Synthesis and Characterization of a Terpene-Based Sustainable

Research Article pubs.acs.org/journal/ascecg

Synthesis and Characterization of a Terpene-Based Sustainable Polymer: Poly-alloocimene Pranabesh Sahu, Preetom Sarkar, and Anil K. Bhowmick* Rubber Technology Centre, Indian Institute of Technology Kharagpur, Kharagpur 721302, West Bengal India S Supporting Information *

ABSTRACT: We present the synthesis and characterization of a biobased sustainable polymer from alloocimene, a monoterpene from renewable resources, by redox emulsion polymerization under ambient conditions. Density functional theory was used to determine the ground-state optimized structure of alloocimene. Poly-alloocimene has a molecular weight of 14 200 Da and subzero glass transition temperature of −17 °C. The microstructure and properties were determined by FT-IR and NMR (1-D, 2-D) spectroscopies, differential thermal analysis, dynamic mechanical analysis, and thermogravimetry. KEYWORDS: Biobased polymer, Terpene, Alloocimene, Emulsion polymerization, Density functional theory

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INTRODUCTION The chemical industry is making great attempts to reduce its dependence on petroleum-based products by developing sustainable material technologies using naturally occurring monomers. The enhanced interest in recent years for natural products has made it desirable to study the chemistry of a large and diverse class of organic compounds produced by a variety of plants and animals. “Sustainable polymers” are polymers produced partially or entirely from natural resources other than petroleum resources. Recent reviews have focused on the potential of biobased compounds as resources for the sustainable creation of polymeric materials. This focus is motivated by several environmental issues, such as dwindling fossil fuel resources and global warming. Compared to today’s synthetic polymers, biopolymers can potentially offer better recyclability and lower process energy requirements, with a smaller environmental footprint overall. These attributes make biopolymers ideal candidates to replace 90 billion pounds of petroleum-based polymers in various applications, such as coatings, packaging, textiles, and automotive items. One of the grand challenges in the polymer industry is to devise green and sustainable technologies for the conversion of biobased materials, such as vegetable oils, sugars, fats, resins, proteins, and amino acids.1−3 The remarkable advances in organic synthesis, catalysis, and functionalization of double bonds in fatty acids can contribute to a more productive synthesis of plant-oil-based polymers.4,5 Although the most widely used renewable compounds are diols, diamines, and dicarboxylic acids (used to synthesize polyesters and polyamides), naturally occurring terpenes comprise another attractive class of biopolymer precursors synthesized mainly by plants, particularly conifers.6,7 They are generally derivative © 2017 American Chemical Society

of isoprene produced biosynthetically, which has the molecular formula C5H8. The basic molecular formulas of terpenes are multiples of that, (C5H8)n where n is the number of linked isoprene units. Among the various monoterpenes present in nature, a series of trienes like myrcene, ocimene, and alloocimene are found in the oils of basil, bay, ylang-ylang, hops, thyme, cannabis, parsley, and several other essential oils.8,9 Although a few research investigations regarding the reaction of terpenes have been proclaimed in synthetic organic chemistry,10 their budding applications as a monomer in the arena of polymer science are still in a rudimentary stage. Thus, due to their abundance in nature, terpene-based olefinic compounds11−13 are promising renewable resources for polymeric materials. The current progresses in the polymerization of monocyclic terpenes are also evolving as high-impact research fields in modern polymer science and technology.14 Limonene obtained from citrus fruit, a very common cyclic terpene, was polymerized via thiol−ene chemistry to obtain a wide range of renewable monomers and polymers like polyamides and polyurethanes.15,16 Coates and co-workers reported a non-petroleum route to carbon dioxide copolymers using limonene.17 Thus, to make use of chemical functionalities (unsaturation, functional groups) present in terpenes, various polymerization techniques have been studied and the hunt for more is under way. Alloocimene (Allo) is a monoterpene produced by thermal isomerization of α-pinene,18 a naturally occurring cyclic terpene found in the oils of several tree species.19 Previously, Jones20 Received: April 1, 2017 Revised: July 24, 2017 Published: July 25, 2017 7659

