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Stereosequenced crystalline polyhydroxyalkanoates from diastereomeric monomer mixtures

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Science  08 Nov 2019:
Vol. 366, Issue 6466, pp. 754-758
DOI: 10.1126/science.aax8466

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Toward less brittle degradable plastic

Bacteria produce a class of polyesters, termed polyhydroxyalkanoates, that are particularly appealing on account of how easily they undergo biodegradation. Unfortunately, these polymers also tend to be overly brittle for many applications. Tang et al. report that a molecular lanthanide catalyst can sequentially polymerize chiral and then achiral diastereomers to form polyhydroxyalkanoate varieties that are substantially more ductile. The catalyst selectively produces these copolymers directly from diastereomeric mixtures of monomers, obviating the need for a wasteful separation process ahead of time.

Science, this issue p. 754

Abstract

Stereoselective polymerization of chiral or prochiral monomers is a powerful method to produce high-performance stereoregular crystalline polymeric materials. However, for monomers with two stereogenic centers, it is generally necessary to separate diastereomers before polymerization, resulting in substantial material loss and added energy cost associated with the separation and purification process. Here we report a diastereoselective polymerization methodology enabled by catalysts that directly polymerize mixtures of eight-membered diolide (8DL) monomers with varying starting ratios of chiral racemic (rac) and achiral meso diastereomers into stereosequenced crystalline polyhydroxyalkanoates with isotactic and syndiotactic stereodiblock or stereotapered block microstructures. These polymers show enhanced ductility and toughness relative to polymers of pure rac-8DL, subject to tuning by variation of the diastereomeric ratio and structure of the 8DL monomers.

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