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

Palladium-catalyzed coordination-insertion copolymerization of ethylene with acrylonitrile (AN) proceeded only by using phosphine-sulfonate (P-SO ₃) 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-SO ₃ 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.