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Catalyst-controlled stereoselective cationic polymerization of vinyl ethers

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Science  29 Mar 2019:
Vol. 363, Issue 6434, pp. 1439-1443
DOI: 10.1126/science.aaw1703

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The right hand lines up vinyl ethers

Well-optimized catalysts produce vast quantities of isotactic polypropylene, in which the side chains all face the same way. Add an oxygen into the monomer, though, and that degree of uniformity becomes harder to enforce. Teator and Leibfarth report a general protocol to polymerize a variety of such vinyl ethers isotactically (see the Perspective by Foster and O'Reilly). They rely on a chiral phosphoric acid in combination with a titanium Lewis acid to bias the monomer orientation during cationic polymerization. The resulting polymers show promising adhesive properties.

Science, this issue p. 1439; see also p. 1394

Abstract

The tacticity of vinyl polymers has a profound effect on their physical properties. Despite the well-developed stereoselective methods for the polymerization of propylene and other nonpolar α-olefins, stereoselective polymerization of polar vinyl monomers has proven more challenging. We have designed chiral counterions that systematically bias the reactivity and chain-end stereochemical environment during cationic polymerization. This approach overrides conventional chain-end stereochemical bias to achieve catalyst-controlled stereoselective polymerization. We demonstrate that this method is general to vinyl ether substrates, providing access to a range of isotactic poly(vinyl ether)s with high degrees of isotacticity. The obtained materials display the tensile properties of commercial polyolefins but adhere more strongly to polar substrates by an order of magnitude, indicating their promise for next-generation engineering applications.

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