DOI: 10.1021/acssuschemeng.7b00990 ACS Sustainable Chem. Eng. 2017, 5, 7659−7669

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ACS Sustainable Chemistry & Engineering

furnished in Table 1, at first DI water, potassium oleate, potassium chloride, and potassium phosphate tribasic buffer were charged into a

reported that boron trifluoride etherate initiates the cyclopolymerization of Allo under anhydrous conditions to yield a low-melting polymer (85−87 °C). The cyclic structure of polyalloocimene was inferred from the iodine numbers, which suggest that two of the alloocimene double bonds are consumed in the polymerization reaction. Then, the cationic homopolymerization of Allo was studied by Marvel et al.21,22 with different catalyst systems. The polymerization yielded mostly 6,7- and 4,7-enchainment, giving an amorphous soluble polymer with a slight amount or no cyclic structures and relatively high glass transition temperature (Tg). Anionic polymerization of alloocimene with a catalyst system comprising a metal, e.g., sodium, and an aliphatic ether, e.g., 1,2-dimethoxyethane, was studied, yielding a mixture of polymers having predominantly 2,3- and 6,7-units with pendant dimethylbutadienyl groups and a minority amount of 4,7alloocimene, displaying a range of glass transition temperatures and weight-average molecular weights. Poly-alloocimene (PAllo) with such microstructures as reactive intermediates can readily form Diels−Alder adducts with activated olefins.23 Later, copolymerization of vinyl monomers with Allo was studied by Puskas et al.24,25 using living carbocationic polymerization. They obtained a copolymer that showed very high molecular weight having an Allo-rich segment displaying thermoplastic elastomer properties. The Allo content of the copolymer was found to have predominantly 4,7- and 2,7enchainment. All of the above polymerization processes used different organic solvents or chemicals and inorganic catalyst. Against this background, it is of great interest to develop a sustainable rubber by employing a green technique. We herein report the redox-initiated emulsion polymerization26 of alloocimene and intend to give an insight into the chemical structure of the synthesized polymer by detailed NMR spectroscopy. Apart from all the past investigations by living polymerization, interestingly, we obtain an amorphous polymer with subzero Tg (just below ambient temperature) by a conventional emulsion polymerization technique. This is new and unique. The mechanism of polymerization in this system is explained later. The optimized structure of alloocimene was determined by density functional theory (DFT) calculation. The microstructure was analyzed by FT-IR and NMR (both 1D and 2D) spectroscopies. The thermal stability and thermal transitions of the synthesized polymer are reported.

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Table 1. Recipe for Redox-Initiated Emulsion Polymerization ingredientsa

amount (g, in phrb)

monomer DI water potassium oleate (C18H33KO2) potassium chloride (KCl) potassium phosphate tribasic (K3PO4) ferrous sulfate heptahydrate (FeSO4·7H2O) ethylenediaminetetraacetic acid sodium salt (Na-EDTA) sodium metabisulfite (Na2S2O5) tert-butyl hydroperoxide (TBHP)

100 134 4.5 0.3 2.0 0.01 0.05 0.05 0.06

a

Chemical reagents: (i) potassium oleate, surfactant; (ii) KCl, electrolyte; (iii) K3PO4, buffer; (iv) Na-EDTA, sequestering agent; (v) FeSO4·7H2O and Na2S2O5, redox couple; (vii) TBHP, redox initiator. bParts per hundred parts of rubber. round-bottom flask and stirred at 280 rpm for 20 min. Thereafter, the reactor was put under vacuum and flushed with nitrogen to make an inert gas blanket. Then, Allo was added into the flask and the solution was allowed to mix thoroughly for a further 20 min. Upon obtaining a stable emulsion, Na2S2O5, FeSO4.7H2O, and Na-EDTA were added to the mixture, followed by the addition of tert-butyl hydroperoxide (TBHP) solution into the flask. The polymerization was allowed to proceed for 16 h at 25 °C (RT). During the course of the reaction, the latex did not form any coagulum, leading to the stability of the polymerization process. Subsequently, the latex was washed with an inert solvent like hexane and the latex was separated using a separating funnel to make it monomer free. The obtained latex was then coagulated using excess ethanol. The polymer was washed thoroughly with DI water and dried in a vacuum desiccator to obtain a pale yellow, amorphous, rubbery-type polymer, as shown in Figure S1 of the Supporting Information (SI). The yield percentage of the polymerization was about 20−25%. Parallelly, a blank reaction was also carried out without the initiator (TBHP) to check the polymerization process, but no polymer was formed. That indicated the formation of a polymer (PAllo) by the redox emulsion polymerization process. Subsequently, the polymerization was explored using two redox initiating systems, (1) FeSO4·7H2O, SHS, and TBHP and (2) FeSO4· 7H2O, Na2S2O5, and APS, and keeping the other ingredients same, as given in Table S1 in the SI. These methods also resulted in the formation of the rubbery-type polymer. The polymerization is shown in Scheme 1. A similar technique was used earlier for emulsion polymerization of other monomers in our laboratory.27 Measurements and Characterization. Polymer molecular weight was measured by gel permeation chromatography (GPC) at 25 °C using an Agilent PL-GPC 50 instrument, having a refractive index detector and equipped with PLgel 5 mm Mixed-D column. Tetrahydrofuran (THF) was used as the eluent (sample concentration 1 mg/mL) at a flow rate of 1 mL/min, and a polystyrene standard was used for calibration. The dissolved sample was filtered before the experiment to remove the impurities and gel present, and the obtained molecular weight was considered relative to linear PAllo polymer only. The particle nature of the latex was measured by the dynamic light scattering (DLS) method using a Malvern Nano ZS instrument employing a 4 mW He−Ne laser (λ = 632.8 nm) at a scattering angle of 90°. The room temperature solubility behavior of the synthesized polymers was evaluated [as 0.1% (w/v)] in common organic solvents, such as tetrahydrofuran (THF) and chloroform (CHCl3). The gel percentage of the polymer was calculated as the ratio of dried polymer weight to its original value after extraction using THF for 10 h at room temperature (25 °C).

