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A Green Approach Towards Sustainable Polymer: Synthesis and Characterization of Poly(myrcene-co-dibutyl itaconate) Preetom Sarkar , and Anil K. Bhowmick ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01591 • Publication Date (Web): 08 Mar 2016 Downloaded from http://pubs.acs.org on March 11, 2016

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ACS Sustainable Chemistry & Engineering

A Green Approach Towards Sustainable Polymer: Synthesis and Characterization of Poly(myrcene-co-dibutyl itaconate)

Preetom Sarkar and Anil K. Bhowmick* Rubber Technology Centre, Indian Institute of Technology Kharagpur Kharagpur- 721302, West Bengal, India

*Corresponding author: Email: [email protected] (AKB) Tel.: +91 (3222) 283180; Fax: +91 (3222) 220312

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ABSTRACT: Motivated by an epidemic spur in developing sustainable polymers, we report here a series of bio-based random rubbery co-polymers from β-myrcene and dibutyl itaconate, which have potential to substitute commercial rubbers. These were prepared by an environmentally benign emulsion polymerization method. The copolymers had predominately 1,4 -cis and –trans polymyrcene in their microstructure and molecular weight in the range of 13, 330 - 64, 700 Da. The copolymers displayed a sub ambient glass transition temperature between -60.3 to -33.5 °C depending on the weight percent of β-myrcene. Morphological studies unveils random placement of the two phases in the copolymer. The copolymers also showed reasonably good thermal stability over the individual homo-polymers and pseudoplastic flow behavior. Monitoring of the polymerization reaction at different times and temperatures revealed first order reaction pathway. A plausible mechanistic insight into the emulsion copolymerization was provided. The reactivity ratio of the monomers indicated quasi ideal copolymerization behavior. Molecular dynamics simulation predicted the spatial arrangements of the macromolecular chains and glass transition temperature of the copolymers, which are in accord with the experimental findings. The current study would thus pave a way for synthesizing terpene-based elastomers as a sustainable material of the future.

KEYWORDS: Bio-based polymer, β-myrcene, Di-butyl itaconate, Emulsion polymerization, Molecular dynamics simulation

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INTRODUCTION The impregnable urge for sustainable development has clinched the attention of macromolecular science and technology in recent years. Unlike the bleak illustration of the future of fossil derived materials, the renewable supplements have been able to invoke an appealing sketch in both academia and industry. Great attempts have been made in the contemporary time to develop new polymers and replace the fossil-derived ones by sustainable alternatives.1-7 Recently, terpenes have emerged as a promising building block in the realm of sustainable polymers, as these are obtained in plenty from various plants.8 Amongst the vast reserve of terpene family, βmyrcene (7-methyl-3-methylene-octa-1, 6-diene), having a conjugated diene structure, is being investigated quite inquisitively. The importance of this topic is revealed by few recent publications. For example, Liu et al. demonstrated homo and co-polymerization of β-myrcene using a highly active Lutetium catalyst.9 Bolton et al. prepared triblock polymer utilizing αmethyl-p-methylstyrene and β-myrcene by a living polymerization method.10 Loughmari and Georges et al. reported stereo-selective polymerization of β-myrcene utilizing a Lanthanide based catalyst and studied their microstructure by precise high-field nuclear magnetic resonance spectroscopy.11-13 We have recently reported a preliminary study of synthesis of polymyrcene by a green technique and structure-property relationship of this polymer.14 The synthesized polymer displayed a glass transition temperature of -73 °C, good thermal stability and pseudoplastic flow behavior. While this polymer has a great future potential to substitute or compete with conventional polymers, a few drawbacks have been observed, viz. non-polar nature, poor tensile strength etc. A host of modification technique, mostly based on non-renewable chemicals, is adopted to enhance the polarity of the otherwise non polar natural rubber. Our objective is to use a bio-based material for the modification of the hydrocarbon polymer. Although attempts have



