Cationic polymerization of aldehydes

The cationic polymerizations of formaldehyde can be carried out in anhydrous media with typical cationic initiators. Initiation takes place by an electrophilic addition of the initiating species to the carbonyl oxygens. This results in formations of oxonium ions [323]  

The oxonium ions may react as oxygen-carbon cations. The propagation steps consist of attacks by the electrophilic carbon atoms upon the carbonyl oxygens of the highly polar formaldehyde molecules  

In these polymerizations, the propagating oxonium ions are probably further solvated [226]  

Terminations of chain growths occur through recombinations, reactions with impurities, and by chain transfers  

Formaldehyde can polymerize in an anhydrous form in all three physical states of matter as a gas, as a liquid, or as a solid [322]. It also polymerizes in water with acid catalysts. Many Lewis acids are efficient catalysts for this reaction. In addition, some protonic acids are also effective. Among them are perchloric and sulfuric acids and monoesters of sulfuric acid [322]. 

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In cationic polymerizations of aldehydes the growing cation can always be solvated by the acetalic oxygens in the polyoxymetiiylene chain, viz. 

The cationic polymerization of cardanol under acidic conditions has been referred to earlier [170,171], NMR studies [16] indicated a carbonium ion initiated mechanism for oligomerization. PCP was found to be highly reactive with aldehydes, amines, and isocyates. Highly insoluble and infusible thermoset products could be obtained. Hexamine-cured PCP showed much superior thermal stability (Fig. 12) at temperatures above 500°C to that of the unmodified cardanol-formaldehyde resins. However, it was definitely inferior to phenolic resins at all temperatures. The difference in thermal stability between phenolic and PCP resins could be understood from the presence of the libile hydrocarbon segment in PCP. 

Cationic polymerization of alkenes and alkene derivatives has been carried out frequently in aqueous media.107 On the other hand, the reaction of simple olefins with aldehydes in the presence of an acid catalyst is referred to as the Prins reaction.108 The reaction can be carried out by using an aqueous solution of the aldehyde, often resulting in a mixture of carbon-carbon bond formation products.109 Recently, Li and co-workers reported a direct formation of tetrahydropyranol derivatives in water using a cerium-salt catalyzed cyclization in aqueous ionic liquids (Eq. 3.24).110... 

Competing side reactions in cationic polymerization of carbonyl monomers include cyclotrimerization and acetal interchange. Cyclotrimerization is minimized by low-polarity solvents, low temperatures, and initiators of low acidity. Acetal interchange reactions among different polymer chains do not occur except at higher temperatures. Acetaldehyde and higher aldehydes are reasonably reactive in cationic polymerization compared to formaldehyde. Haloaldehydes are lower in reactivity compared to their nonhalogen counterparts. 

Polymerization of aldehyde by typical cationic catalysts such as sulfuric acid and titanium tetrachloride is considered to reflect the steric factor. Acetaldehyde gave an isotactic-rich amorphous polymer whereas propionaldehyde and higher aldehydes gave isotactic crystalline ones. The yield of polymer and the stereospecificity of polymerization increased with the increase in the bulkiness of the alkyl group of the aldehyde (Table 7). 

The polymerization of aldehydes is initiated by ionic initiators and the polymerization proceeds by ionic propagation. No radical polymerization of aldehydes has been documented yet. In the case of anionic polymerizations the growing ion is an alkoxide ion. The cationic polymerization has as the propagating species an oxonium ion. Most recent experimental results have shown that haloaldehydes, such as chloral polymerize exclusively by an anionic mechanism. 

The kinetics of cationic polymerization of acetaldehyde or higher aldehydes in solution are even less understood. No rate data on acetaldehyde polymerization with BF3 in ether are available [6]. It is only known that after an induction period of 5—15 min a vigorous polymerization occurs which is completed in a few minutes. No attempts were made to control the temperature during this uncontrolled reaction and polymer precipitated. Other cationic polymerizations of acetaldehyde with less... 

A similar mechanism was also proposed for the cationic polymerization of a number of heterocycles and aldehydes under the action of Lewis acids, but the researchers reasoning was less convincing. 

