Preparation of a Thermoresponsive Lignin-Based Biomaterial through

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Preparation of a Thermoresponsive Lignin-Based Biomaterial through Atom Transfer Radical Polymerization Yong Sik Kim† and John F. Kadla* Biomaterials Chemistry, Faculty of Forestry, University of British Columbia, Vancouver, BC, Canada Received December 22, 2009; Revised Manuscript Received February 4, 2010

Copolymerization of N-isopropylacrylamide (NIPAM) with technical hardwood kraft lignin (HWKL) was achieved by atom transfer radical polymerization (ATRP) using a selectively modified lignin-based macroinitiator. The degree of polymerization (DP) of polyNIPAM graft side chains was affected by varying the ratio of the DMF/ water solvent system from 5:0 to 0:5, and an estimated DPNIPAM of >40 was obtained using a ratio of 1:4 (v/v). The thermal decomposition temperature of the lignin-g-polyNIPAM copolymers significantly increased with increasing DPNIPAM. Likewise, the solubility of the lignin-g-polyNIPAM copolymers in water changed depending on copolymer structure. In both the water-soluble and suspended copolymers, at temperatures above 32 °C, the g-polyNIPAM component underwent the typical hydrophilic-to-hydrophobic transition, resulting in the precipitation of the copolymer.

Introduction Lignin is arguably the second most abundant biopolymer. It is a major component in wood, constituting roughly 15-35% by weight. Lignin is a randomly cross-linked network biopolymer arising from enzymatic dehydrogenative polymerization of hydroxylated and methoxylated phenylpropane units.1-4 It is found between cells and within cell walls, providing both resistance to biological attack and structural rigidity.5 An enormous amount of lignin is produced as a coproduct of “wood-free” papermaking; it is primarily burnt as an energy source and as part of a complex chemical recovery system. Less than 2% of technical lignins are utilized for value-added products, these typically being stabilizers for plastics and rubber as well as in the formulation of dispersants and surfactants.6,7 Much of this can be attributed to its role in chemical recovery, but much of its use has been dictated by its complex irregular macromolecular properties. However, as part of emerging woodto-ethanol biorefinery platforms lignin utilization is becoming more important. Here, value-added lignin-based materials and chemicals are needed to offset the low cost of the commodity fuel. There has been a growing interest in utilizing lignin’s hydrophobic polyol structure to develop novel lignin-based functional materials. Soft, hydrated lignin-based gels have been produced from technical lignins and shown to separate alcohols and other small organic molecules from fermentation broths.8 Temperature-responsive urethane and epoxy gels derived from hydroxypropylcellulose bearing lignin have been shown to have a 5 °C lower LCST (lower critical solution temperature) than those prepared without lignin.9 The blending of lignins with hydrophilic polymers like polyethylene oxide and starch, as well as other thermoplastics have been shown to improve the thermal properties and processability of lignin.7,10-14 The grafting of lignin with synthetic polymers offers the potential of preparing a new class of engineering plastics.15 A number of studies have been performed on grafting using either * To whom correspondence should be addressed. Phone: (604) 827-5254. Fax: (604) 822-9104. E-mail: [email protected]. † Present address: Division of Forest Bioenergy, Bioenergy Research Center, Korea Forest Research Institute, Seoul 130-712, Korea.

irradiation, chemical or chemo-enzymatic initiation.16-19 Atom transfer radical polymerization (ATRP) has been shown to be one of the most successful polymerization methods developed in recent years. ATRP is a useful method to synthesize graft copolymers with well-defined structures utilizing a variety of monomers.20-25 As part of our ongoing research into the utilization of technical lignins, we are investigating lignin-based copolymers using ATRP. Of particular interest is the effect of lignins inherent macromolecular properties on the design and performance of thermoresponsive copolymers. Using selectively modified lignin-based macroinitiators, we have produced ligning-polyNIPAM (N-isopropylacrylamide) copolymers via ATRP (Scheme 1). To our knowledge, lignin-g-polyNIPAM copolymers using ATRP has not been previously reported. In this paper, we present the synthesis and characterization of novel thermoresponsive lignin-based copolymers. The effect of reagents and reaction conditions on the corresponding biomaterial properties is presented and the implications of the copolymer structure on material behavior are discussed.

