Independent Control of Elastomer Properties through Stereocontrolled Synthesis

Abstract In most synthetic elastomers, changing the physical properties by monomer choice also results in a change to the crystallinity of the material, which manifests through alteration of its mechanical performance. Using organocatalyzed stereospecific additions of thiols to activated alkynes, high‐molar‐mass elastomers were isolated via step‐growth polymerization. The resulting controllable double‐bond stereochemistry defines the crystallinity and the concomitant mechanical properties as well as enabling the synthesis of materials that retain their excellent mechanical properties through changing monomer composition. Using this approach to elastomer synthesis, further end group modification and toughening through vulcanization strategies are also possible. The organocatalytic control of stereochemistry opens the realm to a new and easily scalable class of elastomers that will have unique chemical handles for functionalization and post synthetic processing.


Synthetic Procedures
Propane-1,3-diyl dipropiolate, 1. 1,3-Propanediol (20 g, 0.263 mol) was added to a 1 L single neck round bottom flask. To this was added toluene (100 mL) and benzene (100 mL). Two drops of H2SO4 were added and the solution was allowed to stir at room temperature for 5 min before adding propiolic acid (50 g, 0.714 mol). A Dean-Stark apparatus with condenser was fitted and the reaction was then refluxed for 16 h at 120 °C or until the required amount of water was collected. The solution was then cooled to room temperature and solvent extracted with saturated NaHCO3 solution (2 × 200 mL) to remove any residual acids. The organic phase was then collected, dried over MgSO4, filtered and reduced in volume to dryness. The product was then purified on silica gel isocratically using 4:1 hexane/EtOAc and collecting the 1 st fraction. After removal of the solvent, the final product was further purified by distillation under high vacuum at 160 °C to yield colourless oil that slightly crystallised on sitting (24.63 g, 52% yield). Rf (
Variation of molecular weight. The molecular weight of the thiol-yne step growth polymers was varied by changing the amount of dithiol in relation to the dialkyne such that the dialkyne was always in excess. Monomer ratios were determined using the extended Carothers equation for one monomer in excess (assuming p → 100%). 1 Variation of % cis. The % cis in each thiol-yne step growth polymer was tuned based on solvent polarity and base. Truong and Dove 2 have shown that low and high % cis can be achieved by changing the base from Et3N to DBU while maintaining the solvent (CDCl3). However, moderately high % cis can be achieved with Et3N when a more polar solvent such as DMSO is used. All high % cis polymers were formed using DBU/CHCl3 but lower % cis contents were formed by using Et3N and varying compositions of CHCl3 and DMF (17:3, 7:3, and 100% DMF). The double bond stereochemistry was determined by integration of the resonances in the 1H NMR spectrum that are attributed to cis and trans double bond isomers respectively. These signals typically appeared at ca. δ = 5.9 and 7.1 ppm for the cis isomer and δ = 5.8 and 7.6 ppm for the trans isomer (in CDCl3). Critically, they can be distinguished from their different coupling constants (cis: 3 JH-H = 10.2 Hz, trans: 3 JH-H = 15.2 Hz).
General Method for Radical Curing. 2 g of C3A-C6S polymer was dissolved in CH2Cl2 (20 mL) and to this was added 1 wt% of dicumyl peroxide (20 mg). The solution was stirred overnight then reduced to dryness in vacuo. The polymer was placed into a mould and cured at 160 °C for 15 min before being pressed into films.

