Synthesis of UHMWPE by neutral phosphine-phenolate based nickel catalysts.

It is highly interesting to prepare polyethylene with ultra-high molecular weight under homogeneous conditions by nickel based neutral P^O type catalysts. In this contribution, ligand remote electronic effect was explored in some sterically hindered phosphine-phenolate based neutral nickel catalysts Ni1 – Ni3 to afford UHMWPE. The maximal activity of these catalysts was found to be 1.33 × 106 g mol−1 h−1, which is quite active. The nickel catalysts retained considerable activity at high temperature of 120 °C showing their good thermal stability. More importantly, all these catalysts are capable of generating UHMWPE with maximum M n of 2.51 × 106 g mol−1 by Ni2. The minimum distance between the Ni center and the oxygen of the 2,6-dimethoxy-1,1′-biphenyl unit attached with the phosphorous atom in Ni1 was recorded as 3.154 Å, which exactly matches with the van der Waals radii of oxygen and nickel. This provides an effective shield around the metal by preventing chain transfer and resulting to enhance the molecular weight of polymer. Combined with steric the remote substituents effect also crucial both to modulate activity and molecular weight of polymers. 2,6-dimethoxyaryl unit attached on phosphorous provides an effective shield at the axial place of metal to block the chain transfer process. [Display omitted] • The catalysts Ni1-Ni3 were found highly active with activity up to 1.33 x 106 g·mol-1·h-1. • All catalysts remain active even at 120°C showing their good thermal stability whereas previously reported catalysts of this class usually decompose at 60-80°C. • All the catalysts generate UHMWPE at 60°C with maximum M n up to 2.51 x 106 g·mol-1 by Ni2 , representing a major breakthrough in this class of catalysts. • The minimum distance from the Ni to the oxygen of the 2,6-dimethoxy-1,1'-biphenyl unit to the metallacycle in Ni1 recorded as 3.154 Å which exactly matches with the van der Walls radii. • Catalysts also show considerable incorporation of challenging fundamental polar monomer such as methyl acrylate. [ABSTRACT FROM AUTHOR]