A Closed‐Loop Recyclable Low‐Density Polyethylene

Abstract Low‐density polyethylene (LDPE) is one of the most important plastics, which is produced unfortunately under extreme conditions. In addition, it consists of robust aliphatic C─C bonds which are challenging to cleave for plastic recycling. A low‐pressure and ‐temperature (pethylene = 2 bara, T = 70 °C) macromonomer‐based synthesis of long chain branched polyethylene is reported. The introduction of recycle points permits the polymerization (grafting to) of the macromonomers to form the long chain branched polyethylene and its depolymerization (branch cleavage). Coordinative chain transfer polymerization employing ethylene and co‐monomers is used for the synthesis of the macromonomers, permitting a high flexibility of their precise structure and efficient synthesis. The long chain branched polyethylene material matches key properties of low‐density polyethylene.


General Considerations
All manipulations of air and/or moisture sensitive compounds were performed under exclusion of oxygen by using standard Schlenk line techniques or an argon or nitrogen filled glove box (mBraun) with a high capacity circulator (< 0.1 ppm O2).
Characterization Methods 1 H and 13 C NMR -Spectra were performed at a Varian INOVA 300 ( 1 H: 299.9 MHz, 13 C: 75.4 MHz).The 1 H-and 13 C-NMR polymer spectra were recorded at 120 °C in C2D2Cl4 and referenced internally to the residual solvent resonances ( 1 H-NMR: 5.90 ppm; 13 C-NMR: 74.12 ppm).The 1 H-NMR spectra of the low molecular weight compounds were recorded at 25 °C in C6D6 and referenced also internally to the residual solvent resonances ( 1 H-NMR: 7.16 ppm).For the 13 C-NMR measurements 10 000 number of transitions and a relaxation delay of 9 s was used.Chemical shifts (δ) are reported in ppm.Mestre Nova software was used to evaluate the reported NMR spectra.
Gel Permeation Chromatography (GPC) -Analyses were carried out at an Agilent (Polymer Laboratories Ltd.) PL-GPC 220 high temperature chromatographic unit equipped with refractive index and differential pressure detectors, a guard and three linear mixed bed columns (Agilent Olexis).GPC analyses were performed at 150 °C using 1,2,4-trichlorobenzene as mobile phase.The samples were prepared by dissolving the polymer (0.1 wt.-%) in the mobile phase in an external oven and were run without filtration.The molecular weights of the samples were referenced to linear narrow HDPE standards (Mw = 110 -430 000 g/mol, K = 40.6 and α = 0.725) Differential Scanning Calorimetry (DSC) -DSC measurements were performed on a Mettler DSC 3+.Standard aluminum pans with a volume of 40 µl were used.The samples were measured in two cycles from 20 to 210 °C, a heating rate of 10 K/min and under a nitrogen atmosphere.The Tm and Trecryst.was taken from the second heating/cooling curve.
Thermogravimetric Analysis (TGA) -All TGA measurements were performed on a Mettler Toledo TGA / SDTA 851e.Standard aluminum pans with a volume of 40 μL were used.Samples were measured from 20 °C to 700 °C with a heating rate of 10 K/min under a nitrogen atmosphere.
Compression Molding -Compression molding for dogbone-shaped samples was conducted at a Carver Typ 2518 hotpress.Films around 0.5 mm thickness were compression molded at 115 °C or 120 °C for 4 minutes at a pressure of 1 ton.All polymer processing was performed without the addition of additives (stabilizers or antioxidants).
Density Measurements -The density measurements were performed on a Mettler-Toledo weight using the Archimedes principle.Five compression molded samples (30 mg -50 mg) were weighed once on the air and the other time the weight of the sample completely immersed in water (T = 22.4 °C, ρ = 0.9977 g/cm 3 ) was measured.
Tensile Elongation Tests -The tensile tests were performed on an Instron universal testing machine 5565, equipped with a video extensometer using dogbone-shaped samples [0.5 mm (T) x 4 mm (W) x 15 mm (L)].Tensile testing was performed at a ramp speed of 10 mm/min.Six replicates per material were used for the elongation to break tests.
Rheology Measurements -The rheology measurements were conducted on a plate rheometer (Anton Paar MCR 302).Rheology plates (thickness = 1 mm, diameter = 25 mm) were previously compression molded on a Carver Typ 2518 hotpress at 120 °C for 6 minutes at a pressure of 1 ton.The polymer processing for synthesized polymer samples was performed with the addition of commercially available Irganox 1010 as stabilizer.The viscoelastic region was determined prior to the measurements.Frequency sweeps in the melt were performed at 130 °C, 150 °C and 170 °C.