Size-exclusion Chromatography (SEC)
SEC was used to determine the molar masses and molar mass distributions (dispersities, ÐM) of the synthesised polymers. SEC analyses were performed on a system composed of an Agilent 390-LC-Multi detector using an Agilent Polymer Laboratories guard column (PLGel 5 μM, 50 × 7.5 mm), two mixed D Varian Polymer Laboratories columns (PLGel 5μM, 300 × 7.5 mm) and a PLAST RT autosampler. Detection was conducted using a differential refractive index (RI) and an ultraviolet (UV) detector set to 280 nm. The analyses were performed in CHCl3 at 40 °C and containing 0.5% w/w Et3N at a flow rate of 1.0 mL/min. Polystyrene (PS) (162 -2.4 × 105 g.mol -1 ) standards were used to calibrate the system. Polymers containing 2,2′-(ethylenedioxy)diethanethiol and 1,4-dithio-d-threitol were analysed on a system composed of an Agilent 390-LC-Multi detector using an Agilent Polymer Laboratories guard column (PLGel 5 μM, 50 × 7.5 mm), two mixed C Varian Polymer Laboratories columns (PLGel 5μM, 300 × 7.5 mm) and a PLAST RT autosampler. Detection was conducted using a differential refractive index (RI). The analyses were performed in DMF at 50 °C and containing 5 mM NH4BF4 at a flow rate of 1.0 mL/min. Poly(methyl methacrylate) (PMMA) (550 -2.136 × 10 6 g.mol -1 ) standards were used to calibrate the system. Molecular weights and dispersities were determined using Cirrus v2.2 SEC software. Absolute MW size exclusion chromatography (SEC) analysis was performed on an Agilent 390-LC MDS instrument equipped with differential refractive index (DRI), viscometry (VS), dual angle light scatter (LS) and two wavelength UV detectors. The system was equipped with 2 × PLgel Mixed D columns (300 × 7.5 mm) and a PLgel 5 µm guard column. The eluent used was CHCl3 with 2% Et3N. Samples were run at 1 mL.min -1 at 30 °C. Poly(methyl methacrylate), and polystyrene standards (Agilent EasyVials) were used for calibration and ethanol was added as a flow rate marker. Analyte samples were filtered through a PTFE membrane with 0.22 μm pore size before injection.

Thermal Property Measurements
The thermal characteristics of the polymers were determined using differential scanning calorimetry (DSC, TA Q200) from -50 °C to 150 °C at a scanning rate of 10 °C/min. The glass transition temperature (Tg) was determined from the midpoint in the second heating cycle of DSC.

Mechanical Property Measurements Tensile Tests at Different Strain Rates
Thin films of each polymer were fabricated using a vacuum compression machine (TMP Technical Machine Products Corp.). The machine was preheated to 160 °C. Then polymer was added into the 50 × 50 × 0.5 mm mould and put into the compression machine with vacuum on. After 15 minutes of melting, the system was degassed three times. Next, 10 lbs*1000, 15 lbs*1000, 20 lbs*1000, 25 lbs*1000 of pressure were applied for 2 minutes respectively. After that, the mould was cooled down with 1000 psi of pressure to prevent the wrinkle on the film's surface. The films were visually inspected to ensure that no bubbles were present in the films. Dumbbell-shaped samples were cut using a custom ASTM Die D-638 Type V.
Tensile tests at different stretching speed were carried out using Instron (Instron 5543 Universal Testing Machine) at room temperature (25 ± 1 °C). The gauge length was set as 7 mm and the crosshead speed was set as various values (listed in Table S1). The dimensions of the neck of the specimens were 7.11 mm in length, 1.70 mm in width and 0.50 mm in thickness.
Tensile Tests at 20 mm/min Dumbbell-shaped samples were prepared using the same method as stated previously. Tensile tests were carried out using Instron (Instron 5543 Universal Testing Machine) at room temperature (25 ± 1 °C). The gauge length was set as 7 mm and the crosshead speed was set as 20 mm/min. The dimensions of the neck of the specimens were 7.11 mm in length, 1.70 mm in width and 0.50 mm in thickness. The elastic moduli were calculated using the slope of linear fitting of the data from strain of 0% to 0.1%. The reported results are average values from three individual measurements.

