Mechanistic Insights into Ni(II)-Catalyzed Nonalternating Ethylene–Carbon Monoxide Copolymerization

Polyethylene materials with in-chain-incorporated keto groups were recently enabled by nonalternating copolymerization of ethylene with carbon monoxide in the presence of Ni(II) phosphinephenolate catalysts. We elucidate the mechanism of this long-sought-for reaction by a combined theoretical DFT study of catalytically active species and the experimental study of polymer microstructures formed in pressure-reactor copolymerizations with different catalysts. The pathway leading to the desired nonalternating incorporation proceeds via the cis/trans isomerization of an alkyl-olefin intermediate as the rate-determining step. The formation of alternating motifs is determined by the barrier for the opening of the six-membered C,O-chelate by ethylene binding as the decisive step. An η2-coordination of a P-bound aromatic moiety axially oriented to the metal center is a crucial feature of these Ni(II) catalysts, which also modulates the competition between the two pathways. The conformational constraints imposed in a 2′,6′-dimethoxybiphenyl moiety overall result in a desirable combination of disfavoring ethylene coordination along the alternating incorporation pathway, which is primarily governed by electronics, while not overly penalizing the nonalternating chain growth, which is primarily governed by sterics.


General methods and materials
Unless noted otherwise, all manipulations of air and moisture sensitive materials were carried out under inert gas atmosphere using standard glovebox and Schlenk techniques.

Solvents and reagents
Solvents were dried and degassed using standard laboratory techniques. Oxygen was removed from ethyl acetate by freeze-pump-thaw degassing and storage over molecular sieves prior to use. Pentane and diethyl ether were dried and freed from oxygen with a MB-SPS-800 solvent purification system by MBRAUN and molecular sieves. Benzene was distilled from sodium. n-Butyllithium (n-BuLi; 2.5 M solution in n-hexane) and 3,4-dihydro-2H-pyran (≥97 %) were purchased from Sigma-Aldrich.
(2-(Ph)C6H4)2P-6-C6F5-C6H3OH (3b): 3b was prepared by modification of a reported procedure. 5 At 0 o C, n-BuLi (6.9 mL, 1.6 M in hexane, 11 mmol, 1.1 equiv.) was added dropwise to a solution of 3a (5.1 g, 10 mmol, 1.0 equiv.) in THF (30 mL). The reaction mixture was warmed to room temperature and stirred for 2 hours to give a brown suspension. The mixture was cooled to -78 o C, and C6F6 (9.3 g, 50 mmol, 5.0 equiv.) was added dropwise at -78 o C. The mixture was warmed to room temperature slowly and stirred for 12 hours to afford a red solution. The mixture was concentrated in vacuum, the residue was dissolved in 20 mL of degassed ethyl acetate, and 2 mL of conc. HCl were added. The mixture was stirred at room temperature overnight and added slowly to a solution of 5 g NaHCO3 in 60 mL water and stirred for 30 minutes. The organic phase was separated, and the aqueous phase was extracted with ethyl acetate (2 × 20 mL). The combined organic phase was concentrated in vacuo, and the residue was subjected to column chromatography on silica using petrol ether/ethyl acetate = 20/1 as eluent to give pure 3b (4.5 g, 7.6 mmol, 76%).     All gas valves and devices were connected to a HiTec Zang LabBox and operated by HiTec Zang LabVision® software (ver. 2.13). Prior to all polymerization experiments, the reactor was evacuated and heated (thermostat temperature: 90 °C). When the internal reactor temperature exceeded 60 °C, the reactor was flushed with nitrogen and evacuated three times. Then the temperature was adjusted to the desired reaction temperature. 200 mL of dry and degassed toluene were added, stirring was started with 100 rpm and the system was equilibrated for 5 min to reach the desired (internal) temperature. The system was pressurized with 13 C labeled carbon monoxide (starting from between 0.99 to 1.02 bar nitrogen pressure, adding 20 mbar of CO. This corresponds to ca. 8 to 9 mg of CO).
The CO supply was disconnected and the system was pressurized to a total pressure of 10 bar using an ethylene mass flow regulator. The precatalyst was dissolved in 4 mL of toluene and added to the reactor via the liquid dosing pump with 10 mL per minute. The reaction time was started upon begin of addition. The tubing of the pump was flushed with toluene for 3 min to assure the complete addition of the precatalyst. The reaction was kept under constant pressure for 5 minutes (using ethylene massflow regulators), and then the pressure was released. To the reaction mixture methanol was added (~50 mL), followed by evaporation of the solvent. The residue was washed with 50 mL of methanol applying ultrasonication, centrifuged off and then dried at 60 °C/30 mbar overnight.

