Lignin-Inspired Polymers with High Glass Transition Temperature and Solvent Resistance from 4-Hydroxybenzonitrile, Vanillonitrile, and Syringonitrile Methacrylates

We here report on the synthesis and polymerization of nitrile-containing methacrylate monomers, prepared via straightforward nitrilation of the corresponding lignin-inspired aldehyde. The polymethacrylates reached exceptionally high glass transition temperatures (Tg values), i.e., 150, 164, and 238 °C for the 4-hydroxybenzonitrile, vanillonitrile, and syringonitrile derivatives, respectively, and were thermally stable up to above 300 °C. Copolymerizations of the nitrile monomers with styrene and methyl methacrylate, respectively, gave potentially melt processable materials with tunable Tg values and enhanced solvent resistance. The use of lignin-derived nitrile-containing monomers represents an efficient strategy toward well-defined biobased high Tg polymer materials.


Experimental section Materials
All reagents and solvents were obtained from commercial sources and used without further purification. The monomer synthesis steps were monitored by thin-layer chromatography (TLC, silica gel 60 F254), and TLC plates were visualized using a UV lamp. Aluminum oxide (active basic, particle size 0.063-0.200 mm (70-230 mesh ASTM)) was used in the flash chromatography.

Analytical Methods
Nuclear Magnetic Resonance (NMR) spectroscopy was performed on a Bruker DR X400 spectrometer at 400.13 MHz proton frequency and 100.62 MHz carbon frequency. The formation of the polymers was determined by the decrease of the double bond signals of the methacrylate moiety. The molecular weight of the polymers was determined by Size Exclusion Chromatography (SEC) using THF or DMF/0.05 M LiBr as eluent at 40 °C. The instrument included Shodex columns coupled in series (KF-805, -804 and -802.5 for THF and KD-804 and -802.5 for DMF) placed in a Shimadzu CTO-20A prominence column oven, a Shimadzu RID-20A refractive index detector, with Shimadzu LabSolution software. Calibration was done by using poly(ethylene oxide) standards (Mn = 3.86, 21.16, 49.39 and 96.1 kg/mol). All samples were run at an elution rate of 1 mL/min.

Thermogravimetry and calorimetry
Thermogravimetric analysis (TGA) was performed on a TA Instruments TGA Q500. Samples from 1-6 mg were heated to 600 °C at a rate of 10 °C/min under nitrogen flux (60 mL/min). The thermal decomposition temperature (Td,95%) was determined at 5% loss of the total weight. Differential scanning calorimetry (DSC) measurements were performed on TA Instruments DSC Q2000. Samples from 1-10 mg were first heated up to 170-250 °C (depending on their respective thermal decomposition temperature) at a rate of 10 °C/min. They were kept at this temperature for 2 min, then cooled down to -50 °C and kept isothermal for 2 min, before being heated up again to the previous temperature, at the same heating rate.

Melt rheology
Dynamic rheology measurements were performed with TA Instruments Advanced Rheometer AR2000 ETC. The experiments were made using parallel plates (∅ = 15 mm). Discs of PSVM-14, PSVM-28, PSSM-25 and PMVM-18 (∅ = 15 mm, t = 1 mm) were hot-pressed between two aluminum plates using a hydraulic press (Specac, GS15011) at 150 °C during 5 min, and finally cooled to room temperature. A time sweep was carried out during 8 h at 150 °C, at a frequency of 1 Hz at 0.1% strain, which was within the linear viscoelastic region.

4-Cyano-2,6-dimethoxyphenyl methacrylate (syringonitrile methacrylate) SM 3c
To a 100 mL round-bottom flask equipped with a magnetic stirrer, 4-hydroxy-3,5-dimethoxybenzonitrile 2c (2.862 g, 15.97 mmol, 1.0 equiv.) and DMAP (39.4 mg, 0.323 mmol, 0.02 equiv.) were dissolved in EtOAc (26 mL). The flask was sealed and purged with vacuum/N2 cycles. Methacrylic anhydride (2.487 g, 16.13 mmol, 1.01 equiv.) was added dropwise to the reaction mixture. The flask was purged again as described above, and the reaction mixture was then heated at 65 °C for 24 h. The precipitate was filtered and washed with cold EtOAc (25 mL). SM was further purified by aluminum oxide column chromatography, with heptane as eluent, to remove residual acid and phenol. The product was dried carefully under vacuum to afford 2.915 g of a white powder (yield: 74%

Poly(4-cyanophenyl methacrylate) [poly(benzonitrile methacrylate)] PBM
In a schlenk flask, 10 mL of acetonitrile were degassed with vacuum/N2 cycles at 40 °C. To a 25 mL schlenk flask equipped with a magnetic stirrer, 4-cyanophenyl methacrylate BM 3a (0.650 g, 3.475 mmol, 1.0 equiv.) were dissolved in degassed acetone (4.00 mL) and degassed again as before. In a vial, azobisisobutyronitrile (AIBN) (3.0 mg) were dissolved in degassed acetonitrile (2 mL). 1 mL of the AIBN solution is introduced in the flask with the dissolved monomer, and degassed again as before. The reaction mixture was then heated at 60 °C for 24 h. The resulting material was precipitated in MeOH, and the white powder was filtered off and washed repeatedly with MeOH before drying at 50 °C under vacuum, yielding 0.316 g (49%).