EXPERIMENTAL SECTION

Materials. Alloocimene (2,6-dimethyl-2,4,6-octatriene, technical grade, 80%) was purchased from Sigma-Aldrich, and inhibitor was removed by shaking with 2 M NaOH solution. In a few cases mentioned in the appropriate section, this monomer was fractionally distilled at 200−210 °C and the fractions were separated in order to investigate the effect of the fraction on polymerization. Potassium oleate (K-Oleate, 98%) was purchased from Loba Chemie and used as received. Ferrous sulfate heptahydrate (FeSO4·7H2O, 98%) was purchased from Alfa Aesar India. Ethylenediaminetetraacetic acid sodium salt (Na-EDTA), potassium phosphate tribasic (K3O4P), potassium chloride (KCl), sodium metabisulfite (Na2S2O5), sodium hydroxymethanesulfinate (SHS), and tert-butyl hydroperoxide solution (Luperox TBH70X), were purchased from Sigma-Aldrich and used without further purification. Ammonium persulfate (APS, 98%) was obtained from E. Merck. All other chemicals were reagent-grade commercial products and used as received without further purification. Deionized water (DI H2O) was used for all the experiments. Redox-Initiated Emulsion Polymerization of Alloocimene. Redox-initiated emulsion polymerization was carried out by following sequential addition in a round-bottom flask. According to the recipe 7660


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ACS Sustainable Chemistry & Engineering Scheme 1. Emulsion Polymerization of Alloocimenea

Tetramethylsilane (TMS) was used as an internal standard, and the chemical shift values were reported in δ (ppm) relative to the internal standard. A NETZSCH DSC 200F3Maia differential scanning calorimeter (DSC) was used to investigate the thermal transitions of the polymers under a nitrogen atmosphere. The samples were heated from −100 to +50 °C at a heating rate of 5 °C/min. In order to erase the thermal history, the Tg was determined from the second heating run. Tg was reported as the temperature of the midpoint of the heat flow change, as determined from the baseline tangents using NETZSCH proteus thermal analysis software. Dynamic mechanical analysis (DMA) of the polymer was performed using a METRAVIB 50N dynamic mechanical analyzer in tension mode. Temperature sweep experiments were carried out at 1 Hz frequency and 0.1% dynamic strain over a temperature range from −100 to +100 °C, at a heating rate of 3 °C/min. Because of the low strength of the synthesized polymer, the glass fiber cloth was drenched with a solution of the polymer in chloroform and a thin coating of uniform thickness was deposited on it followed by drying at room temperature. The glass fiber cloth was chosen as it did not show a glass transition within the temperature range of interest.28 Only the tan δ peak was taken into consideration from the experiment. Thermogravimetric analysis was carried out for the polymer using an SDT Q600 TA instrument. The experiment was carried out under a nitrogen atmosphere at a heating rate of 10 °C/min. Density functional theory (DFT) calculation was carried out using Gaussian 09 software. The geometry (optimized structure) and frequency of alloocimene were calculated with Becke’s three-parameter hybrid functional (B3LYP) method by using 6-311G(d,p) as a basis set. Electrostatic potential mapping (ESP) of the optimized structure was generated using ArgusLab 4.0.1 software.

a

The asterisks denote the polymerization sites in monomer and polymer.