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been made to alter the chemical structure of synthetic isoprene, there is no attempt in this field so far. Itaconic acid or methylene succinic acid is a prospective sustainable material and thus is considered as one of the most promising building blocks in fabricating renewable polymers. In an industrial scale, it can be produced by fermentation of carbohydrate biomass (glucose, sucrose, molasses, starch etc.) using Aspergillus terreus and Aspergillus itaconicus and also from the distillation of citric acid.15,16 Owing to its unique chemical structure (being a di-carboxylic acid as well as di-substituted olefin), robust large scale productivity and low cost, the US Department of Energy has enlisted itaconic acid as a ‘top value-added chemicals from biomass’.17 The di-esters of itaconic acid can be readily polymerized and has been exploited over time to prepare polyesters, bearing pendent side chains with varying length (from methyl to eicosyl) and structure (linear, branched and cyclic).18-20 The nature and type of side chain has profound effect on the reactivity, glass transition temperature, dielectric relaxation, local crystallinity and tacticity of the itaconate polyesters.21-24 These early investigations pave opportunities to tailor sustainable polymers based on itaconate esters.25-28 Coupled with the efficacy of free radical mechanism (wider range of monomers, low cost initiators, broad choice of dispersion medium), environmentally benign emulsion polymerization (solvent-less and commercially viable) technique is a desirable methodology for preparing such sustainable polymers. In recent times, only handful of literature has been found to deal with this classical polymerization technique for synthesizing sustainable polymers. Wang et al. reported a biobased polymer by emulsion polymerization method, wherein they had utilized petro-derived isoprene as a co-monomer.29 To the best of our knowledge, no one has attempted to prepare copolymer of β-myrcene and itaconate ester by a solvent-less green synthetic route. In


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continuation with our attempt to prepare a fully bio derived polymer, we report herein persulfate initiated emulsion copolymerization of β-myrcene (MY) and dibutyl itaconate (DBI) (both the synthons are based on renewable resources) via emulsion polymerization technique. The structure of the as synthesized copolymers was characterized by various spectroscopic techniques. Attempt has been made to explain the plausible course of the emulsion copolymerization reaction. The reactivity ratios of the two monomers were determined by Fineman-Ross (FR) and Kelen-Tüdös (KT) methods. Thermal and rheological properties as well as the morphology of the copolymers were investigated. In order to have an insight into the spatial disposition of the copolymer chains, molecular dynamics (MD) simulation was performed. The particle nature of the emulsion latices was also examined.

EXPERIMENTAL SECTION Materials. Monomers - dibutyl itaconate (DBI, 98% purity) and β-myrcene (MY, 99% purity) were purchased from Sigma Aldrich Chemical Company. The inhibitor, butylated hydroxytoluene was removed from β-myrcene by shaking with 2(M) NaOH solution. Sodium bicarbonate (NaHCO3, 99% purity) and ammonium persulphate (APS, 98% purity) were procured from E. Merck (India). Sodium dodecyl sulphate (SDS, 99% purity) was obtained from Loba Chemie, India and used without further purification. All other chemicals were reagentgrade commercial products and used as received. Deionised water (DI) was utilized for all the experiments. Characterization. The molecular weight of the synthesized copolymers 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



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(THF) was used as the eluent (sample concentration 1 mg/ml) at a flow rate of 1 ml/min and polystyrene standard was used for calibration. The gel fraction of the polymers was determined by solvent extraction using THF under reflux condition for 8 h. The gel percentage was calculated as the ratio of dried polymer weight to its original value. The particle size and the distribution of the diluted emulsion latex were measured by 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 degree. The particle nature of the emulsion latex was ascertained by a Carl Zeiss MERLIN scanning electron microscope (SEM) equipped with a field emission gun at an accelerating voltage of 5 kV. Diluted emulsion latex was drop casted on an aluminium sheet and coated with gold prior to microscopy. The room temperature solubility behaviour of the synthesised polymers was evaluated (as 0.1 % (w/v)) in various common organic solvents such as tetrahydrofuran (THF), chloroform (CHCl3), dimethyl sulphoxide (DMSO), methyl ethyl ketone (MEK), toluene, N-methyl-2-pyrrolidone (NMP) and ethyl acetate (EA). The Fourier transform infrared (FT-IR) spectra of the synthesized copolymers were recorded in a Perkin Elmer Spectrum 400 machine (resolution 4 cm-1) using universal attenuated total reflectance (UATR) attachment within a spectral range of 4000 - 650 cm-1. A total of ten scans per sample were performed. 1H and

13

C nuclear magnetic resonance (NMR) spectrum was

recorded in an AVANCE III 400 Ascend Bruker instrument operating at 400 MHz at room temperature. The samples were dissolved in deuterated chloroform (CDCl3). Tetramethylsilane (TMS) was used as an internal standard and the chemical shift values were reported in δ(ppm) relative to the internal standard.