Experimental Section Materials. All chemicals were purchased from Aldrich Chemicals and used as received except where noted. N-Isopropylacrylamide (NIPAM 97%) was purified by recrystallization from a 60:40 toluene/ hexane mixture. N,N,N′,N′′,N′′-pentamethylenediethylenetriamine (PMDTA 99%) was purified by passing through a neutral alumina column before use. Cu(I)Br (98%) was purified by stirring in glacial acetic acid overnight, filtering, and washing with dry ethanol. Hardwood Kraft lignin (HWKL) was obtained from Westvaco Corp (Charleston, SC). The lignin was repeatedly washed with diluted HCl to exchange sodium counterions, filtered, washed with water, and vacuum-dried over P2O5. Lignin Characterization. 1H and 13C NMR were measured using a Bruker Avance 300 MHz spectrometer equipped with a BBO probe. For quantitative 1H NMR, 10 mg of lignin was accurately weighed and dissolved in 0.5 mL of DMSO-d6. The NMR spectra were recorded at 300 K, with a 90° pulse width and a 1.3 s acquisition time. A 7 s relaxation delay (d1) was used to ensure complete relaxation of the aldehyde protons. A total of 128 scans were collected. Quantitative 13 C NMR spectroscopy was performed using 200 mg lignin samples in 0.6 mL of DMSO-d6. The sample solutions were filtered prior to NMR analysis. Relaxation was facilitated by the addition of 10 µL of

10.1021/bm901455p  2010 American Chemical Society Published on Web 02/26/2010

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Scheme 1. Synthetic Scheme for the Preparation of Lignin-g-polyNIPAM Copolymers

a chromium acetoacetonate solution (final concentration 10 mM).26 Conditions for analysis included a 90° pulse width with a 1.4 s acquisition time and a 1.7 s of relaxation delay (d1). A total of 20000 scans were collected. Fourier transform infrared (FTIR) analysis was performed using a Perkin-Elmer Spectrum One FTIR spectrophotometer. Samples were analyzed as films cast on a ZnSn window from THF solution (5% w/w). Elemental analysis was performed at Complete Analysis Laboratories (Parsippany, NJ) with an associated error between replicates of 1-3%. The molecular mass distribution of the lignin samples were determined by GPC (Agilent 1100, UV and RI detectors) connected to a multiangle laser light scattering (MALS) detector (DAWN-EOS, Wyatt Technologies) using styragel columns (Styragel HR 4 and HR 2) at 35 °C with THF as the eluting solvent (0.5 mL min-1) and UV detection at 280 nm. Lignin concentration was 1 mg mL-1 and the injection volume was 100 µL. High resolution thermogravimetric analysis (TGA) was performed on a TA Instruments Q500 using approximately 3 mg of sample under nitrogen at a heating rate of 10 °C min-1. Differential scanning calorimetry (DSC) was performed using a TA Instruments Q 1000 DSC. Glass transition temperatures (Tg) were measured using a heating rate of 10 °C min-1 on a 2 mg sample and recorded as the midpoint temperature of the heat capacity transition of the second heating run. Phase transition temperatures were measured by DSC using 2 mg samples in 20 µL of water in sealed aluminum pans. The samples were scanned at 5 °C min-1 over the temperature range of the phase transition, referenced against an empty pan. All temperatures were determined from the second or third heating scan. Acetylation of Hardwood Kraft Lignin. HWKL samples were acetylated to determine aliphatic and phenolic hydroxyl contents using 1 H NMR. Acetylation was performed by dissolving 200 mg of HWKL in 8 mL of pyridine/acetic anhydride (1:1, v/v) and stirring the reaction for 48 h at room temperature. The reaction solution was then added dropwise with stirring to 300 mL of ice-water. The precipitated lignin was collected by filtration through a Nylon membrane (0.45 µm, 47 mm), washed with ice-water, and freeze-dried using a VirTis EX freeze-dryer. Complete acetylation was confirmed by FTIR. Macroinitiator (1A-F) Syntheses. Preferential modification of HWKL phenolic hydroxyl groups was accomplished by adding HWKL (2.5 g, 8.0 mmol phenolic hydroxyl content), triethylamine (8.0 mmol, 810 mg or 16 mmol, 1.62 g), and 75 mL of EtOAc to a 250 mL roundbottom flask. The reaction mixture was stirred and cooled in an ice bath and 2-bromoisobutyryl bromide (8.0 mmol, 1.8 g or 16 mmol, 3.7 g) in 10 mL of EtOAc was slowly added dropwise. The reaction was allowed to react for 1 or 24 h, at which time 20 mL of water/THF (3/1, v/v) was added and stirred for 5 min to quench any unreacted 2-bromoisobutyryl bromide. The solvent was evaporated and the product was dissolved in an aliquot of dioxane and precipitated into a saturated NaHCO3 solution, filtered, and repeatedly washed with water. The crude product was freeze-dried, suspended in diethyl ether, filtered, and dried in a vacuum oven at 40 °C. The recovered amounts of macroinitiatiors

