Why Did Incorporation of Acrylonitrile to a Linear Polyethylene Become Possible? Comparison of Phosphine−Sulfonate Ligand with Diphosphine and Imine−Phenolate Ligands in the Pd-Catalyzed Ethylene/Acrylonitrile Copolymerization

摘要

Palladium-catalyzed coordination−insertion copolymerization of ethylene with acrylonitrile (AN) proceeded only by using phosphine−sulfonate (P−SO₃) as a ligand among the neutral and anionic ligands we examined, those are phosphine−sulfonate (P−SO₃), diphosphine (P−P), and imine−phenolate (N−O). In order to answer a question that is unique for P−SO₃, theoretical and experimental studies were carried out for the three catalyst systems. By comparing P−SO₃ and P−P, it was elucidated that (i) the π-acrylonitrile complex [(L−L′)PdPr(π-AN)] is less stable than the corresponding σ-complex [(L−L′)PdPr(σ-AN)] in both the phosphine−sulfonato complex (L−L′ = P−SO₃) and the diphosphine complex (L−L′ = P−P) and (ii) the energetic difference between the π-complex and the σ-complex is smaller in the P−SO₃ complexes than in the P−P complexes. Thus, the energies of the transition states for both AN insertion and its subsequent ethylene insertion relative to the most stable species [(L−L′)PdPr(σ-AN)] are lower for P−SO₃ than for P−P. The results nicely explain the difference between these two types of ligands. That is, ethylene insertion subsequent to AN insertion was detected for P−SO₃, while aggregate formation was reported for cationic [(L−L)Pd(CHCNCH₂CH₃)] complex. Aggregate formation with the cationic complex can be considered as a result of the retarded ethylene insertion to [(L−L)Pd(CHCNCH₂CH₃)]. In contrast, theoretical comparison between P−SO₃ and N−O did not show a significant energetic difference in both AN insertion and its subsequent ethylene insertion, implying that ethylene/AN copolymerization might be possible. However, our experiment using [(N−O)PdMe(lutidine)] complex revealed that β-hydride elimination terminated the ethylene oligomerization and, more importantly, that the resulting Pd−H species lead to formation of free N−OH and Pd(0) particles. The β-hydride elimination process was further studied theoretically to clarify the difference between the two anionic ligands, P−SO₃ and N−O.