Selective branch formation in ethylene polymerization to access precise ethylene-propylene copolymers

Polyolefins with branches produced by ethylene alone via chain walking are highly desired in industry. Selective branch formation from uncontrolled chain walking is a long-standing challenge to generate exclusively branched polyolefins, however. Here we report such desirable microstructures in ethylene polymerization by using sterically constrained α-diimine nickel(II)/palladium(II) catalysts at 30 °C–90 °C that fall into industrial conditions. Branched polyethylenes with exclusive branch pattern of methyl branches (99%) and notably selective branch distribution of 1,4-Me2 unit (86%) can be generated. The ultrahigh degree of branching (>200 Me/1000 C) enables the well-defined product to mimic ethylene-propylene copolymers. More interestingly, branch distribution is predictable and computable by using a simple statistical model of p(1-p)n (p: the probability of branch formation). Mechanistic insights into the branch formation including branch pattern and branch distribution by an in-depth density functional theory (DFT) calculation are elucidated.


Supplementary Methods
General Procedures: All syntheses involving air-and moisture sensitive compounds were carried out using standard Schlenk-type glassware (or in a glove box) under an atmosphere of nitrogen. All solvents were purified from the MBraun SPS system. NMR spectra for the ligand, complex, and polymers were recorded on a Bruker AV400 ( 1 H: 400 MHz, 13 C: 100 MHz) or a Bruker AV500 ( 1 H: 500 MHz, 13 C: 125 MHz). NMR assignments were confirmed by 1 H− 1 H COSY experiments when necessary. The molecular weights (Mn) and molecular weight distributions (Mw/Mn) of polyethylenes and copolymers were measured by means of gel permeation chromatography (GPC) on a PL-GPC 220-type high-temperature chromatograph equipped with three PL-gel 10 μm Mixed-B LS type columns at 150 °C. Melting temperature (Tm) of copolymers were measured through DSC analyses, which were carried out on a Mettler TOPEM TM DSC Instruments under nitrogen atmosphere at heating and cooling rates of 10 °C/min (temperature range: -100−160 °C). Elemental analysis were performed at the National Analytical Research Centre of Changchun Institute of Applied Chemistry.

X-Ray diffraction:
Data collections were performed at −100 °C on a Bruker SMART APEX diffractometer with a CCD area detector, using graphite-monochromated Cu Kα radiation (λ = 1.54178 Å). The determination of crystal class and unit cell parameters was carried out by the SMART program package. 11 The raw frame data were processed using SAINT and SADABS to yield the reflection data file. 12 All structures were solved and refined by full-matrix least-squares procedures on F 2 using SHELXTL or Olex2. 13 Refinement was performed on F 2 anisotropically for all non-hydrogen atoms by the full-matrix least-squares method. The hydrogen atoms were placed at the calculated positions and were included in the structure calculation without further refinement of the parameters.

Exceptions and special features:
For ipty-Ni, the program SQUEEZE 14 was used to remove mathematically the effect of the solvent. The quoted formula and derived parameters are not included the squeezed solvent molecules.

Explanations of B level alerts on ipty-Ni single crystal structure:
The "THETM01 ALERT 3 B" and "PLAT413 ALERT 2 B" alerts are generated due to that the quality of this single crystal is not very good and there is a large amount of H atom disorder in the structure. We tried to recollect the data from a crystal of higher quality, but attempts to obtain good crystals by using various available solvent systems were failed. The used data was collected from the best crystal that we obtained. We ensured that the two alerts do not affect the structure and the analyzed data was correct.
Materials: -diimine ligand was prepared using the literature procedure. 15

Preparation of Catalyst
Preparation of pentiptycene aminoanisole: To a solution of pentiptycene aminophenol (3 g, 6.5 mmol) in 100 mL dry DMF at r.t. was added NaH (468 mg, 19.5 mmol) under nitrogen. The mixture foamed and was warmed. CH3I (0.6 mL, 9.75 mmol) was added after the bubbling ceased and the mixture was stirred at r.t. for 24 h. The reaction was quenched by adding 400 mL of distilled water and the aqueous layer was extracted with 3 × 50 mL of CH2Cl2. The combined organic phases were washed with 3 ×50 mL of distilled water and 50 mL of brine. Then the organic phase was separated and dried with sodium sulfate. After evaporation of the solvent the pure product was obtained as white solid (2.6 g, 84.1% yield). This method was similar to our previous study. 15

Preparation of Ligand:
A solution of 9,10-dihydro-9,10-ethanoanthracene-11,12-dione (500 mg, 2.12 mmol), pentiptycene aminoanisole (2.5 g, 5.29 mmol) and p-toluenesulfonic acid (5 mg) in toluene (100 mL) was refluxed with Dean-stark trap for 3 days, the solvent was partially evaporated under reduced pressure until the formation of a yellow solid, and the remaining solution was diluted in ethanol (300 mL). The yellow solid was isolated by filtration, washed three times by 20 mL ethanol and dried under high vacuum (1.46 g, 60.2% yield). This method was similar to our previous study.

General Procedure for the Polymerization
A general procedure for ethylene polymerization using Ni catalyst In a typical experiment, a 350 mL glass pressure reactor connected with a high pressure gas line was firstly dried at 90 °C under vacuum for at least 1 h. The reactor was then adjusted to the desired polymerization temperature. 98 mL of toluene and co-catalyst was added to the reactor under N2 atmosphere, then the desired amount of Ni catalyst in 2 mL of CH2Cl2 was injected into the polymerization system via syringe. With a rapid stirring, the reactor was pressurized and maintained at specified pressure of ethylene. After the specified time, the pressure reactor was vented and the polymerization was quenched via the addition of 100 mL acidic EtOH (5% HCl in EtOH) and dried in a vacuum oven to constant weight.

Supplementary Tables
Supplementary Table 1

Supplementary Figures
Supplementary Figure 1