The Fourier transform infrared (FT-IR) spectra of the synthesized homopolymer was recorded in a PerkinElmer Spectrum 400 machine (resolution 4 cm−1) using a universal attenuated total reflectance (UATR) attachment within a spectral range of 4000−400 cm−1. A total of six scans per sample were performed. 1 H and 13C nuclear magnetic resonance (NMR) spectra were recorded on an AVANCE 400 Ascend Bruker instrument operating at 600 MHz at room temperature. Two-dimensional (2-D) NMR COSY and NOESY measurements were made to collect two-dimensional hypercomplex data. After weighing with a shifted sine-bell function, the data were Fourier transformed in the absolute value mode. The samples were dissolved in deuterated chloroform (CDCl3 ).

Figure 1. (a) Time dependence of PAllo synthesis on molecular weight. (b) Temperature dependence of PAllo synthesis on molecular weight. (c) Molecular weight distribution of PAllo. (d) GPC elugram of PAllo. 7661


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RESULTS AND DISCUSSION

Synthesis of Poly-alloocimene (PAllo) by Emulsion Polymerization. Redox-initiated emulsion polymerization of alloocimene was carried out at room temperature (RT) and at atmospheric pressure. RT was preferred for polymerization, to make it more sustainable. Potassium oleate as a surfactant; potassium phosphate tribasic (K3PO4) as a buffer; Na-EDTA as a sequestering agent; and sodium metabisulfite (Na2S2O5), ferrous sulfate heptahydrate (FeSO4·7H2O), and tert-butyl hydroperoxide (TBHP) as the redox system were used, unless otherwise mentioned. The molecular weight of the polymer was confirmed at different times of the reaction. Time and Temperature Dependence of PAllo Synthesis. Effect of Time. In order to study the effect of reaction time on the polymerization, an aliquot was taken out of the reaction mixture at each 4 h time interval up to 20 h. The molecular weight was determined sequentially by GPC measurement. It is apparent from Figure 1a that very low molecular weight was obtained at 4 h, indicating almost no yield. The number-average molecular weight (Mn) reached a maximum value of 14 200 Da at 16 h. Thereafter, a decreasing trend was observed at higher reaction time due to the formation of oligomers. Thus, from the kinetic study and molecular weight, the reaction condition was set at 16 h at RT. Effect of Temperature. The polymerization reactions were carried out at different temperatures between 15 and 35 °C at every 10 °C interval for a fixed reaction time of 16 h with a fixed concentration of TBHP, alloocimene, and other ingredients. It is evident from Figure 1b that with an increase in the temperature of the reaction, there is an initial increase in the molecular weight of the polymer (maximum at 25 °C). It showed a decreasing trend after the maximum. At high temperature, an increase in the rate of propagation and thermal decomposition of the polymer chains prevail, thereby reducing the molecular weight of the polymer. The redox reaction was carried out preferably at room temperature to control the side reactions and to prevent cross-linking between the growing polymer chains at higher temperature and time. We have reported similar observation with polymyrcene.27 From the MWD curve (Figure 1c) and polydispersity index (PDI = 1.6) of the synthesized polymer, it is clear that a narrow distribution or monodisperse polymer formation was observed. Due to continuous generation of primary initiator radicals, the lifetime of growing radicals is low. So, the rate of reaction for free radical polymerization is exceedingly quick. The narrow distribution of polymer formation was due to low conversion and less formation of oligomers and less chain branching. Figure 1d represents the elugram of GPC analysis showing the detector signal height versus the retention time. Over the whole time scale of the experiment, the polymer was eluted at higher retention time, thus indicating a formation of lower molecular weight polymer. Analysis of Particle Size of the Latex. In order to analyze the particle nature of the latex, the dynamic light scattering method was used. The particle size distribution and mean particle size of the PAllo latex were taken at different time intervals. The obtained Z-average particle diameter was 253 nm at 16 h at room temperature. This is higher than the previously reported value of 68.3 nm for polymyrcene.27 FTIR and NMR Spectra of the Monomer (Allo) and Polymer (PAllo). Figure 2 represents the FT-IR spectra of alloocimene and PAllo. The weak peak at 3042 cm−1 in the

Figure 2. FT-IR spectra of alloocimene (Allo) and poly-alloocimene (PAllo).

