Synthesis of Low Density Poly(ethylene) Using Nickel Iminophosphonamide Complexes

Abstract

Ethylene polymerization using a catalyst derived from the reaction of the phosphorane (Me3Si)2NP(NSiMe3)2 (1) with either Ni(COD)2 or bis(π-allyl)Ni complexes affords branched poly(ethylene) (PE) of variable MW (103−106) depending on conditions. The branched PE of high MW is semicrystalline with Tm < 100 °C. High field 13C NMR spectra reveal the presence of methyl branches (ca. 10−15 per 1000 C atoms), branches longer than six C atoms (15−20 per 1000 C atoms) and trace levels of ethyl, propyl, n-butyl, and sec-butyl branches (total <2 per 1000 C atoms). The branching distribution changes modestly in response to changes in ethylene pressure in a manner consistent with a chain-walking mechanism. Analysis of high MW polymers by GPC-light scattering reveals the presence of sparse long-chain branching (gM = 0.78−0.93 with <1 long-chain branch per molecule); the branched PE formed is thus similar to low-density PE. Addition of α-olefin during polymerization leads to enhanced activity but is accompanied by chain transfer. The only evidence of α-olefin incorporation is at the chain-ends in the case of 4-methylpentene, and there is little change to the branching distribution in the presence of α-olefin. A sterically hindered nickel iminophosphonamide (PN2) complex (Me3Si)2NP(Me)(NSiMe3)2NiPh(PPh3) (2) was prepared and characterized by X-ray crystallography. This complex oligomerizes ethylene to branched material with a microstructure very similar to that observed using the catalysts derived from phosphorane 1 and Ni(COD)2 or (π-allyl)2Ni. DFT modeling of the active catalyst, coupled with stochastic simulation of chain growth, reveals that a chain-walking vs insertion mechanism can account for the short-chain branching distributions observed. Kinetic modeling of the observed branching distribution can account for relative intensity of the short branches (≤C5) as well as those of the longer branches. However, in order to fit the intensity of the Hx+ branches, one of the key parameters in the model, the probability of chain-walking for higher secondary Ni−R groups, converges to a value ∼ 1. This finding is not anticipated by the DFT results and suggests that the longer branches present in these materials do not form by a chain-walking vs insertion mechanism.