Poly(4-cyano-2-methoxyphenyl methacrylate) [poly(vanillonitrile methacrylate)] PVM
In a schlenk flask, 6 mL of DMSO were degassed with vacuum/N2 cycles at 40 °C. To a 25 mL schlenk flask equipped with a magnetic stirrer, 4-cyano-2-methoxyphenyl methacrylate VM 3b (0.500 g, 2.302 mmol, 1.0 equiv.) were dissolved in degassed DMSO (2.00 mL) and degassed again as before. In a vial, azobisisobutyronitrile (AIBN) (3.22 mg) were dissolved in degassed DMSO (2 mL). 1.00 mL of the AIBN solution were introduced in the flask with the dissolved monomer, and degassed again as before. The reaction mixture was then heated at 60 °C for 24 h. The resulting material was precipitated in MeOH, and the white powder was filtered off and washed repeatedly with MeOH before drying at 50 °C under vacuum, yielding 0.360 g (72%).

Poly[styrene-co-(4-cyano-2-methoxyphenyl methacrylate)] PSVM-14
In a schlenk flask, 10 mL of DMSO were degassed with vacuum/N2 cycles at 40 °C. To a 25 mL schlenk flask equipped with a magnetic stirrer, 4-cyano-2-methoxyphenyl methacrylate VM 3b (0.094 g, 0.433 mmol) and styrene (0.406 g, 3.897 mmol) were dissolved in degassed DMSO (4.56 mL) and degassed again as before. In a vial, azobisisobutyronitrile (AIBN) (20 mg) were dissolved in degassed DMSO (2 mL). 1.42 mL of the AIBN solution were introduced in the flask with the dissolved monomers, and degassed again as before. The reaction mixture was then heated at 60 °C for 24 h. The resulting material was precipitated in MeOH. The formed white powder was filtered off and washed repeatedly with MeOH before drying at 50 °C under vacuum, yielding 0.337 g (67%).

Poly[styrene-co-(4-cyano-2-methoxyphenyl methacrylate)] PSVM-28
In a schlenk flask, 10 mL of DMSO were degassed with vacuum/N2 cycles at 40 °C. To a 25 mL schlenk flask equipped with a magnetic stirrer, 4-cyano-2-methoxyphenyl methacrylate VM 3b (0.171 g, 0.789 mmol) and styrene (0.329 g, 3.155 mmol) were dissolved in degassed DMSO (4.20 mL) and degassed again as before. In a vial, azobisisobutyronitrile (AIBN) (20 mg) were dissolved in degassed DMSO (2 mL). 1.29 mL of the AIBN solution were introduced in the flask with the dissolved monomers, and degassed again as before. The reaction mixture was then heated at 60 °C for 24 h. The resulting material was precipitated in MeOH, and the white powder was filtered off and washed repeatedly with MeOH before drying at 50 °C under vacuum, yielding 0.361 g (72%).

Poly[styrene-co-(4-cyano-2-methoxyphenyl methacrylate)] PSVM-45
In a schlenk flask, 10 mL of DMSO were degassed with vacuum/N2 cycles at 40 °C. To a 25 mL schlenk flask equipped with a magnetic stirrer, 4-cyano-2-methoxyphenyl methacrylate VM 3b (0.338 g, 1.556 mmol) and styrene (0.162 g, 1.556 mmol) were dissolved in degassed DMSO (3.42 mL) and degassed again as before. In a vial, azobisisobutyronitrile (AIBN) (20 mg) were dissolved in degassed DMSO (2 mL). 1.02 mL of the AIBN solution were introduced in the flask with the dissolved monomers, and degassed again as before. The reaction mixture was then heated at 60 °C for 24 h. The resulting material was precipitated in MeOH, and the white powder was filtered off and washed repeatedly with MeOH before drying at 50 °C under vacuum, yielding 0.487 g (97%).

Poly(4-cyano-2,6-dimethoxyphenyl methacrylate) [poly(syringonitrile methacrylate)] PSM
In a schlenk flask, 6 mL of DMSO were degassed with vacuum/N2 cycles at 40 °C. To a 25 mL schlenk flask equipped with a magnetic stirrer, 4-cyano-2,6-dimethoxyphenyl methacrylate SM 3c (0.500 g, 2.022 mmol, 1.0 equiv.) were dissolved in degassed DMSO (2.56 mL) and degassed again as before. In a vial, azobisisobutyronitrile (AIBN) (30 mg) were dissolved in degassed DMSO (2 mL). 0.44 mL of the AIBN solution were introduced in the flask with the dissolved monomer, and degassed again as before. The reaction mixture was then heated at 60 °C for 24 h. The resulting material was precipitated in MeOH, and the white powder was filtered off and washed repeatedly with MeOH before drying at 50 °C under vacuum, yielding 0.463 g (93%).

S18
SEC Curves (continued on next page). S19 Figure S10. SEC of THF solutions of: (a) homopolymer PVM and PSM, (b) the PSSM series, (c) the PSVM series and (d) the PMVM series. "Intensity" is the differential refractive index (dRI) of the RI detector in arbitrary units. SEC in DMF of (a') homopolymers PBM, PVM and PSM.

Evaluation of the Solubility of Polybenzonitrile methacrylates
The solubility of the different polybenzonitrile methacrylates was investigated by mixing small samples (about 5 mg) with a range of selected solvents (1 mL). The mixture was stirred for 24 h at room temperature. The results of the dissolution tests were divided into two categories, soluble and insoluble, based on visual inspection. If the samples were found to be completely dissolved, they were considered as soluble; if not, they were considered as nonsoluble. Table S1. Solubility of the nitrile-containing polymers at 21 °C. a The symbols "+" and "-" indicate solubility and nonsolubility, respectively. Solubility parameters (δ, MPa 1/2 ) were obtained from the Polymer Handbook, (J. Brandrup, E. H. Immergut, E. A. Grulke, A. Abe, D. Bloch. Polymer Handbook, 4 th ed., John Wiley and Sons, New York, 1999), and the letters s, m and p denote strongly, moderately, and poorly hydrogen-bond-forming solvents, respectively.