β-Myrcene/isobornyl methacrylate SG1 nitroxide-mediated controlled radical polymerization: synthesis and characterization of gradient, diblock and triblock copolymers

Abstract

β-Myrcene (My), a natural 1,3-diene, and isobornyl methacrylate (IBOMA), from partially bio-based raw materials sources, were copolymerized by nitroxide-mediated polymerization (NMP) in bulk using the SG1-based BlocBuilder™ alkoxyamine functionalized with an N-succinimidyl ester group, NHS-BlocBuilder, at T = 100 °C with initial IBOMA molar feed compositions f _IBOMA,0 = 0.10-0.90. Copolymer reactivity ratios were r _My = 1.90-2.16 and r _IBOMA = 0.02-0.07 using Fineman-Ross, Kelen-Tudos and non-linear least-squares fitting to the Mayo-Lewis terminal model and indicated the possibility of gradient My/IBOMA copolymers. A linear increase in molecular weight versus conversion and a low dispersity (Đ ≤ 1.41) were exhibited by My/IBOMA copolymerization with f _IBOMA,0 ≤ 0.80. My-rich and IBOMA-rich copolymers were shown to have a high degree of chain-end fidelity by performing subsequent chain-extensions with IBOMA and/or My, and by ³¹P NMR analysis. The preparation by NMP of My/IBOMA thermoplastic elastomers (TPEs), mostly bio-sourced, was then attempted. IBOMA-My-IBOMA triblock copolymers containing a minor fraction of My or styrene (S) units in the outer hard segments (M _n = 51-95 kg mol^-1, Đ = 1.91-2.23 and F _IBOMA = 0.28-0.36) were synthesized using SG1-terminated poly(ethylene-stat-butylene) dialkoxyamine. The micro-phase separation was suggested by the detection of two distinct T _gs at about -60 °C and +180 °C and confirmed by atomic force microscopy (AFM). A plastic stress-strain behavior (stress at break σ _B = 3.90 ± 0.22 MPa, elongation at break ε _B = 490 ± 31%) associated to an upper service temperature of about 140 °C were also highlighted for these triblock polymers.

Scheme 1. (a) My/IBOMA gradient copolymerization in bulk initiated by NHS-BB and subsequent My or IBOMA/My (∼93/7 mol%) chain-extension in toluene and (b) synthesis of P(My)-(SG1)₂ from PEB-(SG1)₂ difunctional initiator (n ≫ p, q) and subsequent IBOMA/Co chain-extension in toluene (Co = My or S co-monomer, 8–9 mol%).

Fig. 1. (a) Fineman–Ross (FR) and (b) Kelen–Tudos (KT) plots (solid black circles (●) corresponding to experimental outliers not taken into account for the calculations while solid lines refer to the linear trend lines) to determine the binary reactivity ratios for My and IBOMA for copolymerizations done in bulk at 100 °C initiated by NHS-BB (parameters G, H, η and ε defined in the ESI, page S4†). (c) Mayo–Lewis plot of My/IBOMA copolymerizations with respect to IBOMA, using the final molar composition F_IBOMA, and the initial monomer feed composition f_IBOMA,0. The solid straight line indicates the azeotropic composition where f_IBOMA,0 = F_IBOMA while the dashed line is the associated trend line of the experimental data (solid blue circles ()). Table S1 in the ESI lists the samples used for these plots. (d) Individual My () and IBOMA (○) conversions, determined by ¹H NMR in CDCl₃, versus reaction time t for the gradient copolymerization My/IBOMA-50 exhibiting f_My,0 = 0.50.

Fig. 2. (a) Semi-logarithmic kinetic plots of ln((1 − X)⁻¹) (X = overall conversion) versus polymerization time t, (b) Đ versus overall conversion X and (c) M_n determined by GPC relative to PMMA standards in THF at 40 °C, and corrected using the Mark–Houwink relationship, versus overall conversion X for the various My/IBOMA copolymerizations in bulk at 100 °C initiated by NHS-BB. The dashed line indicates the theoretical M_nversus overall conversion based on the monomer to initiator ratio (M_n,theo ∼ 30 kg mol⁻¹ at X = 100% for every experiment). All experimental ID and characterization of experiments are listed in Tables 1 and 2. The same legend at the bottom right of the figure is used for each of the three plots.

Fig. 3. Normalized GPC traces of P(My-grad-IBOMA) with f_My,0 = 0.50, initiated by NHS-BB at 100 °C in bulk targeting M_n,theo = 30 kg mol⁻¹ at X = 100% (experiment My/IBOMA-50).

Fig. 4. F _IBOMA effects on T_g in P(My-grad-IBOMA) gradient copolymers. The DSC traces can be found in the ESI, Fig. S7.T_g = −77.0 °C for F_IBOMA = 0 was determined previously. The black dotted line represents the experimental data fitted to the Gordon–Taylor equation.

Fig. 5. TGA traces (N₂ atmosphere, 10 °C min⁻¹) of final gradient copolymers (a) My/IBOMA-20 and (b) My/IBOMA-80, previously precipitated in excess methanol and dried under vacuum at 50 °C. Sample weight versus temperature is represented by the solid blue line whereas the dotted line represents the derivative of weight relative to the temperature versus temperature in order to determine precisely the temperature at which weight loss is most apparent (T_dec,max).

Fig. 6. Normalized GPC traces for the chain-extensions of (a) My/IBOMA-82 with a IBOMA/My (93/7 mol%) mixture (experiment My/IBOMA-82-IBOMA/My) and (b) My/IBOMA-44 with My (experiment My/IBOMA-44-My) at T = 105–115 °C in 50 wt% toluene. Schematic representations of the final chain-extended copolymers, where solid red and blue circles represent My and IBOMA units respectively, are given below the GPC chromatograms.

Fig. 7. (a) DSC traces (second heating run) of the triblock copolymers My-52-IBOMA/S (blue) and My-35-IBOMA/My (red). The numbers near the changes in slope correspond to the T_gs determined via the inflection method. (b) Dynamic mechanical analysis of the sample My-52-IBOMA/S by torsional oscillation, yielding the storage modulus, the loss modulus and the damping factor versus temperature (0.15 Hz, 1% strain, 5 °C min⁻¹, N₂ atmosphere).

Fig. 8. Atomic force microscopy (AFM, Experimental section) phase image (2 μm × 2 μm) under tapping mode of operation of the surface morphology of the triblock copolymer My-35-IBOMA/My cast film. The dark domain represents the My component (color-coded height scale given to the right of the image).

Fig. 9. Tensile stress–strain curves of five My-52-IBOMA/S samples (same batch, color only used for differentiation) at room temperature and at a cross-head speed of 10 mm min⁻¹. The average Young's modulus E, yield stress σ_Y, tensile strength at break σ_B and tensile elongation at break ε_B, obtained from these curves are also given.