Statistical Analysis
To ensure the reproducibility of the reported results and to enable statistical analysis all batch experiments were conducted independently.For statistical analysis of density measurements mean values and standard deviations (table S10 and table S11) were examined using a sample size of five (n=5).For statistical analysis of tensile tests mean values and standard deviations (table S7-S9) were calculated using a sample size of six (n=6).Highest and lowest values were excluded.Origin Pro software was used to evaluate the reported data.

Polymer Synthesis
All terpolymerizations were conducted in a 300 mL glass autoclave (BüchiGlasUster) with a mechanical stirrer (500-1000 rpm).The temperature of the glass autoclave was controlled via an external water bath.The synthesis of all long chain alcohols (copolymerizations and oxidations) were conducted in a 1000 mL temperature and pressure controlled stainless steel autoclave with a mechanical stirrer (500-1000 rpm).All precatalysts, TEA, comonomers and activator were added as stock solutions in toluene.During the polymerization (semi batch mode) the ethylene pressure was kept constant by replenishing ethylene flow.The ethylene consumption was measured with a Bronkhorst High-Tech EI-Mass-Flow controller in liter normal (Ln), 1 mol gas = 22.41 Ln at p = 1.01325 bar and 0 °C.After the reaction 10 mL ethanol p.a. was added to stop the reaction.

General Synthesis of the unsaturated backbone P1:
For the P1 synthesis the glass autoclave was evacuated (30 minutes) and heated to 30 °C.The autoclave was charged with 50 mL dry toluene, stirred at 1000 rpm and pressured with ethylene (2 bara).20 mmol VNB, 100 mmol 1-hexene, 0.5 mmol TEA and 3 mmol d-MAO were subsequently added as stock solutions in toluene so that the polymerization was conducted in 75 mL toluene/comonomer-solution.6 μmol precatalyst [Ti] were added to start the reaction.After 6 Ln ethylene consumption the polymerization was quenched with ethanol.The polymer solution was washed with H2O and HCl, and the water-phase was extracted with toluene.The organic phases were combined, and all volatiles were removed under reduced pressure.The synthesis of P1 was repeated in three independent batch experiments.The results are summarized in table S1.
Calculation of average vinylic-and 1-hexene-units per chain: As an example, the calculation of incorporated comonomer units per average polymer chain for the VNB/1-hexene/ethylene terpolymers is conducted for Table S1, Batch 1-3.For this purpose, the average polymer chain length Mn of the corresponding GPC data and the 13 C-NMR spectrum of the VNB/1-hexene/ethylene terpolymer (Figure S5) are used.
Calculation of incorporated 1-hexene-units per chain: As an example, the calculation of incorporated 1-hexene units per average polymer chain is conducted for Table S2, Batch 1-2.For this purpose, the average polymer chain length Mn of the corresponding GPC data and the 13 C-NMR spectrum of the 1-hexene/ethylene copolymer (Figure S9) are used.

4 Figure S14: 1 H
Figure S14: 1 H-NMR spectrum of the pure LDPE mimic P4 batch used for rheology experiments before the addition of Irganox 1010 in C2D2Cl4 at 120 °C.