Cyclohexene oxide/carbon dioxide copolymerization by chromium(iii) amino-bis(phenolato) complexes and MALDI-TOF MS analysis of the polycarbonates

Abstract

Amine-bis(phenolate) chromium(III) chloride complexes 1·THF, 2·THF and 1·DMAP catalyze the copolymerization of cyclohexene oxide and carbon dioxide. These catalysts incorporate tetradentate amine-bis (phenolate) ligands [L1] and [L2], (where [L1] = 2-pyridyl-N,N-bis(2-methylene-4-methoxy-6-tert-butylphenolato) and [L2] = dimethylaminoethylamino-N,N-bis(2-methylene-4-methoxy-6-tert-butylphenolato)) and when combined with 4-(N,N-dimethylamino)pyridine (DMAP) or bis(triphenylphosphoranylidene)ammonium chloride or azide, (PPNCl or PPNN3), yield low molecular weight polycarbonate with narrow dispersities. The structure of 1·DMAP incorporates one molecule of 4-(N,Ndimethylamino)pyridine (DMAP) and can be used as a single-component catalyst precursor. Polymer end group analysis by MALDI-TOF mass spectrometry reveals possible initiation pathways. Scheme 3. (A) Proposed initiation pathway for the copolymerization of cyclohexene oxide and carbon dioxide with 1·DMAP and (B) with 1·DMAP/PPNCl

Mechanistic Studies of Cyclohexene Oxide/CO2 Copolymerization by a Chromium(III) Pyridylamine‐Bis(Phenolate) Complex

“…Plot of ln( r obs ) against ln[cat] (Figure ) shows a linear relationship giving a reaction order of 0.96±0.058 ( R 2 =0.9927), corresponding to a first‐order dependence on catalyst concentration. This observation is in good agreement with the previously proposed mechanism indicating there is only one complex participating in the ring‐opening of the epoxide followed by one propagating polymer chain per chromium complex. The mechanism suggested in a previous report from our group proposes a monometallic intermolecular initiation where an external or chromium‐bound nucleophile initiates ring‐opening of a coordinated epoxide.…”

“…If the number of active species formed increases with increased DMAP loading, and if polymer chain growth occurs at each active species, then a decrease in molecular weight is expected. A slight decrease in M n was observed previously when PPNCl and PPNN 3 were utilized together with 1⋅DMAP . When the amount of DMAP is increased, however, the M n remain effectively unchanged (between 20 000 and 25 000 g mol −1 ).…”

“…The yields reported are for polymer obtained through its precipitation from the crude reaction mixture and careful washing to remove catalyst or catalyst decomposition products. Possible byproducts include cyclic carbonates, short chain oligomers, polyethers and cyclohexane diol, but none of these could be observed by 1 H NMR of the crude reaction mixtures resulting from these catalytic conditions . A maximum turnover number (TON) of 468 is achieved at 0.17 mol % catalyst loading, where an acceptably high conversion (78 %) is also maintained.…”

“…We previously reported the activity of chromium amino‐bis(phenolate) and pyridylamino‐bis(phenolate) complexes for the copolymerization of epoxides with CO 2 using ionic and neutral nucleophilic co‐catalysts. Differences in initiation periods and reaction rates were observed for complexes 1⋅THF and 1⋅DMAP and 2 (Figure ) . A longer initiation time (∼17 min) but faster propagation rate (3.5±0.026×10 −2 min −1 ) was observed for 1⋅DMAP than for 1⋅THF (∼5 min initiation time and rate of 1.2±0.020×10 −2 min −1 ).…”

A Rational Investigation of the Lewis Acid-Promoted Coupling of Carbon Dioxide with Cyclohexene Oxide: Towards CO2-Sourced Polycyclohexene Carbonate under Solvent- and Cocatalyst-Free Conditions

“…Thus, for the selective and efficient synthesis of polycarbonates (PCs) from the copolymerization of CO 2 and epoxides, many Lewis acid catalysts such as organometallic complexes usually used in combination with a nucleophile have been developed. In particular, the important characteristics of metal-based catalysts (zinc [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37], aluminum [38,39], chromium [40][41][42][43], cobalt [44][45][46][47], magnesium [48,49], iron [19,[50][51][52][53][54], titanium [55,56], copper [57], ytterbium [58], . .…”