From Tree to Tape: Direct Synthesis of Pressure Sensitive Adhesives from Depolymerized Raw Lignocellulosic Biomass

We report a new and robust strategy toward the development of high-performance pressure sensitive adhesives (PSAs) from chemicals directly obtained from raw biomass deconstruction. A particularly unique and translatable aspect of this work was the use of a monomer obtained from real biomass, as opposed to a model compound or lignin-mimic, to generate well-defined and nanostructure-forming polymers. Herein, poplar wood depolymerization followed by minimal purification steps (filtration and extraction) produced two aromatic compounds, 4-propylsyringol and 4-propylguaiacol, with high purity and yield. Efficient functionalization of those aromatic compounds with either acrylate or methacrylate groups generated monomers that could be easily polymerized by a scalable reversible addition–fragmentation chain-transfer (RAFT) process to yield polymeric materials with high glass transition temperatures and robust thermal stabilities, especially relative to other potentially biobased alternatives. These lignin-derived compounds were used as a major component in low-dispersity triblock polymers composed of 4-propylsyringyl acrylate and n-butyl acrylate (also can be biobased). The resulting PSAs exhibited excellent adhesion to stainless steel without the addition of any tackifier or plasticizer. The 180° peel forces were up to 4 N cm–1, and tack forces were up to 2.5 N cm–1, competitive with commercial Fisherbrand labeling tape and Scotch Magic tape, demonstrating the practical significance of our biomass-derived materials.


Synthesis and characterization of 4-propylsyringyl acrylate, 4-propylguaiacyl acrylate, 4propylsyringyl methacrylate, and 4-propylguaiacyl methacrylate.
The aromatic monomer mixture was acrylated following a procedure adapted from the literature. 1 The aromatic monomers and triethylamine (1.2 mol eq, Fisher Scientific, 99%) were dissolved in dichloromethane (DCM, anhydrous, Fisher Scientific) in a three-neck round bottom flask. The mixture was sparged with argon for 15 min while the flask was immersed in an ice-water bath. A solution of acryloyl chloride (1.2 mol eq, Sigma Aldrich, 97%) in DCM was added drop wise using a constant pressure dropper. The reaction was left to proceed overnight, after which, a white precipitant was filtered by vacuum filtration and discarded. The DCM permeate phase was washed consecutively with solutions of saturated sodium bicarbonate, 1.0 M NaOH (twice), 1.0 M HCl, and deionized water. DCM was removed by rotary evaporation, and the monomers were further purified by flash chromatography using silica gel (Standard Grade, 230 × 400 mesh, 60 Å) with ethyl acetate/hexanes mixture as an eluent (ethyl acetate volume fraction gradually increased from 0% to 10%). Two pure products, 4-propylsyringyl acrylate (4pSA) and 4propylguaiacyl acrylate (4pGA), were obtained. 1  Additional details for the synthesis of poly(4pSA), poly(4pGA), poly(4pSMA), poly(4pGMA), and poly(4pSMA-co-4pGMA).
The polymerization solvent, anisole (Sigma-Aldrich, ≥99.7%) with 5 wt% N,Ndimethylformamide (DMF, Sigma-Aldrich, ≥99.9%), was prepared and stored on molecular sieves to minimize water content. The monomer, BTCBA (or CPB; see Main Text), and AIBN were dissolved in the polymerization solvent and transferred to a pressure vessel. The reaction mixture was degassed by three freeze-pump-thaw cycles, backfilled with argon to a pressure of 3 psi, sealed with a stopcock, and immersed in an oil bath (70 °C) with vigorous stirring. The reaction was quenched at a predetermined time (typical reaction time was 6-7 h) by immersing the pressure vessel in liquid nitrogen. Tetrahydrofuran (THF, Fisher Scientific, certified) was added to the mixture, and the polymer was purified by precipitating into excess hexanes at least two times to ensure no monomer remained (confirmed by 1 H NMR spectroscopy).

Characterization of polymers.
Glass transition temperatures (T g s) of all polymers were determined using a differential scanning calorimeter (DSC, Discovery Series, TA Instruments). The DSC was calibrated using an indium standard. Polymer sample (2-5 mg) was loaded into an aluminum pan and hermetically sealed in air. A heating-cooling-heating cycle was carried out at a rate of 5 °C min -1 under continuous N 2 flow (50 mL min -1 ). For SaBSa, the sample was first heated from 35 °C to 120 °C, held at 120 °C for 2 min, cooled down to -90 °C, held at -90 °C for 2 min, and ramped to 120 °C. The procedure for poly(methyl methacrylate-b-butyl acrylate-b-methyl methacrylate) (MBM, S4 Kurarity LA2140e, Kuraray Co. Ltd.) was the same except that the experimental temperature window was -90 °C to 150 °C. The T g was determined as the midpoint of the inflection in the second heating.
The thermal degradation behavior of SaBSa was characterized using thermogravimetric analysis (TGA, Discovery Series, TA Instruments). 9-11 mg of SaBSa polymer was loaded into a 100 µL platinum pan and heated under continuous airflow (50 mL min -1 sample purge, 20 mL min -1 balance purge). The sample was heated at 20 °C min -1 to 110 °C, annealed at 110 °C for 15 min to remove possible residual water, cooled at 10 °C min -1 to 50 °C, held at 50 °C for 1 min, and heated at 10 °C min -1 to 600 °C.
Dog-bone-shaped testing bars (following ASTM D638, bar type 5, 5.3 mm gauge width, 0.8 mm thickness) were prepared by compression molding into a Teflon PTFE sheet (McMaster Carr) on a PHI Hotpress at 200 °C, with an applied load of 9000 lb. Tensile testing was performed with a RSA-G2 Solids Analyzer (TA Instruments) in tension mode. The lower grip was stationary, and the upper grip was raised at a speed of 10 mm min -1 to obtain tensile strength and elongation at break of SaBSa at room temperature. The measurement was repeated with four test specimens.
Tensile testing on MBM also was performed for comparison, except that the testing specimens were prepared with an aluminum mold, and the pressing temperature was 220 °C.
The micro-phase separation characteristics of SaBSa and MBM were probed by small-angle Xray scattering (SAXS) (Rigaku SAXS instrument at the University of Delaware). The wavelength of the beam was 0.154 nm, and the sample to detector distance was 2 m. The 2D S5 scattering patterns were azimuthally integrated to a 1D profile of intensity [I(q)] vs. scattering vector q, q = 4π sin(θ/2)/λ (θ is the scattering angle, λ is the wavelength). Figure S1. 1 H NMR spectra of 4pSA (a) and 4pGA (b) in chloroform-d (with TMS as an internal standard). Residue water was from the chloroform-d bottle. Figure S2. 1 H NMR spectra of 4pSMA (a) and 4pGMA (b) in chloroform-d (with TMS as an internal standard). In b), peaks labeled with * are from residue un-reacted 4pG (not separated from 4pGMA by flash column chromatography). The existence of 4pG did not affect the polymerization of 4pGMA, thus no attempt was made to remove it until the washes following polymerization.