Pseudo-Cationic Polymerization

Most cationic polymerizations of olefins proceed through carbon cation carriers. There are, however, instances of cationic polymerizations where the evidence was interpreted as suggesting that the propagating species are not carbon cations. Instead, the reactions were said to proceed through covalently propagating species. Such reactions were termed pseudo cationic [109, 110]. In these polymerizations the propagating species may be combinations of ionic (free ions and ion pairs) and covalently bonded species. Reaction conditions were claimed to determine the relative amounts of each. Examples of such polymerizations are cationic polymerizations of styrene or acenaphthene with protonic acids like HClO4 or iodine as the initiators. The propagations were suggested [110] to take place in three successive stages when the reactions are carried out in methylene chloride at 20C. In the first one, rapid, short-lived ionic reactions take place. In the second stage, the ions can no longer be detected by spectroscopy or conductivity measurements. In the third one, rapid increase in the presence of ionic species was shown (detected by conductivity measurements and spectroscopy). At temperatures between 20 and 30C there is effectively no stage one and stage three is shorter. On the other hand, at temperatures as low as 80C there is only stage one. In stages one and three, propagation takes places through combinations of free ions and ion pairs. These combinations of ions were claimed to result in formations of covalent perchlorate esters that are solvated and stabilized by monomer. The propagation in stage two, therefore, is pseudo cationic and covalent because it consists of monomer insertion into the C–O bond of a perchlorate ester [109, 110]:

Whenthere are insufficient amounts of monomer present to stabilize the covalent esters, such as at high conversions, ionizations take place. This leads to rapid ionic, stage three polymerizations. The propagation by free ions and ion pairs in stage three is between 104 and 105 L/mole s at low temperatures, between 60 and 80C. By comparison, polymerizations in stage two are much slower, somewhere between 0.1 and 20 L/mole s. The rates depend, of course, upon the solvent and upon the temperature. While ion–ion pair type propagations yield polymers with molecular weights equal to 104, the covalent propagations only yield oligomers. Differences in molecular weights and molecular weight distributions of the products from the three stages of polymerization can be detected by size exclusion chromatography. The chromatogram shows bimodal distributions. This was taken as supporting evidence for two modes of propagation, ionic and covalent [110]. Similar results were reported in polymerizations of styrene with CH₃COClO₄, CF₃SO₃H, CF₃COOH,ClSO₃H,FSO₃H[109]. Pseudo-cationic mechanism was also claimed in polymerizations of some styrene derivatives, like p-methyl styrene, p-methoxystyrene, and p-chlorostyrene with these protonic acids [109]. Although the concept of pseudo-cationic mechanism may be accepted by some, it is not accepted by many. An alternative mechanism based on ion pairs were offered instead in the past [111–113]. A publication by Scwarc claims that all detailed re-examinations of the evidence for pseudo-cationic polymerizations shows that all the features of styrene polymerization initiated by acids are accounted for satisfactorily and convincingly by the orthodox ionic mechanism. To support his claim, he reported that optical absorption and electric conductance of the styrene solutions undergoing polymerization induced by perchloric acid in stopped-flow studies demonstrated protonations of the monomer by the acid and formation of ionic species [114]. Further proof against pseudo-cationic polymerization mechanism came from an earlier work by Matyjaszewski who successfully synthesized the styryl perchlorate and demonstrated that it hydrolyzes rapidly in reaction with water [115]:

This contradicts the basic claim of pseudo-cationic polymerization mechanism, as originally proposed by Gandini and Plesch, that the hydrolysis of the ester should be negligible [109].

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منصات التواصل تضرّ بنفسية المراهقين… والفتيات الأكثر تأثرا

ابتكار ألماني قد يغيّر مستقبل صناعة البلاستيك

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مواضيع ذات صلة

Living Cationic Polymerizations

Steric Control in Cationic Polymerization

Propagation in Cationic Polymerization

Electro Initiation of Polymerization

Charge Transfer Complexes in Ionic Initiations

One Electron Transposition Initiation Reactions

Metal Alkyls in Initiations of Cationic Polymerizations

Lewis Acids in Cationic Initiations

Initiation by Protonic Acids

Cationic Polymerization

The Chemistry of Ionic Chain-Growth Polymerization

Polymer Preparation Techniques