Mechanistic insights on acrylate insertion polymerization

Complexes [{(PΛO)PdMe]_n] (1_n; PΛO = K²-P, O-Ar₂PC₆H₄SO₂O with Ar = 2-MeOC₆H₄) are a single-component precursor of the (PΛO)PdMe fragment devoid of additional coordinating ligands, which also promotes the catalytic oligomerization of acrylates. Exposure of 1_n to methyl acrylate afforded the two diastereomeric chelate complexes [(PΛO)Pd{k²-C,O-CH(C(O)OMe)CH₂CH(C(O)OMe)CH ₂CH₃}] (3-meso and 3-rac) resulting from two consecutive 2,1-insertions of methyl acrylate into the Pd-Me bond with the same or opposite stereochemistry, respectively, in a 3:2 ratio as demonstrated by comprehensive NMR spectroscopic studies and single crystal X-ray diffraction. These six-membered chelate complexes are direct key models for intermediates of acrylate insertion polymerization, and also ethylene-acrylate copolymerization to high acrylate content copolymers. Studies of the binding of various substrates (pyridine, dmso, ethylene and methyl acrylate) to 3-meso and 3-rac show that hindered displacement of the chelating carbonyl moiety by π-coordination of incoming monomer significantly retards, but does not prohibit, polymerization. For 3-meso,3-rac + C₂H₄ ⇄ 3-meso-C₂H₄ 3-rac-C₂H₄ an equilibrium constant K(353 K) ≈ 2 × 10^-3 L mol_-1 was estimated. Reaction of 3-meso, 3-rac with methyl acrylate afforded higher insertion products [(PΛO)Pd(C₄H₆O₂).,Me] (n = 3, 4) as observed by electrospray ionization mass spectrometry (ESI-MS). Theoretical studies by DFT methods of consecutive acrylate insertion provide relative energies of intermediates and transition states, which are consistent with the aforementioned experimental observations, and give detailed insights to the pathways of multiple consecutive acrylate insertions. Acrylate insertion into 3-meso,3-rac is associated with an overall energy barrier of ca. 100 kJ mol^-1