Determination of CO incorporations from 1 H NMR spectra
Compositions of ethylene-CO copolymers were determined from 1 H NMR spectra according to equation (1). S11 χ CO = 100 ⋅ A + B 2 ⋅ A + 2 ⋅ B + C + D (1) Figure S7. Exemplary 1 H NMR spectrum of an ethylene-CO copolymer produced with catalyst 2 with assignment of typical motifs. S12 Figure S8. Exemplary 13 C NMR spectrum of an ethylene-CO copolymer produced with catalyst 2 with assignment of typical motifs. Figure S9. Exemplary 13 C{ 1 H} NMR spectrum of a copolymer (obtained with catalyst 2, cf. Table   1, entry 2) with assignments of repeat unit motifs. In terms of CO incorporation events during polymerizaton, isolated motifs (I) correspond to an incorporation along the nonalternating pathway, alternating motifs (A) correspond to an incorporation along the alternating pathway, and non-alternating motifs (NA) correspond to a combination of an alt-pathway and a subsequent non-alt pathway. Thus, the relative ratio of non-alternating and alternating carbon monoxide incorporation events can be derived from the microstructure as (I + 0.5 NA) / (0.5 NA + A).

S14 4 Computational details
We performed calculations of all important intermediates and transition states involved in the competitive linear chain growth alternating and non-alternating pathways during the ethylene and CO copolymerization (the 1-cycle5-T intermediate was set as a reference point). All the DFT geometry optimizations were performed at the GGA BP86 7 level with the Gaussian09 package. 8 The electronic configuration of the systems was described with the 6-31G basis set for H, C, N, F, and O while for Ni the quasi relativistic LANL2DZ ECP effective core potential was adopted. 9 All geometries were characterized as minimum or transition state through frequency calculations. The geometry optimizations were performed without symmetry constraints. All transition-state structures were confirmed to connect corresponding reactants and products by intrinsic reaction coordinate (IRC) calculations. 10 The reported free energies were built through single point energy calculations on the (BP86/6-31G/LANL2DZ ECP) geometries using the M06 functional and the triple-ζ TZVP basis set on main group atoms while for Ni the quasi relativistic LANL2DZ ECP effective core potential was adopted. 9,11 Solvent effects were estimated with the PCM model using toluene as solvent. 12 To this (M06/TZVP/LANL2DZ ECP) electronic energy in solvent, thermal corrections were included from the gas-phase frequency calculations at the gas-phase level of theory (BP86/6-31G/LANL2DZ ECP).
The percent buried volume calculations and the steric maps were performed with the SambVca 2.1 package. 13 The radius of the sphere was fixed in the origin of the metal center, while for the atoms, we adopted the Bondi radii scaled by 1.17, and a mesh of 0.1 Å was used to scan the sphere for buried voxels. S15 5 Energetic profiles and Gibbs free energies. 5.1 Energetic profile with the alternative pathways for catalysts 1 and 2. Figure S10. Gibbs free energies (ΔGTol in kcal/mol) of the competitive pathways for non-alternating and alternating carbon monoxide incorporation with catalysts 1 (red, top) and 2 (blue, bottom). S16 5.2 Gibbs free energies of competitive species for catalysts 3, 4, 2' and 3'.   on the BP86 optimized geometries using a) PBEO/D3 functional 14,15 , b) B3PW91/D3 16

= ×
In detail, is the probability to obtain non-alternating segments, whereas is the probability to obtain alternating segments.
The equilibrium and kinetic constants of the equation refer to the following scheme: S18 Since the CO pressure of 0.02 bar is estimated to correspond to a concentration of [CO] ≈ 1.7 × 10 -4 mol L -1 (at 90°C in toluene) 19 and the ethylene (10 bar) concentration is estimated to [E] ≈ 0.67 mol L -1 (from data at 95 °C), 20 the calculated value for catalyst 1 is 20.0.
It worth to note that the contribution of F cor to is negligible because k CO −1 is << K 4 * k C2H4 [C2H4] (indeed F cor is 1).
In other words, the value of corresponds to Brookhart's equation 21  where K'2 is the equilibrium constant between of 1-cycle5-T and 1-cycle6-T.
Comparison of these theoretical value to experimental values from microstructure analysis (Table   1) agrees reasonably (Table S4).  S20 Figure S11. Topographic steric maps of the transition state TSIsom for catalysts 3 (top left) and 3' (top right). The complexes are oriented as shown below (bottom left and right).