1A-E were 1.61, 1.95, 2.02, 2.38, and 2.12 g, respectively (the reaction data and degree of substitution are shown in Table S1). 2-Bromoisobutyryl esterification of both the aliphatic and the phenolic hydroxyl groups was performed by dissolving 2.5 g (16.6 mmol hydroxyl content) of HWKL in a mixture of pyridine (16 mL) and dry THF (10 mL). The mixture was cooled in an ice bath and 8 mL (65 mmol) of 2-bromoisobutyryl bromide was slowly added dropwise. The reaction was brought to room temperature and stirred for 24 h. The lignin-macroinitiator 1F (2.34 g) was recovered and purified as described above. Copolymerization of NIPAM. The copolymerization reactions were performed by first adding Cu(I)Br (e.g., 0.22 mmol, 31.6 mg for macroinitiator 1D) and N,N,N′,N′′,N′′-pentamethylenediethylenetriamine (PMDTA; e.g., 0.22 mmol, 38.1 mg for macroinitiator 1D) to a dry 10 mL Schlenk flask, which was sealed and repeatedly degassed and backfilled with argon four times. Then 2.0 mL of deoxygenated water was added to form the CuI-PMDTA complex, which was further degassed repeatedly by four freeze-pump-thaw cycles. The ligninmacroinitiator (e.g., 0.11 mmol, 50 mg macroinitiator 1D) and NIPAM (e.g., 11 mmol, 1.2 g) were added to a second dry 10 mL Schlenk flask, degassed, and purged with argon. Then 4.0 mL of deoxygenated DMF and water were added, and once the NIPAM was dissolved, the mixture was further degassed by four continuous freeze-pump-thaw cycles and filled with nitrogen. The degassed macroinitiator/NIPAM mixture was stirred at room temperature, and 1.0 mL of the freshly degassed CuI-PMDTA stock solution was slowly added and left stirring under nitrogen for 24 h. The reaction flask was then immersed in a hot water bath (50 °C). The polymerized product was then diluted with hot water, filtered, repeatedly washed with hot water, and finally freezedried. The molecular mass and PDI of the resulting graft copolymers at various ratios of the reaction system, the reaction conditions, and the approximate degree of polymerization of the NIPAM side chain (DPNIPAM) are shown in Table S2. Kinetic Experiments. Kinetic reactions were carried out using a septum-sealed NMR tube placed in a Bruker Avance 300 MHz spectrometer: two stock solutions of macroinitiator and NIPAM in D2O and DMF, and Cu(I)Br and PMDTA in D2O were prepared in Schlenk flasks by freeze-pump-thaw cycles, as described above. To get a better 1 H NMR signal, the concentration of reactants was diluted 25 times as compared to the copolymerization reactions above. In a typical experiment a septum-sealed NMR tube was purged with nitrogen for 5 min, then 0.9 mL of the macroinitiator and NIPAM stock solution was added, and a 1H NMR spectrum was recorded (recorded as the initial reaction signal). Subsequently, 0.1 mL of the Cu(I)Br and PMDTA stock solution was added, the NMR tube was shaken for 10 s, placed in the spectrometer, and multi-1H NMR (200 scans) spectra were recorded at 8.3 min intervals for 5.5 h. The NIPAM signal at 5.65 ppm was integrated against the DMF reference signal.