monomer represents the C−H stretching of C3, C4, C5, which almost disappears on polymerization. The absorption peaks at 2970, 2914, 2857 cm−1 in the monomer are due to −CH 3 , −CH 2 , −CH asymmetric stretching vibrations, respectively, which are broadened in the polymer. The absorption peak at 1646 cm−1 is assigned to the CC stretching frequency. The near disappearance of the CC peak indicates the consumption during the polymerization process followed by simultaneous shifting of the unsaturated CC peak to a higher frequency of 1656 cm−1. The characteristic absorption peaks at 1440 and 1375 cm−1 are due to the bending vibration of the −CH and −CH3 groups, respectively. The strong absorption peaks around 952, 837, and 786 cm−1 in the monomer are due to the sp2 C−H bending vibration of C3, C4, C5, and C7 centers, respectively. The decrease in the intensities of these bands in the polymer is due to the consumption of unsaturated bonds in the monomer. The 1H and 13C NMR spectra of alloocimene are presented in Figure 3a,b. The structure of the monomer contains a continuous conjugated system involving three double bonds. The spectral assignments to various magnetically different protons and carbons are indicated below. The chemical shift of TMS is assigned as the singlet at δ 0 ppm. 1 H NMR Data of Allo. The following are the 1H NMR data of Allo (CDCl3, 600 MHz): δ 6.5 (1H, C-5), 6.4 (1H, C-4) 6.0 (1H, C-3), 5.4 (1H, C-7), 1.88 (6H, C-1, C-2′), 1.81 (3H, C6′), 1.77 (3H, C-8). The olefinic hydrogens of the corresponding carbon (C-5, C-4, C-3, and C-7) appeared in the downfield region (δ 5.4−6.5) and the protons of the methyl groups (C-2′, C-6′, and C-8) were observed in the upfield region (δ 1.77−1.88). 13 C NMR Data of Allo. The following are the 13C NMR data of Allo (CDCl3, 600 MHz): δ 135.2 (C-5), 133.2 (C-6), 127.0 (C-4), 126.1 (C-2), 125.0 (C-3), 124.0 (C-7), 26.1 (C-1), 20.4 (C-6′), 18.4 (C-2′), 13.0 (C-8). The olefin carbon atoms occurred at larger chemical shift, appearing in the downfield region, and the methyl groups attached to the olefinic carbon (C-2, C-6, and C-7) were detected in the upfield region. Figure 4a,b shows the 1H and 13C NMR spectrum of the polymer (PAllo). The polymer may have four possible microstructures like (4,7), (2,7), (6,7), and (2,3)-polyalloocimene, as identified by Marvel et al.21,22 But we could identify only (4,7) and (2,7) microstructures. A different set of notations was used for assigning the respective carbon of (2,7) microstructure in the given figures. The peak assignment reasonably supports the formation of polymer, and the signals 7662


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Figure 3. (a) 1H NMR spectra of Allo. (b) 13C NMR spectra of Allo.

compared with those of the monomer can be ascribed to the generation of the long polymer chain. According to previous NMR studies on polyisoprene,29−33 the peaks above 4.50 ppm indicate olefinic protons of 3,4-addition and 1,2-addition. The olefinic (CH−) protons (C3, C5) for (4,7)-PAllo appear as a single peak at 4.4 ppm. The olefinic (CH−) protons (C3*, C4*, C5*) for (2,7)-PAllo are more deshielded due to a diamagnetic anisotropic effect and the presence of continuous conjugation and appear at the more downfield region. 13 C NMR Data of PAllo. The following are the 13C NMR data of PAllo (CDCl3, 600 MHz): δ 15−30 (C-1*, C-2″, C-7, C-7*, C-8, C-8*, C-6′, C-6″, C-2*, C-1, C-4, C-2′) and 124− 138 (C-2, C-3, C-5, C-6, C-3*, C-4*, C-5*, C-6*). The 13C NMR spectrum shows several overlapped peaks and a few sharp peaks with low intensity. The sharp peaks in the region of δ 15−30 ppm are due to the presence of all of the saturated

of the protons on the polymer were used to confirm the molecular structure of the polymer. Taking these two peaks (δ 6.4 and 4.4 ppm) into account, it is estimated that the redoxinitiated polyalloocimene consists of 30−35% 4,7-PAllo and 65−70% 2,7-PAllo structure. 1 H NMR Data of PAllo. The following are the 1H NMR data of PAllo (CDCl3, 600 MHz): δ 6.4 (4*H), 6.0 (5*H), 5.7 (3*H), 4.4 (3H, 5H), 3.2 (4H), 1.6−1.9 (1H, 2′H, 7H, 7*H, 6′H, 6″H), 1.09−1.45 (8H, 8*H, 1*H, 2″H). The protons of the methyl groups (C8, C8*, C1*, C2″) of both of the microstructures appear as broad peaks typically in the upfield region around δ 1.09−1.45 ppm. The protons attached to carbon (C1, C2′, C7, C7*, C6′, and C6″) in both the microstructures show a chemical shift around δ 1.6−1.9 ppm due to the double bond attached to the respective carbon atoms. The upfield chemical shift of all these protons when 7663


