion increase with the initiator concentration. In fact, the molecular weight of poly(NMM Am) decreased with increasing initiator concentration, as shown in Fig, 7, First order in the initiator concentration should be observed only if the lifetime of the active centres were independent of the initiator concentration. The Rp dependence on the monomer concentration was not linear but increased with increasing NMMAm concentration. Since poly(NMMAm) is considered to have a higher affinity to the monomer than to benzene (nonpolar solvent), swelling of the polymer microspheres is accelerated by increased NMMAm concentration. This... 

When the system Et4N" BF4 /styrene/nitrobenzene was electrolyzed with 0.35 mA for 60 min, some polystyrene was observed around the anode. Another field in which cations are assumed to be the intermediate is styrene polymerization by y-rays [94]. Careful work showed that the rate of polymerization gradually changes from 0.5 order, indicating a radical process, to a first-order process, suggesting an unimolecular termination characteristic for the cationic mechanism [95 99]. 

The phase separation of growing chains decreases the radical activity of the polymerization system known as unimolecular termination (the first order radical-loss process). This event strongly suppresses both termination and propagation events. The result is a decrease in the polymerization rate and the molecular weight of the polymer. Polymerization systems are characterized by limiting conversion caused by the decreased penetration of monomer into polymer coils and/or surroundings of reaction loci. 

Unimolecular termination a short polymer chain breaks up into products, but this rarely is accounted for in the bulk phase. 

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