All-Polycarbonate Thermoplastic Elastomers Based on Triblock Copolymers Derived from Triethylborane-Mediated Sequential Copolymerization of CO2 with Various Epoxides

Various oxirane monomers including alkyl ether or allyl-substituted ones such as 1-butene oxide, 1-hexene oxide, 1-octene oxide, butyl glycidyl ether, allyl glycidyl ether, and 2-ethylhexyl glycidyl ether were anionically copolymerized with CO2 into polycarbonates using onium salts as initiator in the presence of triethylborane. All copolymerizations exhibited a “living” character, and the monomer consumption was monitored by in situ Fourier-transform infrared spectroscopy. The various polycarbonate samples obtained were characterized by 1H NMR, GPC, and differential scanning calorimetry. In a second step, all-polycarbonate triblock copolymers demonstrating elastomeric behavior were obtained in one pot by sequential copolymerization of CO2 with two different epoxides, using a difunctional initiator. 1-Octene oxide was first copolymerized with CO2 to form the central soft poly(octene carbonate) block which was flanked by two external rigid poly(cyclohexene carbonate) blocks obtained through subsequent copolymerization of cyclohexene oxide with CO2. Upon varying the ratio of 1-octene oxide to cyclohexene oxide and their respective ratios to the initiator, three all-polycarbonate triblock samples were prepared with molar masses of about 350 kg/mol and 22, 26, and 29 mol % hard block content, respectively. The resulting triblock copolymers were analyzed using 1H NMR, GPC, thermogravimetric analysis, differential scanning calorimetry, and atomic force microscopy. All three samples demonstrated typical elastomeric behavior characterized by a high elongation at break and ultimate tensile strength in the same range as those of other natural and synthetic rubbers, in particular those used in applications such as tissue engineering.

) at a heating rate of 10 o C/min. Figure S15. DSC curves of the triblock copolymers in high temperature range. Figure S16. Phase angle and tanδ of the triblock copolymers as a function of temperature.

Materials
All the reagents were purchased from Sigma-Aldrich and used as received unless otherwise stated.
BO, HO, OO, BGE, EHGE, AGE and tetrahydrofuran (THF) were purified by distilling firstly over CaH 2 and then over n-butyl lithium for four times using standard Schlenk technique. The nbutyl lithium reacts with both protic impurities such as water and a small quantity of epoxides. In this way, ultra-dry monomers and solvent free of protic impurities can be obtained. The purified monomer and solvent were stored in Schlenk flasks and kept in a glovebox.
Bis(triphenylphosphine)iminium chloride (PPNCl) was purified by dissolving in ethanol and precipitating from diethyl ether three times and then drying under vacuum to remove the solvents.
Tetrabutylammonium chloride (TBACl) was recrystallized from cold n-hexane and dried in a vacuum oven. 1, 4-Di(hydroxymethyl)benzene (DHMB) was purified through sublimation. The traces of water in the initiators were removed by drying the initiators under vacuum in the presence of P 2 O 5 for 2 days. CO 2 was provided by Air Liquid Al Khafran Ind Gases with nominal purity of 99.995%. Super dry CO 2 was obtained by passing it though triisobutylaluminum (TiBA) as reported in our previous work. 1 Instrumentation 1 H NMR spectra were recorded on a Bruker AVANCE III-400 Hz instrument in CDCl 3 . GPC traces were acquired on a VISCOTEK VE2001 system equipped with the Styragel HR2 THF and Styragel HR4 THF using THF (1mL/min) as the eluent. The relative molar masses and distributions were obtained at 35 o C using a RID detector and against linear polystyrene standards.
In situ Fourier-transform infrared spectroscopy (FTIR) study of the TEB-mediated S5 copolymerization of epoxides with CO 2 was conducted using a ReactIR 700 system (Mettler Toledo). The FTIR probe was inserted into the reaction medium through a threaded hole in the reactor lid where air tightness can be guaranteed. The reactor was flame-dried under vacuum before the reagents were added. The monomer was first introduced into the reactor and after equilibration at 60 o C for 20 min. The initiator solution of TEB was then added. Immediately after, the CO 2 was charged to 20 bar. The initial concentration for the epoxide was fixed to be 5 mol/L.
The background was collected at the beginning of the reaction. The FTIR spectra were collected at initial time and at an interval of 3 min; for each spectra 128 scans were conducted. The FTIR data were processed and analyzed using the iC IR 7.1 software. Differential scanning calorimetry (DSC) measurements were performed at a heating rate of 10 o C/min or 20 o C/min on a Mettler Toledo DSC1/TC100 system under nitrogen atmosphere. The curve of the second heating scan was adopted to determine the glass transition temperature (T g ). Thermogravimetric analysis (TGA) experiments were performed on a TGA Q500 analyzer (TA Instruments). Samples were heated from 25 o C to 650 o C at a heating rate of 10 o C/min under N 2 atmosphere. Atomic force microscopy (AFM) was conducted on a Bruker Dimension Icon SPM scanner in a tapping mode. The Bruker RTESPA-300 AFM probe was employed in the study. Thin films were spin-coated on silicon wafers coated with SiO 2 from the THF solutions of the triblock copolymers (2 wt%). The samples were left to dry at room temperature before analysis. The images obtained were processed using the Gwyddion software. The viscoelastic behaviors of the triblock copolymers were determined on a TA Instruments Discovery HR 2 rheometer with parallel plate geometry (diameter of 25 mm, gap of 1 mm). The triblock copolymers were processed at 140 o C in a DSM Xplore twin-screw micro-extruder with a barrel size of 5 mL. Tensile bars with a dimension of 2 mm × 4 mm × 70 mm were produced in a DSM Xplore IM 5.5 micro-injection molder. The barrel temperature was S6 set at 140 o C while the mold temperature was 25 °C. Tensile tests were performed using a Zwick Z100 universal testing machine on dog-bone shape tensile bars. The samples were stretched at a rate of 50 mm/min at room temperature and a 10 kN load cell was employed.