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Figure 4. (a) 1H NMR of PAllo. (b) 13C NMR of PAllo.

carbon atoms and methyl carbons in both of the microstructures. The more downfield peaks in the region of δ 124− 138 ppm arise due to olefinic (CH−) carbon atoms present in the synthesized polymer. A two-dimensional COSY (1H−1H long-range correlation) experiment was carried out to identify all of the assigned protons. As can be seen in Figure 5a, the diagonal peaks have the same frequency coordinate on each axis and appear along the diagonal of the plot, while cross-peaks appear off the diagonal that indicates couplings between pairs of nuclei. We found that the olefinic signal (4*H) was coupled with two other signals (1H each) of 5*H (δ 6.0 ppm) and 3*H (δ 5.7 ppm) and vice versa for the 2,7- PAllo microstructure. The adjacent nuclei are coupled together, giving two symmetrical cross-peaks above and below the diagonal. The correlation spot

between the protons of the methyl group was not well observed due to the overlapped diagonal peak. In the case of the 4,7 microstructure, one signal was coupled with the methyl proton (8H), which corresponds to olefinic signal 3H (δ 4.4 ppm) superimposed with another one, presumably 5H. A two-dimensional NOESY (1H−1H) spectrum was taken to provide information about the spatial coupling between magnetically different protons that are in close proximity to each other in the polymer. Figure 5b presents the NOESY spectrum of PAllo giving both diagonal and cross-peaks. The cross-peaks connect resonances from nuclei that are spatially close to each other. We found that in the 2,7-PAllo microstructure the olefinic protons (3*H, 4*H, and 5*H) are spatially coupled with each other and with nearby methyl protons (2″H, 1*H) to give low-intensity cross-peaks. In the 7664


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Figure 5. (a) COSY (1H−1H) spectra of PAllo. (b) NOESY (1H−1H) spectra of PAllo.

However, to affirm the isomeric forms present in the monomer and their effect, polymerization was carried out with the distilled monomer and the residue. A few representations of polymer were made from the distilled monomer in order to investigate the effect of the fraction in polymerization. The distilled monomer contains mainly a mixture of isomers and is verified from the NMR spectra given in Figure S2 (SI). The residual 20% contains several overlapped peaks that could not be identified accurately from the NMR spectra (Figure S3, SI). The polymerization reaction carried out with the distilled monomer (Allo) resulted

case of 4,7-PAllo, one cross-peak signal was due to a methyl proton (8H) spatially coupled with olefinic proton 3H (superimposed with another one, 5H). The correlation spot between the protons of the methyl group was not well-assigned due to overlapping with the diagonal peak. Polymerization with Distilled Monomer and under Various Conditions. As per the literature report,34 the monomer (alloocimene) mainly exists in two isomeric forms: trans−cis [also called cis,(4E,6Z)] and trans−trans [also called trans,(4E,6E)]. The polymerization products will be a mixture depending on the ratio of trans−cis and trans−trans isomer. 7665


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Figure 6. (a, a′) The ground-state optimized structure of alloocimene (trans−trans and trans−cis) in the B3LYP/6-331G(d,p) basis set. Carbon, gray; hydrogen, white. (b, b′) Charge distribution with colored atoms on different carbons of alloocimene (trans−trans and trans−cis). (c, c′ and d, d′) Molecular orbitals and their energies of two isomers of alloocimene (HOMO and LUMO) in the ground state. (e, e′) Electrostatic potential map of trans−trans and trans−cis Allo molecule.

in the formation of a rubbery-type polymer and the remaining residual 20% follows the formation of a very small amount of the polymer. The NMR spectral assignments of the polymers are presented in the SI (Figures S4 and S5). The polymerization was explored using other redox-initiating systems given in Table S1 (SI). But SHS was not used in the polymerization, as it was not a sustainable reagent due to the evolution of the toxic organic compound formaldehyde. APS was also not used as an initiating system because the conventional redox polymerization uses peroxide as the free radical generator.

