Highly efficient formation of poly(propylene carbonate) can be achieved in the coupling of CO2 and propylene oxide assisted by 4‐(N,N‐dimethylamino)pyridine (DMAP) and catalyzed with salen chromium(III) chloride by using DMAP/Cr ratios of less than 2. Under these conditions a possible backbiting mechanism is suppressed, leading to only minor amounts of cyclic carbonate as a side product. magnified image

The metal-catalyzed synthesis of polyolefins, polyketones, and polycarbonates is well-known in academia and is already successfully applied in industrial processes. Still missing, however, is the metal-catalyzed synthesis of aliphatic polyesters, as one of the most important biodegradeable polymer families. We report here on the cobalt-catalyzed alternating copolymerization of propylene oxide and carbon monoxide, affording atactic and isotactic polyhydroxybutyrates (PHB). The postulated mechanism is supported by online ATR-IR analytics.

Carbonylation of epoxides with a combination of Lewis acids and cobalt carbonyls was studied by both theoretical and experimental methods. Only multisite catalysis opens a low-energy pathway for trans opening of oxirane rings. This ring-opening reaction is not easily achieved with a single-site metal catalyst due to structural and thermodynamic constraints. The overall reaction pathway includes epoxide ring opening, which requires both a Lewis acid and a tetracarbonylcobaltate nucleophile, yielding a cobalt alkyl-alkoxy-Lewis acid moiety. After CO insertion into the Co-C(alkyl) bond, lactone formation results from a nucleophilic attack of the alkoxy Lewis acid entity on the acylium carbon atom. A theoretical study indicates a marked influence of the Lewis acid on both ring-opening and lactone-formation steps, but not on carbonylation. Strong Lewis acids induce fast ring opening, but slow lactone formation, and visa versa: a good balance of Lewis acidity would give the fastest catalytic cycle as all steps have low barriers. Experimentally, carbonylation of propylene oxide to beta-butyrolactone was monitored by online ATR-IR techniques with a mixture of tetracarbonylcobaltate and Lewis acids, namely BF(3), Me(3)Al, Et(2)Al(+).diglyme, and a combination of Me(3)Al/dicobaltoctacarbonyl. We found that the last two mixtures are extremely active in lactone formation.

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The cobalt‐catalyzed alternating copolymerization reaction of racemic propylene oxide and CO affords regioregular but atactic poly(hydroxybutyrates). The application of enantiomerically pure (R)‐ or (S)‐propylene oxide results in the formation of isotactic, optically active and crystalline materials. In contrast to the perfect isotactic microstructures that are available via the microbial pathway, our approach allows to tailor the stereoregularity of the polymer products, and hence their melting points by application of mixtures of the epoxide enantiomers. Investigations into the polymer stereochemistry show that the epoxide units undergo a clean regioregular incorporation into the polymer chain with retention of configuration at the stereogenic carbon centers. MALDI‐TOF and online ATR‐IR experiments give insight into the mechanism of chain termination reactions.

Cover: The cover page shows the formation of polycarbonate formation on solid Zn‐gluterate. Further details can be found in the Full Paper by R. Eberhardt, M. Allmendinger, M. Zintl, C. Troll, G. A. Luinstra, and B. Rieger* on page 42.

Summary: A series of zinc dicarboxylates were synthesized by the reaction of diethylzinc with dicarboxylic acids. Zinc monocarboxylate monoalkyl intermediates were obtained by using a defined excess of diethylzinc over dicarboxylic acid. A subsequent insertion reaction of SO2 into these zinc alkyl bonds resulted in a defined number of Zn‐ethylsulfinate groups, which act as active centers for the copolymerization reaction of CO2 and propylene oxide. Corresponding ethylsulfinic acid ester end groups were detected in the poly(propylene carbonate) (PPC) products. The polymerization activity depended strongly on the number of sulfinate groups incorporated and has been significantly increased compared to conventional zinc dicarboxylates. The obtained PPCs have molecular weights ($\overline M _{\rm w}$) exceeding 80 000 g · mol−1 and polydispersities in the range of 2.3 to 3.3.

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