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A NETZSCH DSC 200F3 Maia® differential scanning calorimeter (DSC) was used to investigate the thermal transitions of the copolymers under nitrogen atmosphere. The samples were heated from -80 °C to +20 °C at a heating rate of 10 °C/min. In order to erase thermal history, the glass transition temperature (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. Molecular dynamics (MD) simulation was executed using Dassault Systèmes BIOVIA Material Studio v8.0.100.21 software.30 At first, the polymer chains were built with respective repeat units, and thereafter cubic simulation boxes were generated by the Amorphous Cell module. Prior to MD simulation, Forcite module was used to optimize the geometry of the structures. X-ray diffraction (XRD) analysis was carried out in the range of 2θ from 10° - 50° (scan rate 3° per min) using X'Pert PRO machine from PANalytical company with CuKα (0.154 nm) as radiation source at 40 kV and 30 mA. Morphology of the copolymers was examined by an atomic force microscope (AFM) (Agilent Technologies USA, model 5500) operating under tapping mode. The cantilever tip was made of etched silicone with a radius of curvature of 10 nm. The resonance frequency and force constant of the tip were 150 kHz and 42 N/m respectively. 0.1% (w/v) solution of the copolymer was prepared in chloroform. The copolymer solution was then drop casted onto small glass wafer followed by drying at ambient condition for 72 h. High resolution transmission electron microscopy (HRTEM) imaging was performed in a JEOL JEM 2100 machine having a point to point resolution of 0.194 nm and lattice resolution of 0.14 nm. A very dilute solution (0.001% (w/v) in CHCl3) of the copolymer was drop casted by an insulin syringe onto a 300 mesh carbon



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coated copper grid and the solvent was allowed to evaporate slowly over 5 days at room temperature. The rheological measurements of the copolymers were performed using a MCR302 Anton Paar modular compact rheometer. PP08 plate was used with a 1.1 mm gap in between the plates. Temperature sweep experiments were carried out at 0.1 % constant strain and 6.28 Hz frequency in between 25 to 125 °C temperature employing a ramp rate of 3 °C/min. Temperature control was done by using P-PTD 200 measuring cells. Frequency sweep experiments were conducted at 25 °C at a constant strain of 0.1 % in the frequency range between 0.0628 to 628 Hz. The temperature sweep experiment of the copolymer was performed in Perkin Elmer DMA 8000 equipment in the temperature range of -90 to +30 °C at a ramp rate of 3 °C/min and 1 Hz frequency. Single cantilever bending mode with material pocket sample holder was used. The sample was of rectangular shape having a dimension of 7mm (width) x 3mm (length) x 1.5mm (thickness). Only glass transition temperature was reported from this experiment. Thermogravimetric analysis (TGA) was carried out using a TA Instrument SDT Q600 equipment under nitrogen purging and at a heating rate of 10 °C/min. Synthesis of MY-DBI copolymers by persulfate initiated emulsion polymerization. Persulfate initiated emulsion co-polymerization of β-myrcene (MY) and dibutyl itaconate (DBI) was carried out following the recipe mentioned in Table 1. At first, the emulsifier (SDS), buffer (NaHCO3) and DI water were charged into a round bottom flask equipped with a magnetic stirring bar. The ingredients were mixed thoroughly at 250 rpm for 20 min at 30 ºC so as to obtain stable micelles. Thereafter, β-myrcene monomer was added into the reaction vessel slowly over a period of 10 min followed by the addition of dibutyl itaconate. The reaction mixture was left as such for a further 20 min to form a stable emulsion. Then, the reactor was sealed and


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flushed with nitrogen to create an inert blanket and the temperature was set to 70 ºC. Subsequently, aqueous solution of ammonium persulfate (APS) was added into the reaction mixture drop-wise over 20 min in two successions. The polymerization reaction was allowed to proceed for 20 h to obtain a stable latex. Poly(MY-co-DBI) copolymers were moved out from the latex by pouring into large volume of acidified ethanol with vigorous stirring. The coagulum was then washed profusely with DI water and dried at 40 ºC in vacuum for 24 h. The polymerization reaction is depicted in Scheme 1. MY and DBI were also polymerized separately under the same reaction condition for comparison. Table 1. Recipe for Persulfate Initiated Emulsion Co-polymerization.