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Figure 1. (a) 1H and (b) 13C NMR spectra of lignin-macroinitiator 1D in DMSO-d6. The signals 1 and 3 and 1* and 3* are from the bromoisobutyryl ester moiety substituted at the phenolic OH and aliphatic OH*, respectively.

Results and Discussion Synthesis of Lignin-Macroinitiator. 2-Bromoisobutyryl ester is an excellent initiator for ATRP,20,24 and has been commonly used to prepare graft copolymers with cellulosic derivatives and synthetic polymers.25,27,28 As lignin is a polyhydroxyl aromatic heteropolymer, there are numerous hydroxyl groups in lignin that can serve as branch points and can be modified to create active initiating sites for controlled radical polymerization. The HWKL used in this study contains 3.2 and 2.9 mmol/g of phenolic and aliphatic hydroxyl groups, respectively. It has been shown that using triethylamine (TEA) in the esterification of functional alcohols can selectively modify phenolic alcohols in the presence of aliphatic alcohols.29 Therefore, hardwood kraft lignin (HWKL) was reacted with 2-bromoisobutyryl bromide in EtOAc with TEA to yield the preferentially phenol-modified lignin-macroinitiators. FTIR analysis of the macroinitiator compounds 1A-D clearly shows the incorporation of the bromoisobutyryl ester moiety as evident from the presence of the CdO and C-O stretching vibrations at 1760 (ν(CdO)) and 1260 (ν(C-O)) cm-1, respectively (data shown in Supporting Information, Figure S1). The methyl group protons and the carbonyl carbon associated with the bromoisobutyryl ester moiety are clearly observed at 2.0 and 169.5 ppm, respectively, as broad signals in the corresponding 1H and 13 C NMR spectra, indicating the bromoisobutyryl esterification occurred selectively at the phenolic OH position (Figure 1). The degree of substitution of the lignin-macroinitiators (Table S1) was calculated from the peak areas of the 1H NMR spectrum and estimated according to eq 1.

DS(%) )

A × 100 AF

(1)

where A is the integral of the methyl protons of the bromoisobutyryl ester moiety at ∼2.0 ppm, and AF is the integral of the methyl protons of the bromoisobutyryl ester moiety from fully substituted lignin-macroinitiator 1F (data shown in Supporting Information, Figure S2). Increasing the TEA and 2-bromoisobutyryl bromide concentrations from 1 to 2 equiv increased the DS from 22 to 44%. Likewise, repeating the reaction twice using the same conditions increased the DS from 22 to 61% and 44 to 80%. However, in the case of the high DS macroinitiator 1E, esterification of the aliphatic alcohol was also observed (Table S1), indicating that increasing the amount of TEA/2bromoisobutyryl bromide still promotes the selective esterification of phenolic alcohol, but the selectivity is less controlled. Graft Copolymerization of Lignin-Macroinitiator with NIPAM. It has been reported that ATRP of NIPAM with various macroinitiators can be performed under mild conditions, such as room temperature and by addition of water to the reaction system.29 Of particular interest is the DMF/water system, which because of the miscibility between water and DMF enables the development of a wide range of copolymers. The number-average molecular mass of the various macroinitiators increased with increasing degree of substitution (1A-F; Table S2). Changing the water/DMF ratio from 5/0 to 1/4 using a constant monomer/CuBr/ligand/macroinitiator concentration ([M]/[CuBr]/[L]/[I] ) 100:1:1:1) increased the resulting Mn of the lignin-g-polyNIPAM copolymer from 15100 to 144400 (Table S2). In addition, the polydispersity index (PDI) decreased from 1.98 to 1.09, indicating that increasing the ratio of water in this system leads to well-controlled ATRP. However, the complete absence of DMF resulted in a slightly higher PDI ∼1.5, despite a relatively high Mn of the lignin-g-polyNIPAM copolymer (2D6). This is likely because the lignin-macroinitiator

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was not soluble in water, so the ATRP may have initiated at the surface of the macroinitiator, becoming less controlled. The DPNIPAM could not be determined by 1H NMR because the signals of the macroinitiator and polyNIPAM side chain overlap. As lignin has a random linkage structure, we were not able to calculate the actual DPNIPAM of the polyNIPAM side chain. However, from elemental analysis, a molecular formula of C9H6.78O1.97 (OCH3)1.14 was calculated, corresponding to a molecular mass of ∼182 g mol-1, which, combined with the degree of substitution of the macroinitiators (Table S1), enabled an approximation of the DPNIPAM according to eq 2.

