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

A DFT-based description is given of the CO2/epoxide copolymerization with a catalyst system consisting of metal (chromium, iron, titanium, aluminum)-salen complexes (salen = N,N'-bis(3,5-di-tert-butylsalicyliden-1,6-diaminophenyl) in combination with either chloride, acetate, or dimethylamino pyridine (DMAP) as external nucleophile. Calculations indicate that initiation proceeds through nucleophilic attack at a metal-coordinated epoxide, and the most likely propagation reaction is a bimolecular process in which a metal-bound nucleophile attacks a metal-bound epoxide. Carbon dioxide insertion occurs at a single metal center and is most likely the rate-determining step at low pressure. The prevalent chain terminating/degradation-the so-called backbiting, a reaction leading to formation of cyclic carbonate from the polymer chain-would involve attack of a carbonate nucleophile rather than an alkoxide at the last unit of the growing chain. The backbiting of a free carbonato chain end is particularly efficient. Anion dissociation from six-coordinate aluminum is appreciably easier than from chromium-salen complexes, indicating the reason why in the former case cyclic carbonate is the sole product. Experimental data were gathered for a series of chromium-, aluminum-, iron-, and zinc-salen complexes, which were used in combination with external nucleophiles like DMAP and mainly (tetraalkyl ammonium) chloride/acetate. Aluminum complexes transform PO (propylene oxide) and CO2 to give exclusively propylene carbonate. This is explained by rapid carbonate anion dissociation from a six-coordinate complex and cyclic formation. CO2 insertion or nucleophilic attack of an external nucleophile at a coordinated epoxide (at higher CO2 pressure) are the rate-determining steps. Catalysis with [Cr(salen)(acetate/chloride)] complexes leads to the formation of both cyclic carbonate and polypropylene carbonate with various quantities of ether linkages. The dependence of the activity and selectivity on the CO2 pressure, added nucleophile, reaction temperature, and catalyst concentration is complex. A mechanistic description for the chromium-salen catalysis is proposed comprising a multistep and multicenter reaction cycle. PO and CO2 were also treated with mixtures of aluminum- and chromium-salen complexes to yield unexpected ratios of polypropylene carbonate and cyclic propylene carbonate.

Four new 1,4-diaza-2,3-dimethylbutadiene ligands (Ar-NdC(CH 3 )-(H 3 C)CdNAr; Ar: 3a ) 2,6-diphenylphenyl; 3b ) 2,6-di(4-OCH 3 -phenyl)phenyl; 3c ) 2,6-di(4-tert-butyl-phenyl)phenyl; 3d ) 2,6-di(3,5-dimethylphenyl)phenyl) as well as the palladium dichloride complexes 4a-c and methyl monochloride derivatives 5a-c were prepared, and their polymerization behavior was investigated. The corresponding nickel species 6a-c were tested for the insertion polymerization of ethene by in situ reactions of 3a-c with (DME)NiBr 2 . The ligands are accessible by a three-step procedure. Aryl boronic acids were prepared by Grignard reactions of substituted aryl bromides and were coupled with 2,6-dibromo aniline according to a Suzuki cross-coupling protocol to give the corresponding terphenyl anilines 2a-d. Condensation of 2,3-butanedione with the corresponding aniline afforded the formation of the diimines 3a-d. The corresponding palladium dichloride complexes 4a-c are accessible by reaction with (PhCN) 2 PdCl 2 . The structures of 4a-c could be determined by X-ray analysis. While the terphenyl complex 4a adopts a C 2v -symmetry, 4c exists in a chiral C 2symmetric coordination geometry, due to the repulsive interactions of the sterically demanding tert-butylphenyl substituents of the aniline moieties. All palladium and nickel complexes are catalysts for the polymerization of ethene. However, the chiral Ni(II) complex 6c shows by far the highest polymerization activity up to 2 × 10 4 kg(PE) [mol (Ni) h] -1 . The polyethenes obtained with the palladium methyl monochloro catalysts activated with Na-[(3,5-(CF 3 ) 2 C 6 H 3 ) 4 B] and the nickel dibromo complexes activated with MAO are linear and show in the case of the Ni(II) derivatives molecular weights up to 4.5 × 10 6 g mol -1 (M w /M n ≈ 2), which can be controlled by addition of hydrogen.

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.

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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.

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.

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.