Copolymerization of CO 2 with Epoxides
In a glovebox, to a pre-dried Parr autoclave were added the initiator (PPNCl, TBACl or DHMB), THF and Et 3 B (1M in THF). In the case of DHMB used as precursor to the initiator, equal equivalents of phosphazene base P 4 -t-Bu (0.8M in hexane) to that of DHMB was added to deprotonate the hydroxyl groups. Then the autoclave was charged with CO 2 to reach a pressure of 10 bar. The epoxide was added to the initiating system either before or after CO 2 charging. The epoxides were added after CO 2 charging when polyethers can form before CO 2 was introduced in the reactor. The procedure of adding epoxide after CO 2 charging is as follows: the reactor was placed in liquid nitrogen; then the epoxide was injected into the reactor through a septum after the pressure inside the reactor dropped to ambient pressure. Another option was to use a separate vial containing the epoxides and placed in the reactor. The vial was broken to free the epoxide after CO 2 charging. Afterwards, the reactor was placed in an oil bath with the temperature set at 60 o C.
After stirring at a rate of 200 rpm for 12 hours, the reaction system was cooled down with an ice/water bath. After residual CO 2 was released, the reaction system was quenched with HCl solution (1M in THF). An aliquot of the crude product was taken for 1 H NMR analysis to determine the selectivity and conversion. The resulting polycarbonate was purified by repeated precipitation from methanol. Then the yield was obtained by weighing the polycarbonate dried under vacuum overnight. The polycarbonate content was determined from 1 H NMR spectra of pure polycarbonates.

S7
The synthesis of the middle block of the triblock copolymer followed the procedure described above. OO was selected as the epoxide monomer and the difunctional initiator DHMB was employed together with equal equivalents of P 4 -t-Bu. After 60 hours, the reactor was placed in liquid nitrogen. CHO was injected into the reaction medium after the pressure inside the reactor dropped to ambient pressure. The reaction medium was stirred for another 12 hours at 60 o C. Then the reactor was cooled with an ice/water bath. After residual CO 2 was released, the reaction system was quenched with HCl solution (1M in THF). An aliquot of the crude product was taken for 1 H NMR analysis to determine the selectivity. The resulting polycarbonate was purified by repeated precipitation from methanol. Then the triblock copolymers were dried under vacuum overnight resulting in an overall yield above 85%. The compositions of the triblock copolymers were determined from the 1 H NMR spectra of the pure products.

S16
We assumed that the soft block and the hard block have the same density and thus the weight fraction of the hard block equals to the volume fraction. We also ignored the dependence of density on the temperature. S17