High-temperature redox polymerization (temperature 65−80 °C) using the recipe listed in Table 1 was studied, but there was no formation of the polymer. Thermal polymerization of monomer was also carried out in the temperature range 75− 200 °C, but again no polymer was formed. It revealed the existence of monomer itself, which was verified from the obtained FTIR spectra (Figure S6, SI) of the resulting solution at 200 °C. The reason was due to the depolymerization reaction at higher temperature or longer time. Density Functional Theory Calculation. The purpose of this study was to investigate the electronic structure (particularly the ground state) and to understand the 7666


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ACS Sustainable Chemistry & Engineering relationship between the chemical structure and the reactivity of the molecule. DFT35 calculation was performed with Becke’s three-parameter hybrid functional (B3LYP) method using 6311G(d,p) as a basis set. E(B3LYP) stands for exchange− correlation energy, Becke’s three-parameter, and Lee−Yang− Parr exchange−correlation function. Figure 6a,a′ represents the optimized ground-state structure of two isomers of alloocimene (trans−trans and trans−cis). The value of the ground-state dipole moment of Allo calculated at the B3LYP/6-311G(d,p) level is 0.3924 D for trans−trans and 0.4404 D for trans−cis isomers. The value on the atoms indicates the Mulliken charges. The more negative value indicates a high charge density on the atoms, making the position avid toward initiation of polymerization. The negative value on the carbon (C-2, C-4, C-7) of the trans−trans isomer of Allo suggests the formation of (2,7)- and (4,7)-PAllo and the negative value on the carbon (C-4, C-7) of the trans−cis isomer predicts the formation of (4,7)-PAllo microstructures. Figure 6b,b′ displays the charge distribution in both the isomers of alloocimene with different colored atoms. It displays a color range with red and dark red atoms indicating high charge density and green atoms with low charge density, specifying the preferable position of polymerization. The molecular orbitals (HOMO and LUMO) are also represented in Figure 6c,c′,d,d′ and indicated that the electron density in both HOMO and LUMO are spread over an entire area of the conjugated system. On the basis of the B3LYP/6-311G(d,p) calculation, the energy of HOMO and LUMO is found to be −0.19855 and −0.04062 eV for trans−trans and −0.20033 and −0.04477 eV for trans− cis isomers, respectively, thereby indicating a possible charge transfer from HOMO to LUMO and making the molecule very keen to polymerize. Figure 6e,e′ presents the electrostatic potential mapping of alloocimene isomers. This method was used to evaluate the electron distribution36 around the molecular surface and to predict sites and relative reactivity toward electrophilic attack. From the ESP mapping, it is clear that the whole conjugated (C2−C7) site is surrounded by an equal density of negative charge, thus making the area potentially more prone toward polymerization. Thus, from DFT analysis it is evident that polymerization proceeds through all the unsaturation present in the molecule, thereby leading to the formation of different possible microstructures, predominantly 4,7 and 2,7-PAllo. The polymerization sites produce linear-shaped 1-D polymer (2,7-PAllo) mostly, and due to the presence of three olefinic double bonds, many side reactions as well as cross-linking between the growing chains (due to the presence of residual unsaturation in the pendent group of 4,7-PAllo) may take place, resulting in the formation of network-type polymer. However, only 2−3% gel fraction was extracted using THF as a solvent. Thermal Analysis of the Synthesized Polymer. Thermal analysis was carried out to study the property of a polymer with a change in temperature. The glass transition temperature of a polymer plays a vital role, whether it is amorphous or crystalline at a specified temperature. The Tg of a material characterizes the range of temperatures over which this glass transition occurs. For elastomers, an ideal Tg should be lower than room temperature, because an elastomer is glassy and not elastic at temperatures lower than Tg. The Tg value of the polymer PAllo was determined by DSC. The samples used for thermal analysis were free of unpolymerized monomer. Figure 7 represents the DSC thermogram after the second heating run, which shows a sharp transition around −17 °C in the baseline of the recorded

Figure 7. DSC thermogram of PAllo.

