Converting natural rubber waste into Ring-opening Metathesis polymers with oligo-1,4-cis-isoprene sidechains

A chemical recycling of natural rubber waste via a degradation/polymerisation approach is described. The vulcanized rubber waste was degraded by cross metathesis with ethyl acrylate as the key-step yielding enoate end-capped oligo-cis-isoprenes, which were subsequently converted into norbornenes via a cycloaddition reaction with cyclopentadiene. Ring-opening Metathesis Polymerisation (ROMP) then yielded main-chain unsaturated polymers bearing oligo-1,4-cis-isoprene side chains with appealing thermal stability and a glass transition temperature of −60 °C.


General
Nuclear magnetic resonance experiments were done on a Bruker Avanze 300 MHz spectrometer ( 1 H: 300.36 MHz; 13 C: 75.53 MHz) at 25 °C.Chemical shifts are given in ppm relative to a tetramethylsilan (TMS) standard.Deuterated solvents were obtained from Cambridge Isotope laboratories Inc. and spectra were referenced against the residual proton signals according to literature. 1 GC-MS analyses were carried out on an Agilent Technologies 7890A GC system equipped with a 5975C mass selective detector (inert MSD with Triple Axis Detector system, EI, 70 eV).Samples were injected by employing autosampler 7683B in a split mode 1/25 (inlet temperature: 250 °C; injection volume: 0.1 μL) and separated on an Agilent Technologies J&W GC HP-5MS capillary column (30 m x 0.2 mm x 0.25 μm) at a constant helium flow rate (He 5.0 (Air Liquide), 1.000 mL/min, average velocity 36.5 cm/sec).A general gradient temperature method was used (initial temperature: 50 °C for 1 min, linear increase to 300 °C (40 °C.min - ), hold for 1 min, 1 min post-run at 300 °C, detecting range: 50.0-550.0amu, solvent delay of 2.80 min).
Differential scanning calorimetry (DSC) analyses were measured on a DSC 8500 instrument from Perkin Elmer in a temperature range from -20 to 100°C with a heating rate of 20°C/min for the first run and with 20°C/min in the second run (T g values were retrieved from the second heating run).
Thermogravimetric analyses (TGA) were performed with a Netzsch Simultaneous Thermal Analyzer STA 449C (crucibles: aluminium from Netzsch).A helium flow of 35 mL min −1 was used in combination with a protective flow of helium of 8 mL min −1 .The heating rate until a final temperature of 550 °C was 10 °C min −1 .
Gel permeation chromatography (GPC) was carried out on a system provided by WGE Dr. Bures operated with THF (separating columns from MZ-Gel SD plus, linear 5µ; UV und RI detector SEC 3010).Poly(styrene) standards purchased from Polymer Standard Service were used for calibration.Purification was done by column chromatography (eluent: cyclohexane:ethyl acetate = 15:1; 600 mL silica) and gave a first fraction of 78.5 mg of a transparent oil (R f =0.94), a second fraction of 1.85 g (yield: 37 % in relation to full conversion towards ethyl 6-methylhepta-2,6dienoate,) of a yellowish oil (R f =0.64-0.60). 1 H-NMR revealed 1 wit m = 3.2 and E/Z = 92/8.The theoretical yield for 1 m=3.2 is 2.70 g, accordingly 69 m% was found in this fraction.a third fraction of 268 mg (R f =0.34) of a yellowish oil which slowly solidifies, identified as diethyl but-2-enedioate

Degradation of natural rubber
The procedure described above was followed using 2.00 g natural rubber, 30 mL toluene, 15 mL ethyl prop-2-enoate and 97.7 mg M51.
After purification as described above the following fractions were obtained: first fraction of 59.5 mg of a transparent oil (R f =0.94) second fraction of 2.05 g (yield: 41 % in relation to full conversion towards ethyl 6-methylhepta-2,6dienoate) of a yellowish oil (R f =0.64-0.60). 1 H-NMR revealed 1 with m = 3.0 and E/Z = 92/8.The theoretical yield for 1 m=3.0 is 2.74 g, accordingly 75 m% was found in this fraction.third fraction of 535 mg (R f =0.34) of a yellowish oil which slowly solidifies, identified as diethyl but-2enedioate Influence of reaction conditions in particular catalyst loading Reactions were performed as described above with changes according to Table S1.Every try was done with 2 g natural rubber (or natural rubber gloves).The crude yield is the weight of the residue after removing volatiles.The crude products were investigated using 1 H-NMR spectroscopy.The integral for the peak at 5.83 ppm was set to 1 and the integral for the peak at 5.24-5.05ppm was read out as factor m (i.e.repeating units of oligo(isoprene), see drawing above.Further, the peak at 6.83 ppm was integrated and its integral was divided by 2 giving the ratio of diethyl fumarate to equivalents of 1 with the m as determined above.This value was used to calculate a percental ratio of 1 : diethyl fumarate given in Table S1 last column.Further, mol-percent were converted to masspercent and with that the weight of 1 (with given m) within the crude product was calculated and is given in the next to last column.n.d. a m is the average number of isoprene repeating groups according to the drawing above; b crude yield describes the weight of the residue after removal of the volatiles; c calculated share of 1 with a given m in the crude product; d molar ratio of 1 to E-fumarate according to 1 H-NMR; e the reaction mixture was first reacted at 80°C without ethyl acrylate for 3 h, afterwards ethyl acrylate was added and the reaction was reacted at 80°C for further 5h; f the crude reaction product from entry 4 was used as starting material; g in this case natural rubber gloves (i.e.vulcanized NR) were used instead of natural rubber; h methyl acrylate was used instead of ethyl acrylate; i reaction performed without acrylate, the residue was analyzed by GPC (THF): M n = 48000 g/mol, M w = 90000 g/mol; * GPC (THF): M n = 1800 g/mol, M w = 4000 g/mol; ** GPC (THF): M w < 500 g/mol

Characterization of representative fractions
Fraction 1 (R f =0.94) was not comprehensively characterized but contained mainly species bearing oligoisoprene units as evident from NMR-spectroscopic investigations (Figs S2 -S3).

Table S1
Degradation of 2 g natural rubber under different reaction conditions