Figure 2. 1H NMR spectra of lignin-g-copolymers; (a) -2D7, (b) -2D5 and

DPNIPAM )

B × (182 + 149 × (1.97 × C)) D × 113

(2)

where B is the Mn of the lignin-g-polyNIPAM copolymer, 182 is the C9 repeat unit molecular weight, C is the degree of substitution of hydroxyl groups to bromoisobutryl moieties (149 is the mass of the bromoisobutyryl ester moiety and 1.97 is the oxygen composition based on a C9), D is the Mn of the corresponding macroinitiator, and 113 is the mass of NIPAM. The solubility of the macroinitiators varied depending on the degree of esterification (substitution). Therefore, to compare the effect of increasing initiation sites copolymerization reactions

13

C NMR spectra of lignin-g-copolymers, (c) -2D7, and (d) 2D5.



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Figure 3. Evolution with time of the 1H NMR spectrum of the monomer conversion during ATRP at 25 °C. [M]/[Cu(I)Br]/[L]/[macroinitiator] ) 100:1:1:1 at 25 °C in the presence of ratio of D2O/DMF 4:1.

were performed in the DMF/water 2:3 (v/v) solvent system ([M]/ [I]/[CuBr]/[L] ) 100:1:1:1). Increasing the amount of bromoisobutyryl ester moieties resulted in an increase in the molecular mass of the copolymer from 76400 (2C4-DPNIPAM ∼ 25) to 89200 (2D4-DPNIPAM ∼ 27) to 215300 (2E4-DPNIPAM ∼ 37; Table S2). However, in the case of the fully substituted macroinitiator 1F, a smaller increase in molecular mass was observed (2F4 - DPNIPAM ∼ 16). In this system, the copolymer quickly precipitated out of solution during the reaction, likely limiting the polymerization process. Detailed structural confirmation of the lignin-copolymers were obtained by 1H and 13C NMR. Figure 2 shows the 1H and 13C spectra of a typical lignin-g-copolymer-2D5 and -2D7. It can be seen that the signals associated with the lignin aromatic ring and methoxyl groups are extremely weak in the lignin-gcopolymer-2D5 (Figure 2b,d), while they are still evident in the spectra of 2D7. In addition, the signal at 58 ppm, belonging to the bromoisobutyryl moiety (C-Br) and methoxyl groups of the lignin-macroinitiator (Figure 1) have disappeared in the lignin-g-copolymer-2D7, indicating there are no unreacted initiating sites (C-Br) in the macroinitiator. Kinetic Studies of Graft Copolymerization of Lignin-Macroinitiator with NIPAM. In situ 1H NMR spectroscopy was employed to evaluate the kinetics of the ATRP of NIPAM with macroinitiators at 25 °C. Initially, two degassed stock solutions were prepared in Schlenk flasks by freeze-pump-thaw cycles. The stock solution of macroinitiator and NIPAM was added into a septum sealed NMR tube under nitrogen atmosphere. The kinetic reaction was initiated by adding the stock solution of Cu(I)Br and PMDTA, shaken for 10 s, and then quickly running multi-1H NMR analyses. The NIPAM signal (5.65 ppm) of the vinylic proton of the monomer was used to determine monomer conversions referenced to the DMF solvent signal (Figure 3). Figure 4a shows the spectral evolution of monomer conversion as a function of time. It was found that the rate of

Figure 4. Kinetics results obtained for lignin-g-polyNIPA from macroinitiator-D grafting NIPAM. (a) NIPAM conversion determined by 1 H NMR. (b) Kinetic plot of ln[(NIPAM)o/(NIPAM)]. [M]/[Cu(I)Br]/[L]/ [macroinitiator] ) 100:1:1:1 at 25 °C; (O) kinetic-2D5, (0) -2D4, (4) -2D3, and (]) -2D2 in the presence of ratios of D2O/DMF 4:1, 2:3, 3:2, and 1:4, respectively.

