Polymerization of acetaldehyde

An accessory proposal was Arthur Michael s hypothesis that many reactions proceed by addition, for example, a polymerization of acetaldehyde (CH3CH = O) in the presence of bases (OH) to an aldol (CH3CHOHCH2CHO), with subsequent loss of water to form crotonaldehyde (CH3CH = CHCHO). Michael, educated in America, Germany, and France, made use of Kekule s idea that two molecules may form a "polymolecule" or molecular compound, which, in turn breaks up to yield the final products.33 Lachman expressed fairly standard misgivings about this proposal of an intermediary and transition form "If we are going to explain reactions by means of addition products which we do not or cannot isolate, our explanation loses its definiteness. It becomes simply a possible explanation, and its conclusions are by no means binding."34... 

Like formaldehyde, acetaldehyde easily forms polymers, in this case paraldehyde and metaldehyde. Paraldehyde will form when hydrochloric or sulfuric acid is added to acetaldehyde. Polymerization of acetaldehyde to mec-aldehyde occurs in the gaseous phase in the presence of aluminum oxide or silicon dioxide catalyst. 

Paraldehyde Paraldehyde, 2,4,6-trimethyl-l,3,5-trioxane (4.3.2), is a trimeric acetaldehyde which is synthesized by the acid-catalyzed polymerization of acetaldehyde and at moderate and high temperatures [32,33]. 

The isoselective polymerization of acetaldehyde has been achieved using initiators such as zinc and aluminum alkyls, Grignard reagents, and lithium alkoxides [Kubisa et al., 1980 Pasquon et al., 1989 Pregaglia and Binaghi, 1967 Tani, 1973 Vogl, 2000], The isoselectivity is high in some systems with isotactic indices of 80-90%. Cationic initiators such as BF3... 

All these organometallic compounds, except for [V], were isolated in pure crystalline state. The stereospedfic polymerization of acetaldehyde was catalyzed by [I] to [IV], and that of propylene oxide by [V] to [VIII]. The crystalline organometallic compound is superior to the liquid one for our purpose, because the former can be purified more easily and extensively than the latter in a small scale operation. 

Stereospecific polymerization of acetaldehyde was discovered in 1960 by Natta 17), Furukawa (18), and Vogl (19) independently. Vogl used an alkali metal compound as a catalyst, while other workers used organoaluminum or organozinc and their derivatives. 

Furukawa and his coworkers also observed that, in the polymerization of acetaldehyde by the in situ triethylaluminum-alcohol (1 1) catalyst system, the bulkiness of the alkoxyl group bonded to an aluminum atom had an influence on the stereospecificity of poly-... 

Fig. 1. The proposed model of stereoregulation in the propagation process of isotactic polymerization of acetaldehyde by organoaluminum catalyst [Natta...

The reaction mixtures of A1R3 with acid amide in various molar ratios were used as catalysts for the polymerization of acetaldehyde. The yield of crystalline polymer was maximum and that of amorphous one was minimum at a molar ratio of 1 1 (Fig. 2). 

Fig. 2. Polymerization of acetaldehyde by the in situ AlEt3—MeCONHPh Catalyst system [Yasuda et al. (42)]...

The AlR3-acid amide (1 1) catalyst system thus proved to be an excellent catalyst for the stereospedfic polymerization of acetaldehyde and to be far superior to the AlR3-ketone (1 1) catalyst system. Both linear and cyclic acid amides gave a similar degree of stereospecificity with good reproducibility (35). We considered that the former catalyst system would be more suitable for our purpose, because this type of... 

Table 1. Polymerization of acetaldehyde by AlEt3-ketone or acid amide (1 1)...

In general, in the stereospecific polymerization of acetaldehyde by organoaluminum catalysts, the temperature at which monomer and... 

Table 4. Effect of monomer charging temperature on polymerization of acetaldehyde...

Table 5. Polymerizations of acetaldehyde by some electron donor complexes...

Table 8. Effect of nature of drying agent on polymerization of acetaldehyde by...

Fig. 11. The proposed model of the initiation of isotactic polymerization of acetaldehyde [Yasuda et cd. (42j ...

In order to find a highly stereospedfic, homogeneous catalyst for the polymerization of propylene oxide, we selected the organometallic compound-primary amine catalyst system which exhibited excellent stereospecificity in the polymerization of acetaldehyde. 

The strategy we adopted for attacking our problem, i.e., the complete understanding of the stereoregulation mechanism in the stereospecific polymerization reaction, has been successfully applied to the stereospecific polymerization of acetaldehyde and propylene oxide. The same strategy should be applicable also to other types of catalysts and monomers, even if the difficulty encountered in the experimental performance is greater. The fruitful harvest must await future investigation. 

Tani,H. Stereospecific polymerization of acetaldehyde and propylene oxide. Kogyo Kagaku Zasshi 70,1895 (1967). 

Yasuda,H, Fujita,K Yamamoto,S, Tani,H. Stereospedfic polymerization of acetaldehyde by diethylaluminum butoxides. J. Polymer Sci. A-l to be published. 

Tani,H, Aoyagi,T, Araki,T. Polymerization of acetaldehyde using AlEt3-alkali metal hydroxide systems. J. Polymer Set B2, 921 (1964). 

Oguni,N. Stereospedfic polymerization of acetaldehyde using RjAl-NR"-AJR2 and Li+[R3AJ-NR -A1R2]- as catalysts. J. Polymer Sd. B3,123 (1965)... 

Araki,T, Oguni,N, Aoyagi.T. A new series of organoaluminum complexes and their use as highly stereospecific catalysts for the polymerization of acetaldehyde. J. Polymer Sd. B4, 97 (1966). 

Tani,H., Yasuda,H., Araki,T. Stereospecific polymerization of acetaldehyde with catalyst systems AlEt3-ketone-H20 and AlEt3-amide-(H20). J. Polymer Sci. B 2,933 (1964). 

Yasuda.H. Stereospedfic polymerization of acetaldehyde by R2A10CR NPh active spedes and essential role of cocatalyst J. Polymer Sd. B 7,17 (1969). 

Yasuda,H., Araki.T., Tani,H. Structure and chemical behavior of an organoaluminum [R2A10CR NPh]2. A stereospedfic catalyst for polymerization of acetaldehyde. J. Organomatal. Chem. in press. 

By 1951 (29) it had been concluded that polyacetaldehyde is an amorphous polyacetal of very high molecular weight. The polymerization of acetaldehyde is initiated by free radicals assisted by the crystallization of the monomer (crystallization polymerization). 

Letort found later that the crystallization polymerization of acetaldehyde is favorably influenced by acids (28)... 

In addition to well-recognized cationic initiators for the solution polymerization of acetaldehyde (50, 51, 52) more specialized systems for preparing elastomeric polyacetaldehyde have been uncovered. They include the polymerization of acetaldehyde by condensing the monomer onto 7-alumina (17, 18, 19, 21) from the vapor phase or polymerization with acid-treated oxide supports (43, 54), the use of phosphines (20) as initiator and polymerization by x-rays (41) and 7-rays (8). 

Rapid polymerization of acetaldehyde to amorphous polymer has been observed with highly active initiators at very low temperatures in solvents, where the polymer is not soluble. Since the polymer is below the Tg at reaction temperature it precipitates as a glass. The rate of polymer formation is almost explosive. With weakly cationic initiators acetaldehyde can be polymerized to low molecular weight polymer where the polymer remains in solution. 

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