Alkyne metathesis polymerization, substituted

Figure 11. Metallocenyl-substituted 1-alkynes polymerized via metathesis polymerization.

A variety of heteroatom substituted 1-alkynes were polymerized with the three carbyne W(VI) complexes. These catalysts tolerate in alkyne polymerization reactions more heteroatom substituents than in olefin metathesis [4, 5, 6, 7], Table 3. 

Abstract Several routes to access ruthenium (Ru)-vinylidene complexes are described, the majority of which employ alkynes and a Ru source as the starting materials. The successful application of Ru-vinylidenes as efficient pre-catalysts for the synthesis of carbocyclic and heterocyclic compoimds by ring-closing metathesis (RCM) of a, -dienes, and in the synthesis of polymers by ring-opening metathesis polymerization (ROMP) of cyclooctene, norbomene, 5-substituted norbomene, and dicyclopentadiene is fully illustrated. Relevant aspects concerning the activity and selectivity of this type of Ru complexes in metathesis reactions are highlighted. 

Scheme XXL A proposed mechanism for the metathesis polymerization of substituted alkynes.

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. 

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. 

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]. 

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... 

The most recent application of olefin metathesis to the synthesis of polyenes has been described by Tao and Wagener [105,117], They use a molybdenum alkylidene catalyst to carry out acyclic diene metathesis (ADMET) (Fig. 10-20) on either 2,4-hexadiene or 2,4,6-octatriene. The Wagener group had earlier demonstrated that, for a number of nonconjugated dienes [118-120], these polymerizations can be driven to high polymer by removal of the volatile product (e. g., 2-butene). To date, insolubility limits the extent of polymerization of unsaturated monomers to polyenes containing 10 to 20 double bonds. However, this route has some potential for the synthesis of new substituted polyacetylenes. Since most of the monomer unit is preformed before polymerization, it is possible that substitution patterns which cannot be incorporated into an alkyne or a cyclic olefin can be built into an ADMET monomer. 

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. 

On the other hand, metathesis catalysts based on group V and VI metals effectively polymerize mono- and disubstituted alkynes to the corresponding substituted PAs. These catalysts are typically the metal chlorides, used with or without main-group organometallic cocatafysts, or metal carbonyls activated with light (Fig. 7) [110]. The latter e of catalyst is known for Mo and W only. Water can even be used as a cocatalyst with these catalysts for some monomers. For example, WQ5 I/2H2O polymerizes phenylacetylene to a soluble, powdery poly(phenylacetylene) with M = 15,000 g/mol and PDI = 2.06 [113]. 

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