Mechanism of oxidative polymerization

On the other hand, the intermediate fraction (G3) shows an increase in TOC. This shift could be partially due to transformation of the HMW to fraction G3. However, the increase in TOC for fraction G3 seems to be too great to be explained solely by migration from Gl to G3. Thus, some of the TOC may have shifted from G5 to G3, either because of the formation of polar oxidation byproducts with elution volumes matching those of G3 (under the experimental conditions used, certain carboxylic acids such as acetic or citric acid would be eluted in fraction G3) or possibly by the mechanism of oxidative polymerization (15,16). 

Many amine-copper complexes, as well as a few amine complexes of other metals, and certain metal oxides have since been shown to induce similar reactions (17, 18, 22, 23, 30). This chapter is concerned largely with the mechanism of oxidative polymerization of phenols to linear polyarylene ethers most of the work reported has dealt with the copper-amine catalyzed oxidation of 2,6-xylenol, which is the basis for the commercial production of the polymer marketed under the trade name PPO, but the principal features of the reaction are common to the oxidative polymerization of other 2,6-disubstituted phenols. 

Figure 51 Mechanism of oxidative polymerization of derivatives of 4,4 -bis(phenoxy)diphenyl sulfone and 4,4 -bis(phenylthio)diphenyI sulfone. (From Ref. 210.)...

Due to the difficulties of studying a system with a heterogeneous, strongly oxidizing catalyst that produces difficult-to-characterize rigid-rod polymers, the mechanism of oxidative polymerization is not easy to decide. However, the RC mechanism seems to be the likely route for PTh synthesis. 

Electrochemical polymerization of pyrrole is the most versatile and widely used method for PPy synthesis. PPy can be polymerized both from an aqueous or organic solvent medium, and at neutral pH [4,10], A general mechanism of oxidative polymerization of pyrrole is shown in Figure 6.8. 

The reaction mechanism of oxidative polymerization of aniline has been a big controversy (Fig. 7). The dimerization step is generally proposed as (i), in which aniline is one-electron-oxidized to a cation radical, followed by coupling of two molecules of the cation radical to a dimer. The subsequent steps of chain extension are under discussion routes involving coupling of cation radicals such as (ii)-(iv) (137-140) and routes via electrophilic attack of a two-electron-oxidized quinodal diiminium ion (v) or nitrenium ion (vi) (141,142) have been proposed. The addition of electron-rich arenes does not inhibit the polymerization, and therefore the route through the nitrenium ion (vi) seems to be rejected (137). 

The mechanism of oxidative dyeing involves a complex system of consecutive, competing, and autocatalytic reactions in which the final color depends on the efficiency with which the various couplers compete with one another for the available diimine. In addition, hydrolysis, oxidation, or polymerization of diimine may take place. Therefore, the color of a mixture caimot readily be predicted and involves trial and error. Though oxidation dyes produce fast colors, some off-shade fading does occur, particularly the development of a red tinge by the slow transformation of the blue indamine dye to a red phenazine dye. 

In contrast to the extensive work of the pure thermal degradation of polymers, less fundamental chemical information is available on the mechanism of oxidative degradation of polymeric materials. As another point of... 

The mechanism of the polymerization was discussed based on electrochemical measurements. Applications of the electro-oxidative polymerization were also described. 

Lewis acids readily isomerize both 1,3-dioxolanes and 1,3-oxathiolanes in ether solution. The reaction proceeds by coordination with the oxygen atom in the latter case since 1,3-dithiolanes do not isomerize under the same conditions. With trityl carbonium ion, an oxidative cleavage reaction takes place as shown in Scheme 6. Hydride extraction from the 4-position of 2,2-disubstituted 1,3-dioxolanes leads to an a-ketol in a preparatively useful reaction. 1,3-Oxathiolanes are reported to undergo similar cleavage but no mention of products other than regeneration of the ketone has been made (71CC861). Cationic polymerization of 1,3-dioxolane has been initiated by a wide variety of proton acids, Lewis acids and complex catalytic systems. The exact mechanism of the polymerization is still the subject of controversy, as is the structure of the polymer itself. It is unclear if polymerization... 

The mechanism of this polymerization has been discussed by Cundy et al. (9). The first step is apparently the insertion of a low-valent metal into the strained C—Si bond to give a silametallacyclopentane. Metallacycles 4 and 5 have in fact been isolated from the reactions of Fe2(CO)9 and CpMn(CO)3 with 1, respectively (10, 11). Complex 4 when treated with phosphines gives polymer and LFe(CO>4. If the metallacycle resulting from insertion (6) is unstable, repeated insertions (oxidative additions) and reductive eliminations lead to polymer. Chain termination results from reductive elimination of =SiH [Eq. (13)]. 

In the usual SjsjZ (A 2) mechanism, the reaction of the water molecule with the protonated compound is rate determining, while in the pure S l (A 1) mechanism, the unimolecular opening of the protonated ring is rate determining. Discussing the mechanism of the polymerization of isobutylene oxide it was eventually concluded that stereochemical studies would be needed to distinguish between these possibilities. 

The kinetics of oxidative polymerization of 1,8-nonadiyne have been studied using oxygen and homogeneous catalysts derived from copper(i) chloride and tertiary amines . A mechanism of the same type as that described above for dimerization was proposed for the polymerization. 

The initial step in the mechanism of ethylene polymerization using Phillips catalysts is believed to occur by way of an oxidation-reduction reaction between Cr (VI) and ethylene as depicted in eq 5.1. This generates Cr (II) and vacant coordination sites. As mentioned above, polymerization may be initially slow because of sluggish reduction or desorption of the oxidation by-products which can coordinate with (and block) active centers. 

It is usually assumed that the mechanism of chemical polymerization is similar to that described earlier in electropolymerization. However, work in our own laboratories71 highlights the fact that it is difficult to duplicate the products of electropolymerization using a chemical oxidant. Studies using 3-methyl-4-carboxy-pyrrole have demonstrated that polymers obtained with both techniques are similar in chemical composition but differ markedly with respect to polymer morphology. 

In a series of papers43 4S), the kinetics of anionic polymerization of ethylene oxide in conjunction with different catalysts were studied. These studies expand our understanding of the mechanism of living polymerization systems and provide new information on the processes of active center association. Herein, primarily, lies the specific nature of the heteroatomic systems, as compared with the vinyl monomers studied earlier 9 ... 

Kinetic experiments and pulse irradiation studies on the mechanism of DHI polymerization (2, 136) revealed that the dominating species formed by DHI oxidation was the quinone methide and that coupling proceeds via oxygen-centered semiquinone radicals, which can also account for the complexity of the later stages of melanogenesis and the heterogeneity of melanin structure. 

Fig. 14.7 Mechanism of oxidative degradation of full-carbon backbone polymeric materials, a General scheme including also the ultimate stage of biodegradation, b Specific transition metal salts able to promote oxidation followed by degradation of full-carbon backbone polymers in a tandem fashion action...

---