Polymerization of substituted alkynes

A growing array of different terminal and internal alkynes have been polymerized [8]. Many polyalkynes are air-stable, soluble materials, and not highly conjugated. As new catalysts allow the polymerization of alkynes with an increasing variety of substituents, an exploration of what properties unsaturated polymers have to offer is warranted. In general, substituted polyacetylenes may or may not be colored, and tend to be more rigid than saturated polymers. Selected materials are described below and compiled in Table 10-1. 

In recent years, the use of metathesis catalysts to polymerize alkynes, instead of Ziegler-Natta catalysts, has increased. This is in part because they have been found to polymerize a wider range of monomers [8], and because the Schrock group has shown that well-defined metathesis catalysts allow some control of alkyne polymerizations (see below) [32, 67, 68]. Metathesis polymerizations differ from Ziegler-Natta polymerizations in that the active species is a metal-carbon double bond, or alkylidene . Alkynes add across this bond, in what may be thought of as a [2 2] cycloaddition [69] to form metallacycles. These in turn 

H or SiMc3 Me CFaSOa -1 R=H 598 (CH3CN) R=SiMej 610 (CH3CN) Soluble [73] 

Poly(phenylacetylenes) with ortho substituents on the phenyl ring and poly(2-ethy-nyl-IV-methylpyridium) derivatives [73] form an interesting subclass of substituted polyacetylenes. It has been found that the absorption maxima of the cis isomers of the phenyl polymers shift to longer wavelength as the size of the substituent is increased. For example, poly(o-me-thylphenylacetylene) absorbs at 440 nm, and poIy(o-trimethyIsiIyIphenyIacetylene) absorbs at 520 nm [74]. Evidently, the steric requirements, of the ortho substituents impose a planar conformation on the backbone (Fig. 10-9). 

Also shown in Table lO-l is an (alkylcyclohexylaryloxy)-substituted polyacetylene [77]. Polymers of this general structure have been found to display liquid-crystalline behavior. In contrast to vinyl-based liquid-crystalline polymers, the geometric isomerism of the main-chain double bonds plays a role in determining the type of phase that is found. Advincula et al. have examined Langmuir films of polyacetylenes at the air-water interface [78]. Polyacetylene derivatives are unusual in that the polymer backbone itself acts as a chromophore therefore, in studies such as these, UV-visible spectroscopy can be a sensitive probe of polymer conformation. 

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The polymerization of substituted alkynes is postulated to proceed either by the metathesis mechanism or by an insertion mechanism (18). Numerous alkyne derivates have been shown to polymerize in the presence of group V, VI, and VIII transition metal catalysts. 

Metals of Groups 5 and 6 (Nb, Ta, Mo and W) are known to form carbene complexes and are widely used in olefin metathesis [99, 100, 111]. Therefore, the polymerization of substituted alkynes with catalysts based on these metals is assumed... 

Finally, one may note the curious behaviour of alkynes. If Mo(CO)6/non-4-yne is irradiated at room temperature and an excess of 3-chlorophenol then added, there is rapid metathesis to give oct-4-yne and dec-5-yne with nearly 100% selectivity (Mortreux 1977). In contrast, W(CO)6/CCl4//iv (350 ran) induces polymerization of hept-2-yne while causing metathesis of pent-2-ene present in the same reaction mixture (Stockel 1978). The difference in behaviour of the two systems presumably lies in the ability of Mo(CO)6/non-4-yne// v to generate a metal carbyne on addition of the phenol, whereas W(CO)6/CCl4/Av gives only a metal carbene. Photoassisted polymerization of terminal alkynes takes place with WCV/iv in hydrocarbon solutions (Landon 1985) photocatalyzed polymerization of substituted alkynes is induced by W(CO)6/SnCl4// v (Tamura 1994). For related systems, see Ch. 10. 

Scheme XX. Possible products from a polymerization of substituted alkynes.

Xiao CS, Zhao CW, He P, Tang ZH, Chen XS, Jing XB (2010) Facile synthesis of glycopo-lypeptides by combination of ring-opening polymerization of an alkyne-substituted N-carboxyanhydride and click glycosylation . Macromol Rapid Commun 31 991-997... 

Polymerization of alkynes by Ni" complexes produces a variety of products which depend on conditions and especially on the particular nickel complex used. If, for instance, O-donor ligands such as acetylacetone or salicaldehyde are employed in a solvent such as tetrahydrofuran or dioxan, 4 coordination sites are available and cyclotetramerization occurs to give mainly cyclo-octatetraene (cot). If a less-labile ligand such as PPhj is incorporated, the coordination sites required for tetramerization are not available and cyclic trimerization to benzene predominates (Fig. A). These syntheses are amenable to extensive variation and adaptation. Substituted ring systems can be obtained from the appropriately substituted alkynes while linear polymers can also be produced. 

On the other hand polysilylalkynes with phenyl or allyl substituents are converted with triflic acid into polymeric alkynylsilyltriflates. These polymers react with many acidic element hydrogen compounds or lithium element compounds with formation of silicon element bonds. Thus we found an easy approach to numerous new functional substituted alkynes [12], Eq.(9) shows selected examples of this reaction type. 

Scheme 1 ROMP of a 2,3-disubstituted norbornadiene, a 2-substituted norborn-5-ene and polymerization of a 1-alkyne via a- and /3-addition, respectively. A and B are initiator-and termination-derived endgroups, respectively...

In 1975, it was discovered that WCk, which is a typical metathesis catalyst, is capable to catalyze the polymerization of phenylacetyl-ene. Subsequently, various substituted acetylenes have been polymerized by this type of catalyst. In 1983, poly(l-(trimethylsilyl)-l-propyne)) was synthesized in the presence of Tads and NbCls (35). The alkyne polymerization has many similarities with ROMP. 

Electron-withdrawing m-substituents will decrease the electron density at the aromatic terminus of the [3,3]-rearrangement, thereby retarding the reaction. This allows polymerization of the terminal alkyne to compete successfully with the cyclization. However, the presence of a m -substituent has a much more significant effect the cyclization of m-substituted aryl propargyl ethers can lead to two isomeric products (46a and b). 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

The stable silylenes 83-85 do not react with conventional C=C double bonds however, diazasilole 83 is an efficient catalyst for the polymerization of alkenes, terminal alkynes, and 1,3-butadienes <2000ACR704, 2002USP028920, 2004JOM4165>. The stable bisaminosilylene 85 reacts with the activated double bond in 177-phosphirenes 134. The heterobicyclobutane 135 is however only a transient species and after addition of a second silylene 85 phosphasiletes 136 were isolated. Use of more sterically demanding substituted phosphirenes hampered the attack of the second silylene and the phosphasiletes 137 and 138, which are valence isomers of bicyclobutane 135, were obtained (Scheme 14) <2004AGE3474>. 

In the course of time it appeared that many olefinic substrates could undergo this reaction in the presence of a transition metal compound, such as substituted alkenes, dienes, polyenes, and cyclic alkenes, and even alkynes. Calderon et al. were the first to realize that the ring-opening polymerization of cycloalkenes, which they observed with their tungsten-based catalyst system [4], and the disproportionation of acyclic olefins are, in fact, the same type of reaction. They introduced the more general name metathesis [2], The metathesis reaction has now become a common tool for the conversion of unsaturated compounds. In view of the limited space this intriguing reaction is reviewed only briefly more information can be found in a detailed and extensive monograph [5]. 

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