Pd(ii)-catalyzed ethylene/methyl acrylate copolymerization: toward catalyst recovery and recycling

The paradigm shift from a linear to a circular economy is a very important goal, which involves several fields of chemistry, including the recycling of plastic materials and of critical raw materials, including platinum-group metal catalysts. The introduction of polar functional groups into the polyolefin skeleton, to yield functionalized polyolefins (FPOs), might open new and easier pathways for plastic depolymerization processes compared to simple polyolefins. However, the controlled copolymerization of ethylene with polar vinyl monomers via homogeneous catalysis to produce FPOs is a highly challenging reaction; at present, Pd–α-diimine complexes are the most promising catalysts for this reaction. In this contribution, we have revisited the catalytic behaviors of three known palladium complexes with benchmark α-diimines (N–N), with the general formula [Pd(N–N)(Me)(NCMe)][PF6], by carrying out, for the first time, the copolymerization of ethylene with methyl acrylate in trifluoroethanol/dichloromethane mixtures of different compositions. We found that the solvent composition had an unprecedented effect on catalyst productivity, the content of inserted polar monomers and the mode of incorporation. Detailed NMR investigations in CD2Cl2/TFE-d3 mixtures of the reaction of one of the Pd-complexes with methyl acrylate allowed us to correlate the observed catalytic behavior with the organometallic intermediates present in solution. Moreover, at specific solvent compositions, the spontaneous formation of two phases occurred at the end of the catalytic runs. We exploited this phenomenon to successfully perform, for the first time in this field, the recovery and recycling of the catalyst.