Metathesis polymerization of acetylene

The reaction between acetylene and ethene, catalyzed by WCle in benzene at 20°C, gives polybutadiene (85% 1,2-, 15% 1,4-) of high MW. The initial formation of butadiene probably occurs by alternate addition of acetylene to [W]=CH2 to form [W]=CHCH=CH2, and chain transfer with ethene (Zaliznaya 1990). 

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Examples for catalysts are listed Table 1.5 and shown in Figure 1.6. For the metathesis polymerization of acetylene related compounds, catalysts with a metal carbyne bond have been introduced, such as... 

In principle, conjugated materials may either be directly synthesized via metathesis polymerization of acetylene or 1-alkynes, via ROMP of various cyclooctatetraenes (COTs) or via ROMP of polyene precursors as realized in the Durham route [107-111]. The first direct polymerization of acetylene to yield black untreatable unsubstituted polyacetylene was achieved with W(N-2,6-i-Pr2-C6H3)(CH-t-Bu)(OC-t-Bu)2 [112]. In order to obtain soluble polymers, polyenes were prepared via the ROMP of a polyene-precursor, 7,8-bis(trifluoromethyl)tricyclo[4.2.2.02 5]deca-3,7,9-triene (TCDTF6), using initiators such as W(N-2,6-i-Pr2-C6H3)(CH-t-Bu)(OC-t-Bu)2) (Scheme 5.10) [113, 114]. 

The direct metathesis polymerization of acetylenes is not the only route to polyacetylenes using olefin metathesis chemistry. Below are summarized some of the other methods that have been developed in recent years. 

Poly (acetylenes) [16], There are several catalysts available for polymerization of substituted acetylenes. Whereas Ziegler-Natta catalysts are quite effective for polymerization of acetylene itself and simple alkylacetylenes, they are not active towards other substituted acetylenes, e.g. phenylacetylenes. Olefin-metathesis catalysts (Masuda, 1985 Masuda and Higashimura, 1984, 1986) and Rh(i) catalysts (Furlani et al., 1986 Tabata, 1987) are often employed. In our experience, however, many persistent radicals and typical nitrogen-containing functional groups serve as good poisons for these catalysts. Therefore, radical centres have to be introduced after construction of the polymer skeletons. Fortunately, the polymers obtained with these catalysts are often soluble in one or other organic solvent. For example, methyl p-ethynylbenzoate can be polymerized to a brick-coloured amorph- See the Appendix on p. 245 of suffixes to structural formula numbers. 

Considering the metathesis mechanism (Scheme 1) in the polymerization of acetylene compounds, an ideal initiator composed of transition metals has a carbene ligand, which promisingly achieves precisely controlled polymerization. 

Ziegler-Natta catalysts are not active at all in polymerization of disubstituted acetylenes.415 Mo- and W-based systems (for alkynes with small substituents) and Nb- and Ta-based catalysts (for alkynes with bulky groups), in turn, are very effective catalysts used to convert disubstituted acetylenes into polymers with very high molecular weight.414 415 A polymerization mechanism similar to that of metathesis polymerization of cycloalkenes are supported by most experimental observations.414 423 424... 

X. POLYMERIZATION OF ACETYLENES BY OLEFIN METATHESIS CATALYSTS A. Proof of Mechanism... 

Thus the synthesis, detailed structural analysis and reactivity of complexes II and III make an important contribution to the mechanisms suggested previously for the metathesis of olefins and for the polymerization of acetylenes by metal-carbene complexes . 

Alkyne metathesis is catalysed by alkylidene complexes of tungsten and molybdenum, but not by all alkylidene complexes of these metals. Complexes which do not catalyse metathesis catalyse polymerization of acetylenes to give polypropynes. 

Katz and coworkers suggested that the polymerization of acetylenes in this way results from catalysis by metal carbenes. The authors speculate that metal carbynes may well be the source of metal carbenes if the metal carbyne is coordinatively unsaturated, then metathesis can occur, but if not, it must acquire coordinative unsaturation in order to react with the alkyne, which it can do only by transforming into a metal carbene and catalysing 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. 

Polymerization of acetylenes by metathesis-type catalysts such as M0CI5 and WCle/PluSn was first observed in the 1970s (Woon 1974 Masuda 1974, 1976), but the nature of the chain carrier was then in some doubt. However, it was soon found (i) that metal carbene complexes would initiate the polymerization of MesCC=CH at 60°C (Katz 1980a) (ii) that end-groups derived from such initiators could be detected in polymers of PhC=CH (Kunzler 1988b) and (iii) that triblock copolymers could be made by successive addition of norbomene, acetylene and norbomene to such initiators (Schlund 1989). All types of acetylene can be polymerized in this way and the reactions proceed by a ROMP-type mechanism see Ch. 10. 

Rather surprisingly some metal carbyne complexes bring about the polymerization of acetylenes by a reaction of type (2) rather than their metathesis by a reaction... 

It was first observed by Woon (1974) and Farona (1974) that acetylenes could be polymerized by catalysts of the type Mo(CO)3(toluene). This was followed by the discovery that conventional metathesis catalysts such as M0CI5 (Masuda 1974) and WCls (Navarro 1976 Masuda 1976), with or without a cocatalyst, could also bring about polymerization of acetylenes. At first there was some doubt as to whether these polymerizations were being propagated by the metathesis mechanism (Scheme 10.2) or whether a Ziegler-Natta mechanism was operating. However, the observation that metal carbene complexes could react with acetylenic molecules to form simple adducts as in reaction (20) (Fischer, H. 1980), and the fact that such complexes could initiate the polymerization of acetylenes, albeit somewhat slowly, but cleanly and in fair yield, soon allayed these doubts. 

Table 10.4 References to the metathesis polymerization of disubstituted acetylenes"... 

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