#

ingredients

amount (g, in phr #)

β-myrcene (MY)

variable

dibutyl itaconate (DBI)

variable

deionised (DI) water

250

sodium dodecyl sulphate (SDS)

2.5

ammonium persulfate (APS)

0.35

sodium bicarbonate (NaHCO3)

1.5

phr is the abbreviation for parts per hundred parts of rubber



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Scheme 1. Synthetic Route Towards Poly(MY-co-DBI).

RESULTS AND DISCUSSION Optimization of the reaction conditions. Persulfate initiated emulsion copolymerization of βmyrcene and dibutyl itaconate was carried out at 70 °C for 20 h. In order to find out the optimized set of reaction conditions, the reaction time and temperature were varied. Figure 1a-b

Figure 1. (a) Temperature and (b) Time dependence of poly(MY50DBI50) synthesis on yield and molecular weight. 10 ACS Paragon Plus Environment

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represents the variation of percentage yield and molecular weight with varying temperature and time respectively for poly(MY50DBI50) copolymer synthesis. At higher reaction temperature (greater than 70 °C) chain depolymerization prevails, thereby reducing the percent yield and molecular weight values of the copolymer (Figure 1a). The optimum reaction temperature was thus set as 70 °C. In order to study the effect of time, the polymerization reaction mixture (at 70 °C) was taken out at different time intervals. The molecular weight and yield percentage of the copolymer samples were then measured. With increasing reaction time, the molecular weight and yield percentage were found to increase and thereafter showed a decreasing trend after a maximum. As in case of temperature, the decreasing trend of molecular weight at higher reaction time can be attributed to the scission of the macromolecular chains. Accordingly, the optimum reaction conditions were 70 °C for 20 h, which were used to prepare all the copolymers. The homopolymers were also checked and the same optimization conditions were obtained (see Figures S1-S2, Supporting Information). We have thus prepared a series of copolymers having different weight percentage of the β-myrcene moiety. Figure S3 (Supporting Information) depicts the kinetic plot for the polymerization of MY, DBI and their 50/50 copolymer. It is evident that the rate for homo-polymerization for β-myrcene alone is the slowest (kapp = 6.9×10-3 min-1) and the copolymer shows reasonably higher rate (kapp = 2.51×10-2 min-1), whereas the rate for homo polymerization of DBI (kapp = 1.22×10-2 min-1) is in between the two. This is explained later with the help of size of the micelles and relative solubility of the monomers in the aqueous phase. The kinetic plots follow the first order reaction pathway. The molecular weight, PDI, percentage yield, gel content and latex particle size are presented in Table 2, where the subscript denotes the weight percent of the respective comonomer.



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Table 2. Molecular Weight, PDI, Gel Content, Yield and Latex Particle Size of Persulfate Initiated Poly(MY-co-DBI) Copolymers. a

polymer samples

# a

Mn (Da)

a

b

PDI

% gel

% yield

Z-average diameter (nm)

Poly(MY100DBI0)

92, 900 (±205)#

2.50

12

95 (±0.5)#

91.4 (±3)#

Poly(MY90DBI10)

13, 330 (±270)

1.04

10

80 (±0.4)

61.7 (±2)

Poly(MY80DBI20)

17, 400 (±265)

1.18

8

85 (±0.5)

61.0 (±3)

Poly(MY70DBI30)

29, 820 (±240)

1.17

7

88 (±0.6)

62.6 (±2)

Poly(MY60DBI40)

55, 260 (±190)

1.46

6

90 (±0.8)

67.6 (±4)

Poly(MY50DBI50)

64, 700 (±220)

1.74

6

93 (±0.5)

87.3 (±4)

Poly(MY30DBI70)

44, 200 (±210)

1.72

4

79 (±0.7)

64.0 (±5)

Poly(MY0DBI100)

22, 500 (±175)

1.52

0

89 (±0.4)

59.7 (±2)

Values in the parenthesis indicate the standard deviation based on three measurements. Values obtained from GPC measurement; b Values obtained from DLS measurement.

