Alkyl-substituted acetylene polymerization

Two mechanisms have been proposed for acetylene and substituted acetylene polymerization by transition metal catalysts one is the metal-alkyl mechanism and the other is the metal-carbene mechanism. In general, it has been proposed that the polymerization of acetylenes by Ziegler-Natta catalysts proceeds by the metal-alkyl mechanism, while the metal-carbene mechanism has been accepted for the polymerization of substituted acetylenes by metathesis catalysts whose main components are halides or complexes of group 5 and 6 transition metals. The latter will be discussed in Section III. 

Well-controlled polymerization of substituted acetylenes was also reported. A tetracoordinate organorhodium complex induces the stereospecific living polymerization of phenylacetylene.600 The polymerization proceeds via a 2-1 -insertion mechanism to provide stereoregular poly(phenylacetylene) with m-transoidal backbone structure. Rh complexes were also used in the same process in supercritical C02601 and in the polymerization of terminal alkyl- and arylacetylenes.602 Single-component transition-metal catalysts based on Ni acetylides603 and Pd acet-ylides604 were used in the polymerization of p-diethynylbenzene. 

Figure 2. Schematic representation of a possible mode of polymerization of an alkyl substituted terminal acetylene.

With catalyst 26, 1-chloro-l-alkynes with different alkyl lengths also undergo living polymerizations. Consequently, the sequential addition of 1-chloro-l-hexadecyne (A), 1-chloro-l-hexyne (B) and 1-chloro-l-hexadecyne (A) in this order provides an A-B-A-type triblock copolymer. Similarly, one can obtain a B-A-B-type triblock copolymer. These are the first examples of block copolymers from substituted acetylenes. 

The polymerization of alkyl-substituted, terminal acetylenes was carried out at 20°C, using rare earth naphthenates with tri-isobutylaluminum or triethyl-aluminum as the catalyst in cyclohexane or chlorobenzene by Shen and Farona... 

The assignment of the monomer solution spectrum was performed by using an NMR spectral database system (SDBS-NMR)54. The signals of the six acetylene carbons from 60.34 to 81.91 ppm in the solution spectrum indicated the monomer structure of a dodec-ahexyne derivative substituted symmetrically by alkyl groups. Since the spectral patterns in Figure 36A are almost the same as those of the monomer, only a small extent of polymerization had occurred during the 30 min after recrystallization. The signal at about... 

The work of Nazarov on vinyl ethynyl carbinols involves condensation of vinylacetylene with ketones in the presence of caustic potash and also their conversions, many of which are catalytic in nature. A review of his work involving polymerization, isomerization, hydrogenation, and other conversions was published by him (252). Hydration of divinylacetylenes in methanol solution in the presence of mercuric sulfate and sulfuric acid gave vinyl alkyl ketones. These can be reacted with hydrogen sulfide, amines, etc., to yield heterocyclic compounds. Substituted vinyl alkyl ketones underwent spontaneous cyclization to cyclopentenones. Nazarov summarized a decade of this research in this field in 1951 (253). His general review of organic syntheses based on acetylene is also of interest in this connection (254). 

Table 5 lists representative examples of polymerization of monosubstituted acetylenes, catalysts, and MWs of the polymers formed. Mo, W, and Rh catalysts, all of which involve transition metals, are particularly effective. Whereas Mo and W catalysts are sensitive to polar groups in the monomers, Rh catalysts are tolerant to such groups. Mo and W catalysts are effective toward sterically aowded monomers, while Rh catalysts are rather restricted to a particular type of monomers including propargyl esters, N-propargylamides, alkyl propio-lates, and PAs. Fe and Pd complexes are also useful in some cases. It is noted that not only sterically unhindered monomers but also very aowded ones afford high-MW polymers with W and Mo catalysts. An overview of typical monosubstituted acetylene monomers such as aliphatic acetylenes, ring-substituted PAs, and other aryl acetylenes is presented below. 

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