Wide Angle X-ray Diffraction Measurement (WAXD).
The WAXD experiments were conducted using a Rigaku Ultima IV X-ray Diffractometer at room temperature. The generator was operated at 40 kV and 40 mA with a beam monochromatized to Cu Kα radiation. Samples were scanned at 1.0 degree/min continuously over a scan range of 5.0 -70.0 degree. The air scattering was subtracted.

Small Angle X-ray Scattering Measurement (SAXS).
The SAXS experiments were performed on a Rigaku MicroMax 002+ instrument equipped with a two-dimensional multiwire area detector and a microfocus sealed copper tube. The working voltage and current used were 45 kV and 0.88 mA, with the wavelength of the X-ray is 0.154 nm. The scattering vector (q) was calibrated using standard of silver behenate with the primary reflection peak at (q = 1.067 nm -1 ). The recording time for each sample was 10−20 min, depending on the scattering intensity. The data analysis was done with SAXSgui software.

Network Mechanical Properties
According to Boltzmann superposition principle, 3 we can describe stress evolution in polymeric networks undergoing uniaxial elongation at a constant strain rate ̇ as where E(t) is time dependent network Young's modulus. In entangled amorphous polymeric networks, dynamics of entangled strands forming a network on the time scales t smaller than the strands' Rouse time, = 0 2 (Ne -degree of polymerization of network strands between entanglements and 0 -characteristic monomer time), the dynamics of network strands is not influenced by entanglements or crosslinks and network modulus decays with time as, ( ) ∝ ( 0 ) 1/2 , where  is a monomer number density, k is the Boltzmann constant and T is the absolute temperature. However, at the time scales t larger than the Rouse time of the entangled polymeric strands the network response is pure elastic with Young's modulus ∝ / . We will use the following approximation for time dependent modulus to describe network relaxation: For the stress relaxation modulus given by eq S2 integration of eq S1 results in where we introduced Em=3 kT/v with monomer volume v. For experiments at constant strain rate, we can substitute = /̇ into eq S3 to express t as a function of strain as In the opposite limit, t> , Analysis of the eq S5 shows that in this network deformation regime we can collapse data sets obtained at different strain rates by plotting ( ) vs /. The slope will provide value of the network Young's modulus at small deformations, 0 = / . Note, that in the case of composite materials with a spectrum of relaxation times, eq S5 is valid on the time scales longer than the longest relaxation time of stress relaxation in a system.
Network tensile toughness reported in Table I was calculated from the area under the stress-strain curve where break is the strain upon failure.

Surface Energy Measurement.
The surface energies of the polymers were estimated from contact angle measurements. By measuring contact angle with four different probe liquids (propylene glycol, ethylene glycol, glycerol and water) using an Advanced Goniometer (Rame-Hart Instrument Ćo., Model 500) at 25 °C and fitting the data using the Owen's equation, the dispersion and polar components of the polymer surface energy before stretching and after stretching were obtained from the slope and the intercept (Table S6). [4][5][6] At least five measurements were performed on different spots and averaged.
Owen's equation:         Table S2.  Table S2.  Table S2.  Table S3.  Table S3.  Table S3.  Table S3. , where is density (assume 1.0 g/cm 3 for all these elastomers) , R is the gas constant and T is the absolute temperature. Contour length of the chain can be determined as , where M0 is monomer molecular weight (330.10 g/mol) and l0 is the length of monomer in cis/trans configuration (shown in Figure 1 and results listed in Table 1). l0 is calculated as the end-to-end distance by √(∑ ) 2 + (∑ sin ) 2 , where r is the bond length and is the angle between x axis and the bond.
The maximum extension ratio max can be estimated (Table S3)   3.0 ± 0.1 -16.0 ---Mechanical data were determined by tensile testing and are presented as a mean ± s.d. of three independent measurements. Thermal data is reported from DSC 2 nd run. Tg, Tm and ΔHm are obtained from heating curves and Tc from cooling curves. Mw determined by SEC analysis, %cis content determined by 1 H NMR analysis.