Although the addition of DBI has an incremental effect on the rate of copolymerization (rate gets almost doubled compared to the individual homo-polymerization reaction), the molecular weight values of some copolymers are lower. As the DBI monomer was introduced into the polymerization system [Poly(MY90DBI10)], the molecular weight of the resulting co-polymers was found to decrease compared to the individual homopolymers [Poly(MY0DBI100) and Poly(MY100DBI0)]. Thereafter, with increasing amount of DBI percentage, the molecular weight of the copolymers first increases up to a maximum of 64,700 Da for poly(MY50DBI50), and then 12 ACS Paragon Plus Environment

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decreases for poly(MY30DBI70), which are below the molecular weight of poly(MY100DBI0) (92,900 Da). Despite the relative simplicity of the process, emulsion co-polymerization is a combination of several complicated mechanistic events and is governed by the interplay of various factors. Different parameters including polarity, partitioning nature and water solubility of the monomers greatly influence the emulsion co-polymerization behaviour.31 Figure 2 represents a schematic illustration of various plausible intervals of the emulsion copolymerization reaction. The reaction medium comprises of the following components: emulsified MY and DBI droplets (monomer reservoirs), water-soluble APS initiator, free surfactant molecules (SDS) and monomer-swollen micelles (miceller monomer). It was believed that due to hydrocarbon nature, the interiors of most of these micelles were predominantly occupied by MY monomers. On the other hand, being a polar monomer, DBI has relatively higher propensity to solubilize in water. After the thermal decomposition of the initiator, the generated water-born free radical preferentially propagates with the more water-soluble DBI monomer and produces the oligomeric radicals (Interval I). Upon attaining a critical chain length, these oligomeric radicals become relatively water insoluble and tend to enter monomer-swollen micelles to continue propagation therein (micelle nucleation).32,33 Owing to the polar DBI unit, the oligomers would be bit reluctant to diffuse into the micelles, thereby delaying (less facilitated) the nucleation tendency and hence chain propagation (Table 2). Nevertheless, once the active radical makes an entry within the micelle, the monomers would be consumed by the radical and consequently the monomerswollen micelles are successfully converted into particle nuclei (Interval II).



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Figure 2. Schematic representation of emulsion copolymerization of MY and DBI in three different intervals.

Further propagation at this stage requires aqueous phase diffusion of the monomer from the monomer reservoirs to these nuclei. Due to polar nature, the diffusion of DBI is believed to take place with more ease than MY. However, it is to be noted that, at the same time the polar DBI molecules will be less prone to penetrate into the hydrophobic core of the particle nuclei. On the contrary, the presence of two electron withdrawing carbonyl groups make the DBI free radical


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more stable than that of the MY. This makes the DBI free radicals comparatively less avid towards chain propagation. Thus, pictorially, after the nucleation process, the polymerization locus gets crowded by DBI, and restricts the participation of MY unit in the polymerization process. This phenomenon impedes the growth of the polymer chain and thus reducing the molecular weight of the copolymer lower than that of polymyrcene. Once the monomer reservoirs are exhausted, the remaining monomers within the growing particles are polymerized (Interval III). This is further supported by the lower Z-average diameter values of the poly(MY90DBI10) copolymer micelles. With further increment in percent DBI i.e. from poly(MY80DBI20) to poly(MY50DBI50), an increased flux of active radicals is expected in the reaction medium. Despite the fact that a few of these radicals will tend to undergo mutual termination, the increased overall radical flux would be able to draw the MY monomers to the growing nuclei. Once this is done, due to hydrocarbon nature the penetration of the MY molecules would be facilitated and hence higher growth of the polymer chains. Thus, the molecular weight increases monotonously and so does the average diameter of the latex particles (Table 2 and Figure S4, Supporting Information). It is anticipated that with still higher percentage of DBI monomer [poly(MY30DBI70)], homogeneous nucleation predominates over the miceller nucleation due to the presence of more polar species in the system. This reduces the molecular weight of poly(MY30DBI70) to 44,200 Da. The polydispersity index (PDI) of the synthesized polymers was in between 1.04 - 2.50. The gel content of the copolymers was also found to increase with increase in weight percent of βmyrcene moiety. Presence of double bond in the β-myrcene unit triggers the formation of microcrosslinks in between the polymer chains and hence increases the gel percent. The yield of the



