High Molecular Weight Poly(p-phenyleneethynylene)s by Alkyne Metathesis Utilizing “Instant” Catalysts:  A Synthetic Study

In this contribution, alkyne metathesis of 1,4-dipropynylated benzenes 3 is reported. High-molecular weight poly( p -phenyleneethynylenes) 4 form from 3 under concomitant evolution of butyne. The highly active catalyst system employed in this polymerization reaction forms from commercially available Mo(CO) 6 and 4-chlorophenol or 4-trifluoromethylphenol in situ at 140 °C in off-the-shelf quality 1,2-dichlorobenzene. Introduction of dry nitrogen into the reaction vessel serves to remove the byproduct, 2-butyne. Several dipropynylated benzenes 3 carrying solubilizing alkyl (hexyl, nonyl, dodecyl, 2-(ethyl)hexyl, ethylbutyl, isopentyl, 3,7-dimethyloctyl, 2,5,5-trimethylhexyl, cyclohexyl, isopentyloxy) or alkoxy substituents were prepared starting from 1,4-dichlorobenzene (alkyl substituted) or hydroquinone (alkoxy substituted). Polymerization of the monomers 3 furnished the yellow and highly fluorescent poly( p- phenyleneethynylene)s 4 in excellent to quantitative yields and high purity. For soluble derivatives, the degree of polymerization ( P n ) is observed in the range 1 × 10 2 ≤ P n ≤ 13 × 10 2, while the polydispersities ( M w / M n ) appear in a bracket, 2 ≤ M w / M n ≤ 4.5. Miscellaneous optimizations (time, temperature) were performed. Maximum molecular weights are obtained at reaction temperatures around 130−150 °C and reaction times of 25−30 h.