DSC signal. No effect on Tg of the polymer was observed due to a negligible amount of gel, when the samples with and without 2−3% gel were compared. So, the glass transition temperature (Tg) of the PAllo was taken as −17 °C, thereby implying the amorphous and rubbery nature of the polymer. PAllo was reported earlier as a low-melting (85−87 °C), semicrystalline, and highly cross-linked polymer in nature and the polymerization yielded mostly 6,7- and 4,7-enchainment with little cyclic structures, while our present work with the synthesized polymer reports an amorphous, rubbery polymer (Tg = −17 °C). This makes this work novel. Anionic polymerization of Allo yielded a mixture of polymers with 2,3- and 6,7-units and a minority amount of 4,7-PAllo, displaying a wide range of Tg. The product of the living carbocationic polymerization of Allo showed a high molecular weight with 4,7- and 2,7-enchainment, contradicting the results of our study of free radical polymerization, yielding predominantly 2,7- and 4,7-PAllo units. Despite all of the results obtained, the difference might be due to the microstructure of the polymer unit and of course a matter of polymerization technique. One was due to the regioselective formation of polymer microstructures and cyclopolymer (carbocationic polymerization) yielding a semicrystalline nature, as reported earlier,20−22 and the other was due to 1,4 and vinyl-type polymerization (emulsion polymerization) involving the conjugated system yielding an amorphous nature, as reported now. The mechanism is shown in Scheme 2. It is interesting to note that the same monomer gives different polymer depending on the polymerization technique. In one case, it is a rubbery polymer, while it is a plastic in the other case. In order to confirm the Tg, dynamic mechanical analysis of the synthesized polymer (PAllo) was carried out. The tan δ vs temperature plot (Figure S7, SI) shows a tan δ peak at around 6 °C, which is lower than the room temperature. The numerical value is a little higher than that in the DSC analysis. The higher value of Tg than from DSC analysis is due to the dynamic heating rate in the experiment. Similar observations have been made by many researchers on this subject.37 The degradation property of the polymer was evaluated using thermogravimetric analysis (TGA). Figure 8 represents the thermal decomposition curves for the polymer. The synthesized polymer showed a two-step degradation pattern. The sample weight dropped after the onset of degradation (corresponding to 5% weight loss) until it reached a temperature of ∼130 °C, where the polymer showed the maximum weight loss due to the degradation of the 2,7-structure (more unsaturation in the main chain). This is followed by degradation of the 4,7structure over 200 °C, beyond which the polymer degrades 7667


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Scheme 2. (a) Mechanism of Free Radical Emulsion Polymerization and (b) Cationic Cyclo-Polymerization of Alloocimene20

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Tel: +91 3222 283180. Fax: +91 3222 220312. ORCID

Anil K. Bhowmick: 0000-0002-8229-5353 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank IIT Kharagpur for providing the necessary facilities. Pra.S. is thankful to CSIR (HRDG), New Delhi, for providing financial assistantship in the form of a junior research fellowship [Ref. No. 20/12/2015(ii) EU-V]. Pra.S. and A.K.B. also acknowledge the partial support of Central Research, Bridgestone Corp., Japan.

Figure 8. TGA thermogram of PAllo.

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quickly. The initial low-temperature weight loss or degradation was due to the faster degradation of the 2,7-structure because of the presence of more unsaturation in the main chain of the polymer.

(1) Gandini, A. Polymers from Renewable Resources: A Challenge for the Future of Macromolecular Materials. Macromolecules 2008, 41, 9491−9504. (2) Yao, K.; Tang, C. Controlled Polymerization of Next-Generation Renewable Monomers and Beyond. Macromolecules 2013, 46, 1689− 1712. (3) Coates, G. W.; Hillmyer, M. A. Polymers from Renewable Resources. Macromolecules 2009, 42, 7987−7989. (4) Biermann, U.; Bornscheuer, U.; Meier, M. A. R.; Metzger, J. O.; Schäfer, H. J. Oils and Fats as Renewable Raw Materials in Chemistry. Angew. Chem., Int. Ed. 2011, 50, 3854−3871. (5) Montero de Espinosa, L.; Meier, M. A. R. Plant oils: The perfect renewable resource for polymer science?! Eur. Polym. J. 2011, 47, 837− 852. (6) Belgacem, M. N.; Gandini, A. Monomers, Polymers and Composites from Renewable Resources; Elsevier: Amsterdam, 2008; Chapter 2. (7) Erman, W. F. Chemistry of Monoterpenes; Marcel Dekker, New York, 1985. (8) Breitmaier, E. Terpenes: Flavors, Fragrances, Pharmaca, Pheromones; Wiley-VCH Verlag GmbH: Weinheim, 2006. (9) Connolly, J. D.; Hill, R. A. Dictionary of Terpenoids; Chapman & Hall: London, 1991. (10) Finar, I. L. Organic Chemistry. Stereochemistry and The Chemistry of Natural Products; Longmans Green and Co. Ltd.: London, 1964; Vol. 2, Chapter 8.

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CONCLUSIONS On the basis of the above evidence, it is concluded that PAllo was synthesized via redox emulsion polymerization of Allo using TBHP as an initiator. The PAllo synthesized displayed the molecular weight of 14 200 Da and a subzero glass transition temperature of −17 °C with an amorphous, rubbery nature. Spectroscopic measurements combined with DFT calculation confirmed the participation of conjugated double bond in the polymerization process.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00990. Picture of PAllo polymer; alternate recipe used for redox emulsion polymerization; 1H NMR spectra of distilled Allo, Allo residue, distilled PAllo, and PAllo residue; FTIR spectra of Allo and Allo at 200 °C; and DMA thermogram of PAllo (PDF) 7668


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