Polymerization of Disubstituted Acetylenes

Ring-substituted PhAs have also been polymerized by well-defined Mo carbenes ligated by less bulky alkoxy groups (4, Table 15.2) [38]. Like metal halide-induced living polymerizations, bulky ring substituents at the ortho position are required for controlled polymerization. The advantage of the Mo carbene-catalyzed polymerization system is that the initiation and propagation steps can be readily monitored by an NMR technique. 

In general, Ru carbenes do not show high catalytic activity for the polymerization of substituted acetylenes, and therefore have not been studied in detail. Nuyken and coworkers [63,64] developed a well-defined Grubbs-type Ru carbene for living polymerization of diethyl dipropargylmalonate (5, Table 15.2). 

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Since only Ta and Nb catalysts, which are not tolerant to polar groups, are available for the polymerization of disubstituted acetylenes, it is generally difficult to synthesize disubstituted acetylene polymers having such a highly polar substituent as a hydroxy group. Recently, synthesis of poly[l-phenyl-2-( -hydroxyphenyl)acetylene] has been achieved by the polymerization of 1-phenyl-2-(p-siloxyphenyl)acetylene and the subsequent acid-catalyzed deprotection reaction. 

Rh complexes are examples of the most effective catalysts for the polymerization of monosubstituted acetylenes, whose mechanism is proposed as insertion type. Since Rh catalysts and their active species for polymerization have tolerance toward polar functional groups, they can widely be applied to the polymerization of both non-polar and polar monomers such as phenylacetylenes, propiolic acid esters, A-propargyl amides, and other acetylenic compounds involving amino, hydroxy, azo, radical groups (see Table 3). It should be noted that, in the case of phenylacetylene as monomer, Rh catalysts generally achieve quantitative yield of the polymer and almost perfect stereoregularity of the polymer main chain (m-transoidal). Some of Rh catalysts can achieve living polymerization of certain acetylenic monomers. The only one defect of Rh catalysts is that they are usually inapplicable to the polymerization of disubstituted acetylenes. Only one exception has been reported which is described below. 

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... 

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. 

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

Group 5 Transition Motals. The most probable side reaction in the polymerization of acetylenes is cyclooligomerization that is well promoted by Group 5 transition metals. For example, cyclotrimerization of 1-alkynes readily occurs in the presence of NbCls (135-137). Thus, bulky substituents must be incorporated into the monomers for the successful formation of polymers by Group 5 transition metals. In other words, Ta and Nb catalysts suit the polymerization of disubstituted acetylenes. 

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... 

Ta- and Nb-based catalysts are good for the polymerization of disubstituted acetylenes but they are not tolerant of polar groups. It is generally difficult to synthesize disubstituted acetylene polymers having protic and/or highly polar substituents such as hydroxy, carboxy, and sulfonic acid groups. Therefore, polymer reactions have been employed to introduce polar groups, and examples of such polymer reactions are illustrated in Scheme 15.3. 

Polymer Modification Reactions 383 Table 15.3 Recent examples of polymerization of disubstituted acetylenes. 

Some of the Rh catalysts can achieve living polymerization of certain acetylenic monomers. The only defect of Rh catalysts is that they are usually inapplicable to the polymerization of disubstituted acetylenes. Only one exception has been reported and is described below. 

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