Diphenylacetylene polymerization

There are, to date, no reports of discretely isolated neutral alkene complexes of the type [TpxNi(ri2-alkene)], though one must presume their (transient) intermediacy in the polymerization of ethylene, which has been widely effected with a variety of nickel systems (Section IV.A). A single 7i-alkyne complex has been reported, the g r 2 r 2-alkyne-bridged, metal-metal bonded xenophilic complex 19 (Scheme 1), obtained by the interaction of diphenylacetylene with TpMe2 4BrNi—Co(CO)4 (20).17... 

The evidence for the formation of dimethylsilylene from 7-silanoboradiene includes reactions with diphenylacetylene, ethylene, etc., and the observed polymerization to dimethylpolysilane. 

Reduction of benzoyl chloride and benzoyl fluoride at carbon and platinum cathodes in MeCN containing TEAP has been carried out by Cheek and Horine [217]. Macroelectrolysis of benzoyl chloride at carbon yields cis- and tra 5-stilbenediol dibenzoate, as reported earlier [213], but this product is reducible to diphenylacetylene at a more negative potential. Major electrolysis products derived from benzoyl fluoride are benzyl benzoate (15%) and diphenylacetylene (15%) along with stilbenol benzoate and other (polymeric) species of high molecular mass. 

Diphenylacetylene has been polymerized by Cp2TiCl2 with irradiation of light.1223... 

For instance, propanol is converted into propyl propionate (conversion 93%, selectivity 98%) in acetone at 145°C with Ru3(CO)l2 and diphenylacetylene, the catalytic turnover being 140 (124,125). This reaction can be extended to other alcohols with excellent yields and selectivity (125). The intermediacy of the unsaturated mononuclear complex Ru(CO)2(C4Ph4CO) seems to be the key step in this catalysis (126) 1,4-and 1.5-diols give rise to lactones while 1.6- and 1,10-diols are polymerized in this reaction (127). 

For the polymerization of disubstituted acetylenes, M0CI5 and WCl6 alone are inactive, and it is necessary to use the catalyst/cocatalyst mixtures (16), which are active for sterically less crowded monomers (e.g., 2-octyne and 1-chloro-l-octyne). In contrast, NbCls and TaCls by themselves polymerize disubstituted acetylenes with bulky substituents such as 1-(trimethylsilyl)-l-propyne. Diphenylacetylene and its derivatives, however, are polymerizable only with the TaCls-cocatalyst systems. The Nb and Ta catalysts selectively afford cyclotrimers from most monosubstituted acetylenes. 

Example 15 Polymerization of Diphenylacetylene with a TaCls-Based Catalyst... 

Polymerization of diphenylacetylene is carried out under dry nitrogen. Polymerization conditions are as follows in toluene, 80 °C, 24 h. 

Reactions between alkynes and transition metal compounds yield a surprising variety of products (76, 77), indicating nonspecific mechanisms of formation. At least for the reaction of alkynes with metal carbonyls any simple polar mechanism must be excluded, in view of the insensitivity of the reactions to the degree of polarity of the solvents. A radical mechanism would perhaps be better suited for a general description but this has so far been rejected, since inhibition of the reactions with f-butylphenol or hydroquinone proved unsuccessful (78). Likewise, iron carbonyls react with diphenylacetylene, using ethyl acrylate, vinyl methyl ketone or vinyl acetate as the solvent, without polymerization of the vinyl compounds (79). These experiments, however, do not fully eliminate the possibility of a radical mechanism. 

It is noteworthy that the Grubbs-Hoveyda catalyst can polymerize monosub-stituted and diphenylacetylenes [67]. The Grubbs-Hoveyda catalyst and a series of Ru carbene complexes catalyze the polymerization of o-substituted PhAs, as represented by (o-isopropoxy)phenylacetylene [68]. The substituents at the ortho position of monomers are assumed to serve as supportive ligands that maintain and prolong the life of unstable propagating carbene species. 

Recent examples of polymerization of disubstituted acetylenes are shown in Figure 15.1 and Table 15.3. The monomers are mainly diphenylacetylene (DPhA) and l-alkyl-2-phenylacetylenes, while l-chloro-2-arylacetylenes (or-2-alkylacetylene) are also polymerizable. Although DPhA derivatives are steri-cally very crowded, they polymerize in good yields into high molecular weight polymers in the presence of Ta catalysts, typically TaClg-w-Bu Sn. It is easy to introduce various relatively nonpolar substituents (e.g., alkyl, MejSi) into DPhA. l-Alkyl-2-phenylacetylenes are sterically less hindered, and they polymerize... 

PPh2) (PhP)GsH4N, and (/i-H)Ru3(GO)9 (PhN)(GsH4N) the latter catalyze the polymerization of phenyl-acetylene and the hydroformylation of diphenylacetylene as shown in Figure 23. 

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