Abstract
The influence of composition of liquid phase on composition of poly(propylene ether carbonates) in the copolymerization of CO2 with propylene oxide (PO), mediated by a zinc chloride cobalt double metal cyanide, was monitored by FT-IR/CO2 uptake/size exclusion chromatography in batch and semi-batch mode. The ratio of mol fractions of carbonate to ether linkages F (~0.15) was found virtually independent on the feed between 60 and 120 °C. The presence of CO2 lowers the catalytic activity but yields more narrowly distributed poly(propylene ether carbonates). Hints on diffusion and chemistry-related restrictions were found underlying, broadening the distribution. The incorporation of CO2 seems to proceed in a metal-based insertion chain process, ether linkages are generated stepwise after external nucleophilic attack. The presence of amines resulted in lower activities and no change in F. An exchange of chloride for nitrate in the catalyst led to a higher F of max. 0.45. The observations are interpreted in a mechanistic scheme, comprising surface-base-assisted nucleophilic attack of external weak nucleophiles and of mobile surface-bound carboxylato entities on activated PO in competition to protonation of surface-bound alkoxide intermediates by poly(propylene ether carbonate) glycols or by surface-bound protons. Basic entities on the catalyst may promote CO2 incorporation.
Highlights
The utilization of carbon dioxide (CO2 ) as a feedstock in the chemical industry has been propagated over decades and as a kind of a soft type of green washing: no current chemical process involvingCO2 has a negative carbon footprint [1]
Care is taken to keep the heat content of the reaction mixture below the reactors’ safety limits; it always needs to be assured that the catalyst has been activated and propylene oxide (PO) is consumed before additional feeding of PO, e.g., to get to higher molecular weights
The catalytic action of a double metal cyanide (DMC) complex with chloride ligands was mapped in batch and semi batch
Summary
The utilization of carbon dioxide (CO2 ) as a feedstock in the chemical industry has been propagated over decades and as a kind of a soft type of green washing: no current chemical process involvingCO2 has a negative carbon footprint [1]. Apart from its negative image as a greenhouse gas, it is a readily available waste product of several large scale processes and a utilization in a “chemical Verbund concept” may be economically interesting because of future taxations [5,6,7]. As it has a high thermodynamic stability (∆f H◦ = −393.51 kJ/mol at 298.15 K) [8], economically processing CO2 is challenging; reactions with it are bound to be endothermic and are in need of supplementary energy.
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