Switch Catalysis To Deliver Multi‐Block Polyesters from Mixtures of Propene Oxide, Lactide, and Phthalic Anhydride

Abstract Switchable polymerisation catalysis enables block polymer sequence selectivity from monomer mixtures, resulting in the formation of multiblock polyesters. The aluminium salphen catalyst switches between two different polymerisation mechanisms and selectively enchains mixtures of commercially available monomers: lactide, phthalic anhydride, and propene oxide. Sequential monomer mixture additions yield multi‐block polyesters featuring 3, 7, 11, 15, 19, 23, and 27 blocks. The unparalleled catalytic selectivity can be used to access completely new multi‐block polyesters relevant for future applications.


List of Figures
.. 27 Figure S. 24 -13 C{ 1 H} NMR Spectra of polymer obtained from 'switch' catalysis and polymer synthesized via sequential monomer addition (n. b.: the polymer from 'switch' catalysis used for 13 C{ 1 H} NMR spectroscopy had a higher molar mass than the polymer obtained from sequential monomer addition).

Materials
(±)-Propylene oxide (99%) was purchased from Sigma Aldrich and purified by fractional distillation after drying over calcium hydride overnight. Phthalic anhydride (99%, Aldrich) was purified by stirring in dry benzene overnight, filtering off insoluble impurities, evaporating of the remaining solution and recrystallization of the obtained solid in hot (anhydrous) chloroform. Sublimation under high vacuum (10 -2 mbar, 80°C) yielded phthalic anhydride, which was stored in a nitrogen-filled glovebox. Trans-1,2-cyclohexanediol (CHD, 98%, Aldrich) was recrystallized from anhydrous ethyl acetate, dried at 40°C under high vacuum overnight and stored inside the glovebox. (Rac)-Lactide (99%, Aldrich) was recrystallized in anhydrous toluene and sublimed under high vacuum three times. The obtained solid was stored in the glovebox at -30°C. TCA1 was synthesized according to literature procedures, recrystallized from acetone and sublimed under high vacuum before use. [1] Bis(triphenylphosphoranylidene)ammonium chloride (PPNCl, 97%, Aldrich) was recrystallized from anhydrous acetonitrile / diethyl ether, dried at 40°C overnight and stored in a glovebox.
[SalphenFAlCl] was prepared according to literature procedures, dried under high vacuum for several days and stored inside the glovebox. [2] Dry toluene was obtained from a SPS-800 system by M Braun, degassed by bubbling with nitrogen for one hour and stored over activated molecular sieves. Toluene-d 8 and CDCl 3 were stirred over calcium hydride overnight, transferred under high vacuum, degassed with three freeze-pump-thaw cycles and stored over activated molecular sieves. Mesitylene (98%) was dried over calcium hydride, transferred under high vacuum, degassed by three freeze-pump-thaw cycles and stored over activated molecular sieves. All other chemicals were obtained from obtained from several commercial suppliers (Sigma Aldrich, Fischer, VWR, Alfa Aesar, Acros Organics).

In situ ATR-IR Spectroscopy
For monitoring by ATR-IR, a Mettler-Toledo ReactIR 4000 spectrometer with a MCT detector and a silver halide DiComp probe was used. The reported plots were obtained after one or two point baseline corrections and the monitored bands were selected from the overlap of individual IR spectra of monomer and polymers (Figure S

DSC
Differential scanning calometry (DSC) was performed on a Mettler Toledo DSC 3 Star calorimeter under nitrogen at a heating rate of 10 °C/min. First, the sample was kept at 25 °C for 1 minute, then heated to 160 °C at 10 °C/min, kept for 1 minute and cooled down to -90 °C and kept for 1 minute. This heating / cooling cycle was then repeated twice and the data from the second or third heating cycle is reported. The flow rate of nitrogen was kept at 80 mL/min throughout the measurement. 1 H-NMR spectra were measured on a Bruker Avance III HD nanobay NMR equipped with a 9.4T magnet ( 1 H: 400.2MHz, 31 P 162.0MHz) and a Bruker Avance III NMR equipped with a 11.75T magnet ( 1 H NMR: 500 MHz). 13 C{ 1 H}-NMR spectra were measured on a Bruker Avance NMR equipped with a 11.75T magnet and a 13 C{ 1 H} detect cryoprobe ( 1 H: 500.3MHz, 13 C: 125.8MHz). All spectra were recorded in CDCl 3 . [3] SEC Size exclusion chromatography (SEC) was performed on an Agilent PL GPC-50 instrument, with HPLC grade THF, at 30°C and a flow rate of 1.0 mL/min. In all cases, near monodisperse polystyrene standards were used for calibration. The samples were prepared by dissolving ca. 20 mg of polymer in THF, and filtering through a 2 μm PTFE filter before injection. [4] The procedure was previously reported by Spyros and co-workers. [4] Frist, a stock solution was prepared from bisphenol A (400 mg), Cr(acac) 3 (5.5 mg) and pyridine (10 mL) and stored over molecular sieves. The polymer was dissolved in CDCl 3 (50 mg/0.5 mL) and 40 μL of this stock solution were added before the phosphorus agent (30 μL) was added. The mixture was allowed to react for at least 30 minutes and then analysed by 31 P { 1 H} NMR spectroscopy after calibration with the internal standard (bisphenol A, 138.57 ppm).

Synthesis of ABA Triblock Polyesters
Inside the glovebox, [Salphen F AlCl] (1 equiv., 10 mg, 16.5 μmol), PPNCl (0.9 equiv., 8 mg, 14.8 μmol), CHD (10 equiv., 19 mg, 165 μmol), PA (244 mg, 100 equiv., 1.65 mmol) and LA (237 mg, 100 equiv., 1.65 mmol) were weighed in a pre-dried vial and 1.05 mL of PO were added. The resulting suspension was sealed with PVC tape and stirred in a preheated aluminium block at 60°C for the specified time. Once full conversions were detected by 1 H NMR spectroscopy (taken inside the glovebox), the reaction was quenched by exposure to air, excess PO was evaporated and the obtained solid was analysed by GPC. The polyester was further purified by precipitation in methanol.

Synthesis of Multiblock Polyesters
The general procedure outlined above was followed after mesitylene (10 equiv.) were added to reaction mixture. After each monomer addition, the reaction was left to react for at least 20 hours before aliquots were taken inside the glovebox and conversions were analysed by 1 H NMR spectroscopy and GPC. It should be noted that the solution became highly viscous, particularly at high block numbers, and additional PO (1 mL) was added, the reaction was transferred to a larger vial and a rare-earth stirring bar was used. The final polymer was purified as described above.

Illustration of Possible Polymeric Products From Mixed Feedstocks Containing Anhydrides, Epoxides and Lactones
Scheme S. 1 -Possible outcomes for a Polymerisation of a mixed feedstock containing three monomers. Scheme S. 2 -Possible materials in 'switch' catalysis and recommendations of how to distinguish between them. [5]

1 H NMR Spectra of Multiblock Copolyesters
Figure S. 9 -1 H NMR spectra of crude reaction mixture after each monomer addition; typical reaction times: 6 hours -2 days. Conversions are based on mesitylene (6.80 ppm) as internal standard.