Polymers Containing Aromatic C—H Bonds

The most common method used to introduce a functional group to aromatic polymers is modification via electrophific aromatic substitution—e.g., bromination and sulfbnation. However, the formation of highly reactive carbocation intermediates frequently causes side reactions in polymer chains and changes in molecular weight properties between the functionalized polymers and the precursor polymers.  

As an alternative to the electrophific functionalization of PS, the iridium-catalyzed borylation of aromatic C—H bonds to functionalize PSs with different tacticities (atactic, isotactic, and syndiotactic Fig. 24A) was reported. Because the borylation did not occur at the C—H bonds of sp carbons, it did not affect the tacticity of PSs. Similar to the case of molecular arenes, this C—H borylation of PS was achievable with bis(pinacolato) diboron (B2pin2) or pinacolborane (HBpin) and generated a mixture of meta- and para-substituted borylated PSs. However, the ratio of the two isomers was different than that of isopropylbenzene because a phenyl ring was attached to every other carbon in the PS backbone, the meta C—H bonds of PS were more sterically hindered than the para C—H bonds. Thus, while the ratio of meta- and para-substituted isomers in the C—H borylation of isopropylbenzene was 7 3, the C—H borylation of PS under identical 

The boryl groups installed on the aromatic polymers are useful synthetic intermediates for conversion to other polar functional groups. For example, they can be oxidized to give hydroxylated polymers and a variety of polar group-containing polymers via either direct functional group transformations or palladium-catalyzed Suzuki cross-coupHng reactions with appropriate phenyl bromides (Fig. 25). This transition metal-catalyzed polymer 

Using the results of previous studies of palladium- and platinum-catalyzed hydroarylation of alkynes (Fig. 26A), Kitamura and coworkers extended this chemistry to functionahze PS and polystyrene-l)/ocfe-poly(ethylene-ra -butylene)-l /ocfe-polystyrene (SEES) with acrylic acid moieties (Fig. 26B). Reactions of PS with propionic acid or methyl ester in the presence of K2PtCl4 and Ag(CF3S03) afforded maximums of 7% and 30% alkenylated styrene unit for PS and SEES, respectively. Although 

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