polymerization was dependent on solvent. Increasing the D2O ratio of the DMF/D2O solvent system from 4:1 to 1:4 using a constant [M]/[I]/[CuBr]/[L] ) 100:1:1:1 concentration at 25 °C

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Figure 5. Plot of NIPAM conversion (%) vs Mn [M]/[Cu(I)Br]/[L]/ [macroinitiator] ) 100:1:1:1 at 25 °C in the presence of ratio of D2O/ DMF 4:1.

dramatically increased monomer uptake, consistent with the Mn results (Table S2). A relatively fast polymerization was observed in the 1:4 DMF/D2O solvent system (kinetic-2D5), with close to 90% conversion within 5 h. This is likely because the polymerization rate increases as polarity of the solvent system was increased.30 In kinetic-2D4 and kinetic-2D5 systems, the first-order kinetic plots of ln[(NIPAM)o/(NIPAM)] as function of time were linear up to ∼60-80% conversion (Figure 4b). This is consistent with the concentration of the propagating radical species being constant throughout the polymerization process, and that termination is negligible. However, it was found that the reactions of kinetic-2D3 and kinetic-2D2 systems (high DMF containing systems) were substantially slower, leveling off at only 10-20% conversion (Figure 4a). This may indicate that the intermediate radicals may be significantly stabilized as the DMF ratio of the DMF/D2O solvent system was increased, such that the rate of deactivation is greater than that of activation. In a controlled living polymerization the number-average molecular mass increases linearly with conversion.31-33 Figure 5 shows the linear increase in copolymer molecular mass with NIPAM conversion for the lignin-g-polyNIPAM copolymer (2D5) in the 1:4 DMF/D2O solvent system: batch reactions were run in Schlenk flasks under a nitrogen atmosphere and were stopped at different times by bubbling air into the reaction. This linear relationship indicates that all (or nearly all) propagating chains have the same lifetime, starting at essentially the same time (i.e., fast initiation) and growing with negligible chainbreaking reactions until high conversion of monomer is reached. This further illustrates that the free phenolic hydroxyl groups in these lignin-based macromolecules do not appear to affect or compete with the ATRP mechanism. First-order kinetics were also observed in the copolymerization of macroinitiators 1A and 1C using a DMF/D2O 1:4 (v/v) solvent system at 25 °C (Figure 6). The polymerization rates of the lignin-macroinitiator with NIPAM in the presence of CuI-PMEDTA complex became faster in the order of macroinitiators 1D > 1C > 1A (Figure 6). All of the systems were run using the same ratio of NIPAM to initiator, but based on the difference in initiation sites (DS), the concentration of NIPAM was greatest in 1D > 1C > 1A. This trend in polymerization rates follows the regiochemistry of the macroinitiator, specifically, the reaction kinetics are faster in the system with the highest DS of bromoisobutyryl initiator groups. We are currently investigating this further. Unfortunately, the copolymerization kinetics for macroinitiators 1E and 1F could not be performed using 1H NMR due to the insolubility of the macroinitiators in the DMF/D2O solvent system.

Kim and Kadla

Figure 6. Kinetic plots of ln[(NIPAM)o/(NIPAM)] for lignin-g-polyNIPA from macroinitiators 1D, 2C, and 1A grafting NIPAM. [M]/[I]/[Cu(I)Br]/ [L]/[macroinitiator] ) 100:1:1:1 at 25 °C in the presence of a DMF/ H2O 1:4 (v/v) solvent system.

Figure 7. TGA analysis of lignin-g-polyNIPAM copolymers (numbers in parentheses represent DPNIPAM).

