Anionic polymerization of aldehydes

Many patents describe polymerizations of anhydrous formaldehyde by anionic mechanism. The initiators included amines, phosphines, and metal alcoholates. Kern pictured initiations of formaldehyde polymerizations by tertiary amines as direct addition reactions  

Earlier, however, Machacek suggested that the initiations take place with the help of protonic impurities  

Much of the evidence presented since favors the Machacek mechanism of initiation. By contrast. 

The propagation reactions in tertiaiy amine initiated polymerizations can be pictured as follows  

The terminations probably result from chain transferring  

Much of the evidence presented since favors the Machacek mechanism of initiation [335], By contrast, tertiary phosphines apparently do initiate such polymerizations by a zwitter ion mechanism [338]. This may, perhaps, be due to higher nucleophihcity and lower basicity than that of the tertiary amines. Phosphorus incorporates into the polymer [338] in the process. 

The terminations, probably, result from chain transferring [241]  

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What is the mechanism of control in anionic polymerizations of aldehydes ... 

What is the mechanism control in anionic polymerization of aldehydes polymerizations ... 

Polyphenylsilane, (PhSiH) , can be derivatized by free-radical hydrosilylation in the presence of a radical initiator. Alkenes, ketones and aldehydes react readily, to replace up to 93% of the Si-H bonds. This route can be employed to make polysilanes with hydrophilic groups, such as hydroxy, amino and carboxylic acid functions.43 Dialkylamino substituted polysilanes, made by the anionic polymerization of masked disilenes (see equation (17)), when treated with acetyl chloride give chloro-substituted poly silanes. The chlorine can then be displaced by other nucleophiles.27... 

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. 

Polyacetals form a different subclass of compounds with oxygen in the backbone chain. In this group are included polymers that contain the group -0-C(R2)-0- and can be formed from the polymerization of aldehydes or ketones. A typical example of a polymer from this class is paraformaldehyde or polyformaldehyde or polyoxymethylene (CH20)n. Polyoxymethylene can be prepared by anionic catalysis from formaldehyde in an inert solvent. Acetylation of the -OH end groups of the polymeric chain is common since it improves the thermal stability of the polymer. Some results reported in literature regarding thermal decomposition of these polymers are indicated in Table 9.2.1 [1]. 

The polymerization of aldehydes to give polyacetal is readily undertaken anionically in the presence of base, due to the susceptibility of formaldehyde to nucleophilic attack (Odian, 1991) as shown in Scheme 1.25. It should be noted that the resulting polymer is thermally unstable and stabilization is achieved by an esterification reaction of the unstable hemiacetal end groups after polymerization. 

There are indications that many aldehyde polymerizations result in formations of living polymers, similarly to anionic polymerizations of vinyl compounds. Termination can occur through hydride transfer via a form of a crossed Cannizzaro reaction ... 

A stereospecific anionic polymerization of acetaldehyde was originally reported in 1960 [343, 344]. Two alkali metal compounds [341] and an organozinc [342] one were used as the initiators. Trialkylaluminum and triarylaluminum in heptane also yield crystalline, isotactic polymers from acetaldehyde, heptaldehyde, and propionaldehyde at -80°C [343]. Aluminum oxide, activated by diethylzinc, yields stereoblock crystalline polymers from various aldehydes [342, 344], Lithium alkoxide formed polyacetaldehyde is insoluble in common solvents. It melts at 165°C [341],... 

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). 

They proposed a polymerization scheme closely related to other well-known chemical reactions of metal alkoxide with carbonyl compounds (20). In Scheme 2, complex [A] is converted to [B] by hydride ion transfer from the alkoxyl group to the carbon atom of aldehyde (Meerwein-Ponndorf reduction). Addition of one molecule of monomer to the growing chain requires transfer of the alkoxide anion to the carbonyl group to form a new alkoxide [C]. Repetition of these two consecutive processes, i.e., coordination of aldehyde and transfer of the alkoxide anion, constitutes the chain propagation step. 

Under mild conditions, hydroformylation of olefins with rhodium carbonyl complexes selectively produces aldehydes. A one-step synthesis of oxo alcohols is possible using monomeric or polymeric amines, such as dimethylbenzylamine or anion exchange resin analog to hydrogenate the aldehyde. The rate of aldehyde hydrogenation passes through a maximum as amine basicity and concentration increase. IR data of the reaction reveal that anionic rhodium carbonyl clusters, normally absent, are formed on addition of amine. Aldehyde hydrogenation is attributed to enhanced hydridic character of a Rh-H intermediate via amine coordination to rhodium. 

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. 

We will discuss the scarce data that are available on the rates of polymerization in an attempt to critically review their usefulness at the present time. The subject is divided into four types of polymerization that lead to higher aldehyde polymers (a) the so-called crystallization polymerization (b) cationic polymerization (c) anionic polymerization and (d) polymerization with aluminium alkyls and related compounds. 

Volume 15 deals with those polymerization processes which do not involve free radicals as intermediates. Chapters 1 and 2 cover homogeneous anionic and cationic polymerization, respectively, and Chapter 3 polymerizations initiated by Zeigler-Natta and related organometallic catalysts. Chapters 4, 5 and 6 deal with the polymerization of cyclic ethers and sulphides, of aldehydes and of lactams, respectively. Finally, in Chapter 7 polycondensation reactions, and in Chapter 8 the polymerization of AT-carboxy-a-amino acid anhydrides, are discussed. 

The polymerization of the higher aliphatic aldehydes has many similarities with formaldehyde polymerization. Notable differences are a lower ceilii temperature and the possibility of different steric configurations due to the substituted carbon atom. Especially anionic catalysts such as alkali metal alkoxides, soluble hydrides, and organo metal compounds lead to polymerizations during which crystalline isotactic polymer is produced 96). Little is known about the morphology and the detailed crystallization mechanism of the polyaldehydes. 

Problem 8.1 Contrary to the high selectivity shown in cationic and anionic polymerization, radical initiators can bring about the polymerization of almost any carbon-carbon double bond. However, aldehydes and ketones are not activated by free radicals. Explain, giving reasons. 

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