Aldehydes anionic polymerization

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). 

Aldehyde Polymers Asymmetric anionic polymerization can lead trichloroacetalde-hyde (chloral) to a one-handed helical, isotactic polymer having a 4/1-helical conformation with... 

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. 

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. 

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. 

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 ... 

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

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],... 

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

Polybutadiene belongs to the most important rubbers for technical purposes. In 1999 more that 2 million tons were produced worldwide, that is about 20% of all synthetic rubbers [11,12]. The cis type made by 1,4-addition is economically the most important polybutadiene [13,14]. Trans- as well as isotactic, syndiotactic, or atactic 1,2-polybutadiene can also be synthesized in good purity with suitable catalysts. For anionic polymerization with butyllithium or the coordinative process with Ziegler catalysts, 1,3-butadiene must be carefully purified from reactive contaminants such as acetylene, aldehydes, or hydrogen sulfide. 

Anionenaustauscher (starker/schwacher) anionic polymerization anionische Polymerisation anisic aldehyde/ anisaldehyde Anisaldehyd anneal tempem ... 

Needless to say, the electrophilic aldehyde function of 4-formylstyrene (14) must be protected prior to anionic polymerization. The most suitable protected functionality for aldehydes may be the acydic and cydic acetals that are stable to highly reactive basic reagents and nudeophiles. Indeed, a styrene para-substituted with D-gJucose (7a), whose diol functions were protected as cydic acetals, successfully underwent living anionic polymerization, as mentioned in Section 3.18.2.1. The cydic acetal-protected functionalities were suffidently stable in THF at -78 °C, but were readily deaved to quantitatively regenerate the diol functions under mild acidic hydrolysis. 

Aldehydes can be protected as N,N-acetals, which are stable to highly basic reagents such as RLi. Therefore, the aldehyde function of 14 was treated with N,N -dimethylethylenediamine to convert it to a N,N-acetal-protected styrene (14e). ° Unlike the oxygen analogues, 14e was anionically polymerized without problem to afford a living polymer, which was stable in THF at -78 °C even after 24 h (Table 3). The protective group was quantitatively removed by acid hydrolysis with 2 N HCl in THF (Scheme 8). The SEC trace of the poly(14) thus obtained... 

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