Thermal Characterization of Lignin-Copolymer. Figure 7 shows the TGA curves of lignin-copolymer derivatives with a series of DPNIPAM from 0 to ∼40, as determined by GPC. The thermal decomposition temperature of the starting material showed two peaks at 252 and 364 °C. The TGA curve drastically changed after ATRP. With increasing DPNIPAM, the bimodal peaks disappear and a peak at 406 °C appeared, which is in good agreement with polyNIPAM.25 Thus, the thermal decomposition properties of the lignin-copolymer dramatically improve with increasing DPNIPAM and appear to be trending toward those of polyNIPAM. In water, polyNIPAM exhibits a thermally activated phase transition: it is a clear solution at room temperature and undergoes a hydrated coil to dehydrated globule transition at higher temperature (LCST ∼32 °C) in aqueous solutions.24 The solubility of the lignin-g-polyNIPAM was found to be dependent on the macroinitiator. The lignin-g-polyNIPAM made with the fully substituted macroinitiator 1F (2F4, both phenolic and aliphatic hydroxyl groups bromoisobutyrylated) was soluble in water, but those from macroinitiator 1D (i.e., 2Dx) were not, forming stable turbid suspensions. When the temperature was increased to above ∼32 °C (LCST for polyNIPAM), a white precipitate was observed in all solutions, indicating that the



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constant at that of polyNIPAM, 32 °C. This is in contrast to other work in which hydroxyprolylated pulps containing lignin show a dramatic decrease in the LCST (5 °C) as compared to a similar molecular weight HPC (hydroxypropylcellulose) sample.9 Thus, the grafting of the thermoresponsive polymer to lignin, as oppose to “blending” does not appear to impact the thermoresponsive polymers LCST.

Conclusions

Figure 8. Visual appearance of the phase transition of (a) lignin-gcopolymer 2D5 and (b) lignin-g-copolymer 2F4.

Lignin-macroinitiators were prepared by bromoisobutyryl esterification of the HWKL phenolic hydroxyl groups using TEA. The degree of substitution (esterification) increased with increasing TEA and 2-bromoisobutyryl bromide concentration. Increasing the phenolic hydroxyl group esterification beyond 60% DS led to decreased selectivity, that is, substantial esterification of the aliphatic hydroxyl groups. Lignin-gpolyNIPAm copolymers were prepared by atom transfer radical polymerization (ATRP) with NIPAM in various ratios of DMF/ water, from 5/0 to 0/5. The resulting copolymerization kinetics and copolymer molecular weights were very much dependent on the DMF/water ratio. Kinetics studies showed that ATRP of lignin-macroinitiators with NIPAM was well-controlled using a DMF/water 1:4 (v/v) solvent system and copolymers with polyNIPAM side-chains of DPNIPAM up to ∼43 were synthesized. The thermal decomposition temperature of the lignin-g-polyNIPAM copolymers significantly increased with increasing DPNIPAM. Depending on the degree of substitution of the macroinitiator, the lignin-gpolyNIPAM copolymers were either fully or partially soluble in water. Both the soluble and suspended lignin-g-polyNIPAM copolymers precipitated from aqueous solution at temperatures above 32 °C, likely arising from the hydrophilic-to-hydrophobic transition of the polyNIPAM graft side chains. Acknowledgment. The authors thank the Natural Science and Engineering Research Council of Canada for financial support. Supporting Information Available. Reaction data, degree of substitution, molecular mass (Mn) and degree of polymerization data for the macroinitiators 1A-1F and graft copolymers 2C-2F. FT-IR and 1H NMR spectra of hardwood kraft lignin (HWKL) and macroinitiators 1A-1F. This material is available free of charge via the Internet at http://pubs.acs.org.

References and Notes (1) (2) (3) (4) Figure 9. DSC analysis of lignin-g-polyNIPAM copolymers made from lignin-macroinitiator 1D in water (numbers in parentheses represent DPNIPAM).

(5) (6)

polyNIPAM side chain is dehydrated and becomes hydrophobic above this temperature (Figure 8). In this system, the polyNIPAM maintains an expanded form below 32 °C, resulting in either a soluble copolymer aqueous solution (2F) or suspension (2Dx) and attains a contracted form beyond it leading to copolymer precipitation. The thermal transition properties of the various lignin-gpolyNIPAM copolymers were investigated using DSC (Figure 9). There is a clear increase in the calorimetric enthalpy with increasing DPNIPAM in going from the copolymers with DPNIPAM ) 8 to DPNIPAM ) 43 at 32 °C. However, the transition remains

(7) (8) (9) (10) (11) (12) (13)

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