Solutions of maleic anhydride in ether will initiate cationic polymerizations of isobutyl vinyl ether or Af-vinyl carbazole, if subjected to attacks by free radicals. The same is true if the solutions are irradiated with ultraviolet light or gamma rays. Also, active species are generated from reactions of aldehydes or ketones with maleic anhydride when attacked by free radicals or irradiated by UV light, or gamma rays from These active species are presumed to be formed through... 

J. Furukawa and T. Saegusa, Polymerization of Aldehydes and Oxides, Interscience, Wley, New York, 1963 S. Penczek and P. Kubisa, Cationic Ring Opening Polymerization Ethers, Chapt. 48 in Comprehensive Polymer Science, Vol. 3 (G.C. Eastmond, A. Ledwith, S. Russo, and P. Sigwalt, eds.), Peigamon Press, London, 1989. 

Many papers relate to the cationic polymerization of acrolein but the unambiguous estimation of this has not been made. Copolymerization of acrolein with styrene initiated by BFjOEt at - 78 °C produces a copolymer, which by n.m.r. analysis contains a large proportion of acrolein incorporated into the copolymer through aldehyde addition (92%). Kinetic equations have been proposed to describe the cationic copolymerization of 1,3-dioxolane with formaldehyde. ... 

It should be noted that ring-opening polymerization of cyclic acetals is not the only route to polyacetals. Polyacetals are also formed by ionic polymerization of aldehydes, by polycondensation of aldehydes and diols, or by polyaddition of divinyl ethers to diols. " In this chapter, however, only cationic ring-opening polymerization of cyclic acetals will be discussed. 

Cationic polymerization of cyclic acetals yields polyacetals, that is, polymers containing acetal bonds -OCR R O- in the main chain (the class name polyacetals should not be confused with the name poly (vinyl acetal )s, which is the class name for a group of polymers that are products of the reaction between poly (vinyl alcohol) and an aldehyde). Homopolymers of cyclic acetals are at the same time perfectly alternating copolymers composed of oxymethylene and oxyalkylene units, as shown for DXL polymerization in Scheme 2. 

The differing features of the base-assisting catalysts are opening possibilities for living cationic polymerization of new monomers, which had been considered difScult to polymerize in a controlled way. Aromatic aldehydes are difficult to polymerize cationically, and there have only been a few copolymerization... 

Other cationic processes include ring opening polymerization of epoxies, discussed below, and acid-catalyzed polymerization of aldehydes, discussed above. 

By organic chemistry formalism, polyacetals are reaction products of aldehydes with polyhydric alcohols. Polymers generated from aldehydes, however, either via cationic or anionic polymerization are generally known as polyacetals because of repeating acetal linkages. Formaldehyde polymers, which are commercially known as acetal resins, are produced by the cationic ring opening polymerization of the cyclic trimer of formaldehyde, viz., trioxane [29-30] (Fig. 1.5). 

The importance of the electrophilic character of the cation in organo-alkali compounds has been discussed by Morton (793,194) for a variety of reactions. Roha (195) reviewed the polymerization of diolefins with emphasis on the electrophilic metal component of the catalyst. In essence, this review willattempt to treat coordination polymerization with a wide variety of organometallic catalysts in a similar manner irrespective of the initiation and propagation mechanisms. The discussion will be restricted to the polymerization of olefins, vinyl monomers and diolefins, although it is evident that coordinated anionic and cationic mechanisms apply equally well to alkyl metal catalyzed polymerizations of polar monomers such as aldehydes and ketones. 

Table IV gives an example of our own work on the polymerization of a number of higher aldehydes. Potassium triphenylmethoxide—a soluble initiator—polymerized a number of higher aldehydes to crystalline isotactic poly aldehydes. Table V lists a number of alkali alkoxides and other related compounds used as initiators for the n-butyraldehyde polymerization. Neither the type of the alkoxide nor the cation is of any great importance for the polymerization rate, the polymer yield, and stereoregularity of the resulting polyaldehyde as long as the initiator is adequately soluble in the reaction mixture.

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