A Highly Active and Recyclable Catalytic System for CO₂/Propylene Oxide Copolymerization

Abstract

Copolymerization of CO 2 and epoxide catalyzed by metal complexes has drawn much attention recently.[1] In the initial report by Inoue and co-workers in 1969 the catalyst activity was poor (turnover number (TON): 6; turnover frequency (TOF): 0.12 h À1 ), [2] but by the early 2000s it had been significantly improved by using binary catalytic systems of [Co(salen)] [3] or [Cr(salen)] complexes [4] with an onium salt or base (1; salen = N,N'-bis(salicylidene)ethylenediamine,[5] It was proposed that the two components or the two zinc centers participated in the propagation reaction, and therefore binding of the two metal centers or the two components further improved catalysis to the maximum reported TON and TOF of 14 500 and 4600 h À1 , respectively. [6,7] These values, which were achieved with 2 in the copolymerization of CO 2 and propylene oxide, are still low enough to warrant further improvement, [7] because low activity means higher catalyst cost and higher levels of catalyst-derived metal residue in the resin.[8] This metal residue either colors the resin or causes toxicity. For the TON of 14 500 attained with 2, the residual cobalt level in the resin reached 40 ppm unless it was removed.[9] Herein, we report a highly active catalytic system for CO 2 /propylene oxide copolymerization and an efficient recovery strategy of the catalyst.We prepared various cobalt-salen complexes by varying either the ortho substituent or the number of attached quaternary ammonium groups (Scheme 1). Thus, the Friedel-Crafts acylation of ortho-substituted phenol using 4-chlorobutyryl chloride and subsequent hydrogenation attached a (CH 2 ) 4 Cl group in the para position. In a similar manner, attack by 1,7-dichloroheptan-4-one on lithium 2-alkyl-4-lithiophenolate yielded a tertiary benzylic alcohol, and subsequent hydrogenation attached CH[(CH 2 ) 3 Cl] 2 . After formylation, the chloro substituent, that was not susceptible to nucleophilic substitution with tributylamine, was transformed into a more reactive iodo substituent. Formation of the quaternary ammonium salt occurred in nearly quantitative yields after generation of the salen-type ligand. Because the halide ion interfered with the metalation reaction, we replaced it with inert BF 4 À . Following metalation by a routine method, an active 2,4-dinitrophenolate anion replaced this BF 4 À ion.[3a]We screened the newly prepared complexes for CO 2 / propylene oxide copolymerization under the following conditions: [propylene oxide (PO)]/[catalyst (cat.)] = 25 000, 80 8C, and CO 2 pressure 2.0-1.7 MPa (Table 1). The polymerization was terminated before 25 % conversion was reached, because after this time the solution became highly viscous, thus making stirring and diffusion difficult. The identity of the ortho substituent strongly influenced the activity. Contrary to other reports, [3] the bulky tert-butyl group was not the best choice in this study, and replacing it with a small methyl group significantly enhanced TOF. The TOF also increased dramat-

Ring‐Opening Polymerization Reactions of Propylene Oxide Catalyzed by Porphyrin Metal (3+) Complexes of Aluminum, Chromium and Cobalt

“…These have achieved TOFs of ca. 10 3 h −1 and the right combination of steric and electronic effects of the salen ligand frame have yielded regioregular and even stereoselective polymerizations, starting from racemic epoxides . The formation of polycarbonates including PPC have been the topic of several excellent recent reviews, and it is not the purpose of this article to update these, but rather to report and summarize the work in this laboratory that has been devoted to the study of the Inoue's porphyrin systems with the three metals Al, Cr and Co, in their trivalent states .…”

Šezdesetgodišnjica Ziegler-Nattinih katalizatora i stereospecifičnih polimerizacija

“…Također, kopolimerizacijom etilena s ugljikovim monoksidom uz Pd-komplekse nastaje alternirajući kopolimer sa svojstvima poliamida. 82 Veliko područje istraživanja i primjene organometalnih katalizatora primjer je nedavnih rezultata vrlo značajne sinteze amonijaka izravno od dušika i vodika pri blagim reakcijskim uvjetima, za razliku od poznatog Haber-Boschovog postupka. Ta su istraživanja pokazala da cirkonijevi kompleksi s pentadienilnim ligandima omogućuju stvaranje veze N-H iz smjese N 2 i H 2 , pa tako primjerice već pri 85 °C uz stalnu hidrogenaciju nastaje NH 3 , za sada u malom iskorištenju.…”

Aluminum porphyrins with quaternary ammonium halides as catalysts for copolymerization of cyclohexene oxide and CO2: metal–ligand cooperative catalysis

“… 2 Since the discovery of the ring-opening copolymerization of epoxides and CO 2 by Inoue in 1969, 3 a variety of metal complex catalysts have been developed, 4 – 20 such as Co(salen), 4 – 6 Cr(salen), 7 – 9 Zn(β-diiminate), 10 and metalloporphyrins. 11 , 12 , 20 Among them, multinuclear metal complexes 4b , 6c , 9 , 10b , 13 – 17 and bifunctional monometallic complexes 4a , 6b , 18 – 20 have exhibited notable catalytic performances. Among several effective metal elements, aluminum is fascinating from the viewpoint of low toxicity, low price, and abundance on the earth.…”