Acetylene sterically crowded

Typical examples of the polymerization of monosubstituted acetylenes are shown in Table 3. Transition metal catalysts that involve Mo, W, and Rh are particularly effective. Whereas Mo and W catalysts are sensitive to polar groups in the monomer, Rh catalysts are tolerant to such groups. Another point is that Mo and W catalysts are effective to various sterically crowded monomers, while Rh catalysts are useful to rather restricted kinds of monomers including... 

It is rather amazing that 1-adamantylacetylene, an acetylene sterically even more crowded than ferf-butylacetylene, also polymerizes in high yield20. Poly(l-adamantylacetylene) is insoluble probably because the bulky rigid substituent makes the main chain very stiff. Its copolymerization with /ert-butylacetylene produces a soluble copolymer. 3,3-Dimethyl-1-pentyne and 3,3-dimethyl-1-nonyne produce polymers as well20). 

In Table 19 are collected typical monomers that polymerize with group 5 and 6 transition metal catalysts to produce high-molecular-weight (Mw > 1 x 10s) polyacetylenes. Among them, tert-butylacetylene and 3-(trimethylsilyl)-l-octyne are monosubstituted acetylenes, while the others are disubstituted ones. It is noteworthy that all of these monomers are considerably sterically crowded. By judicious choice of polymerization conditions, the polymer yield becomes fair to quantitative in every case. The Rtw s of the polymers reach ca. 3 x 10s 2 x 106. 

The importance of group 5 and 6 transition metal catalysts stems from the fact that they can polymerize sterically crowded acetylenes for which Ziegler catalysts are inactive. On the contrary, the present catalysts 12,13) (e.g, WC16—Ph4Sn) are much less effective toward the unsubstituted acetylene than is Ti(0-n-Bu)4—Et3AL, a Ziegler catalyst this is because the present catalysts are prone to cyclotrimerize acetylene. 

A relationship between the structure and polymerizability of sterically crowded acetylenes can be deduced as follows CH3C=CSi(CH3)3, CH3C=CSi(CH3)2(n-C6Hi3), CH3C CSi(CH3)2(C6H5), and UC C tert-C4H9) are polymerizable, whereas C2H5C = CSi(CH3)3,... 

M0CI5 and WCl6 alone can induce the polymerization of various monosubstituted acetylenes. The use of a suitable organometallic cocatalyst enhances the catalytic activity. With these catalysts, polymer molecular weight is low or medium (<10 ) for 1-hexyne and phenylacetylene but reaches one million for sterically crowded monomers like t-butylacetylene and or /2o-substituted phenylacetylenes. 

Since only Ta and Nb catalysts, which are not tolerant to polar groups, are available for the polymerization of sterically crowded disubstituted acetylenes, it is generally difficult to directly synthesize disubstituted acetylene polymers that have a highly polar substituent such as a hydroxy group. Recently, the synthesis of poly[l-phenyl-2-(p-hydroxyphenyl)acetylene] has been achieved by the polymerization of l-phenyl-2-(p-r-butyldimethylsiloxyphenyl)acetylene (11) and the subsequent add-catalyzed deprotection reaction. ... 

Monosubstituted acetylenes generally prefer cyclotrimerization to polymerization in the presence of halides of Group 5 metals as described earlier (135-137). Tbe polymerization of monosubstituted acetylenes by NbCls and TaCls catalysts is possible only in the case of sterically crowded monomers, which is exemplified by the polymerization of 3-trialkylsilyl-l-alkynes with the formula of HC CH(Si(CH3)2R)R (R = CHg, n-CeUis, CgHs R = ra-CgHT, n-C5H11, ra-CyHis) (45). Even tert-butylacetylene affords a low yield of polymer in the presence of TaCls or NbCls. Additionally, the molecular weights of these Ta- and Nb-based poly(tert-butylacetylene)s are lower than those of the W-based ones. However, there has been a demonstration of the imique ability of... 

In general, Rh catalysts are not very effective in stereoseleetively polymerizing sterically crowded monosubstituted acetylenes such as t rt-butylacetylene and ortho-substituted phenylacetylenes. Rh catalysts are also not capable of polymerizing disubstituted acetylenes. One exception is cyclooctyne, whose very large ring strain ( 38 kJ/mol) enables fast polymerization with [(nbd)RhCl]2, giving an insoluble polymer in good yield. ... 

Compound (578) is effectively a tetra(t-butyl)cyclobutadiene, and has been made by dimerization of the acetylenic compound (577) in the presence of PdCl2(PhCN)2. Removal of the metal with the more powerful ligand allowed isolation of (578) as a crystalline solid. The material is remarkably stable and, besides allowing full spectroscopic characterization, it gives the expected reactions of a cyclobutadiene whose dimerization tendency has been frustrated by severe steric crowding. The u.v. spectrum has = 227 nm, at... 

Head-to-tair unsymmetrical 1,3-dienes (73) are obtained in excellent yield on dimerization of even sterically crowded terminal and internal acetylenes, via the vinyl mercurial derivatives (72). Other diene syntheses of interest are the... 

The protonated ligand is displaced again by methyl acetylene to give 4.25. In the next step, because of steric crowding, insertion of the alkyne into the Pd-CO Me bond takes place in an anti-Markovnikov fashion to give 4.26. 

However, because of steric crowding, insertion of methyl acetylene into the Pd-H bond would be expected to give the anti-Markovnikov isomer 4.29. Coordination of CO (not shown) followed by insertion into the palladium methoxy bond would lead to the formation of 4.30, which can then reductively eliminate the product. Had this mechanism been operative, the methyl ester of crotonic acid rather than MMA would have been the main product. 

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. 

Typical examples of the polymerization of monosubstituted acetylenes are shown in Table 5. As seen, Fe, Mo, W and Rh catalysts, all of which involve transition metals, are particularly effective. It is noted that not only sterically unhindered monomers but also very crowded ones afford high molecular weight polymers. 

In general, disubstituted acetylenes are sterically more crowded than their monosubstituted counterparts and, consequently, their effective polymerization catalysts are restricted virtually to group 5 and 6 transition-metal catalysts. Among 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... 

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