Ethylene polymerisation in aqueous media
Guironnet and co-workers have presented an interesting piecework in Nature Communications entitled “Encapsulation of catalyst in block copolymer micelles for the polymerization of ethylene in aqueous medium.” The work explores ethylene polymerisation within block copolymer micelles that are present in an aqueous environment.
Olefin polymerisation (ethylene and propylene) accounts for approximately 40% of the annual polymer production worldwide. Therefore polyethylene is one of the most important polymer commodities that we as a society rely on heavily. However, the catalysts used in these polymerisation reactions are largely water sensitive because they are mostly based on early transition metals. Therefore, the polymerisation reactions need to be carried out under water free conditions and involve organic solvents.
A vast amount of research has been carried out to overcome this problem. A couple of solutions have been proposed. Some academics have produced catalysts consisting of late transition metals for such polymerisations because they have higher water tolerance. Others have conducted miniemulsion polymerisations, where the catalyst is soluble in an organic solvent that is dispersed into water by sonication. However, these polymerisations have lower activities compared to the analogous ethylene polymerisation reactions in organic solvents.
Here the authors have circumvented this water problem by incorporating ethylene polymerisation catalysts in block copolymer micelles. Three block copolymers were explored (PEG-b-PEHA (PEHA: poly(ethyl)hexylacrylate), PEG-b-PCF3 (PCF3: poly(2,2,2-trifluoroethyl methacrylate)) and PEG-b-PS (PS: polystyrene)) and four different phosphinosulfonate palladium catalysts were investigated.
It was found that the performances of the encapsulated catalysts were significantly higher when compared with the same catalysts in miniemulsion polymerisation reactions. The protection of the catalyst by the block copolymer micelle improved the stability of the catalyst in dispersion. However, the authors acknowledged the fact that the catalysts activities in micelles were still orders of magnitude lower than when in organic solvents.
After thorough investigations, along with kinetic studies, the authors surmise that the lower activity could be due to the lower solubility of ethylene in the micelle. Additionally, the competition at the catalyst active site between ethylene and either the PEG block of the micelle or the amine present in the reaction also hamper the activity of the catalyst.
However, it was shown that by adding acid, the chemistry of the block copolymer micelle changed, which improved the activities of the catalysts. Thus this should motivate further work on the development of next generation block copolymer micelles and investigate the relationship between catalyst activity and block copolymer microstructure.