Synthesis of Substituted PA

The catalyst Fe(acac)3/AlEt3 will polymerize 1° or 2° alkyl-acetylenes (HC=CR) to soluble high-molecular-weight polymers [112]. However, when the R groups are either 3°-alkyl or aromatic or contain a heteroatom, only oligomers and insoluble polymers are obtained. In general, Ziegler-Natta catalysts are not known to polymerize disubstituted acetylenes (RC=CR ). 

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]. 

In a ical polymerization using metathesis catalysts, the metal salt and the cocatalysts were mixed together in an organic solvent followed by an aging period. If no cocatalyst was used, the cat-afyst required no aging and was used directly. Subsequent, the substituted acetylene monomer was added, and the extent of conversion was monitored by gas chromatography. The reaction was typically terminated with a mixture of toluene and methanol (5 1 by volume). The reaction mixture was then poured into MeOH to precipitate the polymers. 

For more bulky disubstituted alkynes (e.g., l-(trimethylsilyl)-l-propyne), Nb- and Ta-based metathesis catalysts perform better than Mo- and W-based catalysts [114], For example, TkCls and a main-group organometallic cocatalyst will polymerize dipheny-lacetylene (PhC CPh). With less crowded monomers, the and Th catalysts tend to give only cyclotrimer product. 

Catalyst HC=CH Unhindered HC=CR Bulky HCsCR Unhindered RC=CR Bulky RCsCR  

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Scheme XXVII. An indirect synthesis of substituted PA using the dehydrobromination route.

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