Catalytic Ring-Opening Polymerisation of Cyclic Ethylene Carbonate: Importance of Elementary Steps for Determining Polymer Properties Revealed via DFT-MTD Simulations Validated Using Kinetic Measurements

Abstract

The production of CO2-containing polymers is still very demanding in terms of controlling the synthesis of products with pre-defined CO2 content and molecular weight. An elegant way of synthesising these polymers is via CO2-containing building blocks, such as cyclic ethylene carbonate (cEC), via catalytic ring-opening polymerisation. However, to date, the mechanism of this reaction and control parameters have not been elucidated. In this work, using DFT-metadynamics simulations for exploiting the potential of the polymerisation process, we aim to shed more light on the mechanisms of the interaction between catalysts (in particular, the catalysts K3VO4, K3PO4, and Na2SnO3) and the cEC monomer in the propagation step of the polymeric chain and the occurring CO2 release. Confirming the simulation results via subsequent kinetics measurements indicates that, depending on the catalyst’s characteristics, it can be attached reversibly to the polymeric chain during polymerisation, resulting in a defined lifetime of the activated polymer chain. The second anionic oxygen of the catalyst can promote the catalyst’s transfer to another electrophilic cEC monomer, terminating the growth of the first chain and initiating the propagation of the new polymer chain. This transfer reaction is an essential step in controlling the molecular weight of the products.

Investigation and Comparison of Catalytic Methods to Produce Green CO2-Containing Monomers for Polycarbonates

“…Although replacing ethylene oxide with carbon dioxide makes ecological sense, this approach has not made its way to the market so far [2][3][4]. Further catalysts and variations in the product structures are still being researched today [2,3,[5][6][7][8][9][10][11][12][13][14]. The synthesis of CO 2 -containing polymers is still very challenging, aiming at the avoidance of high-energy costs during copolymerization and extreme conditions such as high temperatures and pressures.…”

“…The synthesis of CO 2 -containing polymers is still very challenging, aiming at the avoidance of high-energy costs during copolymerization and extreme conditions such as high temperatures and pressures. Using pre-formed CO 2 -containing monomers such as cyclic ethylene carbonate (cEC) opens up new possibilities in this area with ring-opening polymerizations (ROPs), whose reaction pathways are already better understood [7,8,10,15]. By using ROP, the incorporation of CO 2 is substantially increased in comparison to CO 2 /epoxide copolymerization, and therefore ensures significant savings in energy and raw materials.…”

“…There are various very efficient ways of producing CO 2 -based monomers [2,3,13,16,17]. One of the most important monomers is cEC, which is produced in large quantities via the so-called Omega process as an intermediate in ethylene glycol production [7,8,15,[18][19][20] (see Figure 1). This monomer is suitable not only for producing CO 2 -containing polymers employing controllable ring-opening polymerization but also for producing CO 2 -containing surfactants, if a long-chain alcohol is used as starter for the ROP [8,9].…”

“…It is desirable to produce these polymers and, at the same time, control the product properties by using a CO 2 building block such as cEC or cSC. The understanding of the control possibilities [8] in ring-opening polymerization (ROP) results in a tunability of the reaction, which leads to the adjustment of both the melting point and the CO 2 content [6,13,33,34]. However, the melting point is also determined to some extent by the structure of the polymers [33].…”

“…Still, both methods have disadvantages that make them unattractive for larger scale applications. High-pressure autoclave reactions of epoxides and CO 2 sometimes yield very impure products, some of which are already polymerized and have to be separated [6,13,[35][36][37], as some of the catalyst degradation products themselves act as catalysts for the ROP [8,13,33,38,39]. This leads to purification processes with high equipment and energy costs.…”