On the Formation of Aliphatic Polycarbonates from Epoxides with Chromium(III) and Aluminum(III) Metal–Salen Complexes

Abstract

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.

Flexibly Tethered Dinuclear Zinc Complexes: A Solution to the Entropy Problem in CO2/Epoxide Copolymerization Catalysis?

“…These polymers show interesting properties, as they are biodegradable, highly transparent, UV stable, and have a high Youngs modulus. [9][10][11][12] Latest research even led to isotactic poly(cyclohexene carbonates), opening a route to semi-crystalline thermoplastic materials. [13] Despite the fact that aliphatic polycarbonates were already introduced 40 years ago by Inoue et al, the development of an economic production process is still hampered by the availability of efficient catalysts.…”

“…The main focus of numerous publications in this field was based on a dinuclear catalysts, which accommodate the epoxide ring opening and a CO 2 insertion in a single molecule. [8,10,12,[14][15][16][17][18][19][20][21] Zinc as a catalytically active species has tremendous advantages compared to other transition metals. It is an economic, ecofriendly element, and its ions are colorless.…”

A bimetallic iron(iii) catalyst for CO2/epoxide coupling

“…45 Furthermore, although Co(III)/Cr(III) salen complexes are excellent catalysts, the Fe(III) analogues were inactive. 16,17 The macrocyclic pro-ligand, H 2 L, was prepared following a two-step procedure, in 84% overall yield. 9 It was deprotonated, using potassium hydride, and reacted with two 50 equivalents of [FeCl 3 .DME] (DME = dimethoxyethane) to yield an air stable blue di-iron tetrachloride complex 1 (85%, unoptimized) (Scheme 2).…”

Aluminium coordination complexes in copolymerization reactions of carbon dioxide and epoxides

“…47 A detailed computational study has been reported by Luinstra, Reiger and co-workers on M III -salen complexes in the reactions of PO with carbon dioxide. 46 In the theoretical and experimental study, Al-salen species exclusively formed PC and not PPC whereas Cr-salen complexes could form PPC exclusively but typically gave a mixture of products dependent on the ligand, co-catalyst and reaction conditions. In this study, activation of the incoming epoxide group by a metal complex (not bearing the growing polymer chain) was proposed to play an important role in the polymerization process.…”