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copolymers remains in the range from 80% to 93%. The yield increases with increase in DBI content in the copolymers. The average particle size of the poly(MY-co-DBI) latices were measured and the Z-average diameter of the samples was in between 61.7 - 87.3 nm. The histogram plots based on intensity measurement are presented in Figure S5 (Supporting Information). Further, the particle nature of the copolymer latex was supported by FESEM image (Figure S6, Supporting Information). Based on 25 measurements, the average diameter of the poly(MY50DBI50) latex particles was found to be ~ 90 nm, which is in line with the value obtained from the DLS measurements (87 nm). The solubility behavior and the theoretical solubility parameter, calculated based on the method proposed by Hoftyzer and van Krevelen34 (see calculation of theoretical solubility parameter in Table S1, Supporting Information) of the synthesized copolymer - poly(MY50DBI50) and the homo polymers are presented in Table 3. Owing to the inclusion of polar di-butyl itaconate moiety, the 50/50 copolymer - poly(MY50DBI50) shows improved solubility in common polar aprotic solvents like MEK and NMP over pristine polymyrcene. However, the copolymers also showed reasonably good solubility in CHCl3 at room temperature and thus for all the morphological measurements, the CHCl3 solution of the copolymer was used. Chloroform as a solvent also facilitates easy drying of the samples at room temperature. It was observed that the calculated solubility parameter (Table 3) for the 50/50 copolymer lies in between the two homopolymers. The calculated value also resides close to those of THF (18.6 MPa1/2), CHCl3 (19.0 MPa1/2), MEK (19.0 MPa1/2) and toluene (18.2 MPa1/2) in which the polymers display reasonable solubility.


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Table 3. Solubility Behavior and Theoretical Solubility Parameter of the Synthesized Polymers. solvents (solubility parameter, MPa1/2)b

polymer samples (theoretical solubility parameter, MPa1/2)a

THF

CHCl3

DMSO

MEK

Toluene

NMP

EA

(18.6)

(19.0)

(29.7)

(19.0)

(18.2)

(22.9)

(18.6)

Poly(MY100DBI0) (16.8)

±

±

-

-

s

-

-


+

+

±

+

±

+

+


±

±

s

s, ±

±

±

±

+: completely soluble, ±: partially soluble, -: insoluble, s: swells but not soluble a

Values are calculated by van Krevelen method, reference 34.

b

Values are adapted from reference 35.

FTIR and NMR characterization of the synthesized poly(MY-DBI) copolymers. The chemical structure of the poly(MY-DBI) copolymers were confirmed by FTIR and NMR spectroscopy. Figure 3 represents the FTIR spectra of the synthesized homo- and co-polymers and Table S2 (Supporting Information) collates the characteristic FTIR peaks of the copolymer. The characteristic C=O vibration peak of the ester bond of di-butyl itaconate was obtained at 1730 cm-1 (Figure 3a). The retention of this peak in the poly(MY50DBI50) copolymer indicates the successful inclusion of the itaconate moiety into the copolymer structure. Due to the presence of double bond in DBI monomer, two peaks arise for C-O-C bond, viz. 1143 cm-1 (C=O group in conjugation with C=C) and 1187 cm-1 (C=O group without conjugation with C=C).



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Figure 3. (a) FTIR spectra of homo and copolymers (b) FTIR spectra of of poly(MY-DBI) copolymers with varying DBI content.

After polymerization, the two peaks merge to a single broad one at 1171 cm-1. The characteristic stretching peak for C=C at 1641 and 816 cm-1 almost disappears in the copolymer, confirming the exhaustion of double bond during the polymerization process. However, as the copolymer contains residual unsaturation, small hump of these peaks is retained in the spectrum. The peak for conjugated double bonds at 1597 cm-1 in MY also disappears upon polymerization (Figure 3a). The absorption frequencies at 890 and 988 cm-1 in MY monomer are due to sp2 C-H bending of the two conjugated double bonds. The former peak arises for the germinal di-substituted olefinic centre, while the latter one arises for the mono-substituted alkene centre. Upon polymerization, these un-saturations are consumed and thus their signature peak decreases sharply. The weak peak at 3094 cm-1 for =C-H stretching in MY also disappears upon polymerization. The broad absorption peaks at 2960, 2925 and 2857 cm-1 are assigned to the stretching vibrations of –CH3, –CH2, and –CH groups respectively in the copolymer. These peaks arise due to the cumulative contribution of both MY and DBI units. Figure 3b illustrates 18 ACS Paragon Plus Environment

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overlay plot of the copolymers with varying DBI content. It is evident that the peak intensity at 1730 cm-1 (C=O stretching of ester bond) and 1171 cm-1 (C-O-C stretching after polymerization) increases with increase in DBI content. The 1H and

13

C NMR spectra of a representative copolymer - poly(MY50DBI50) is presented in

Figure 4a-b. For understanding the structure of the copolymer, NMR spectra of the monomers and their homo-polymers were also taken (Figure S7-S9, Supporting Information). The spectral signals are well assigned to various magnetically different protons and carbons. The chemical shift values of the relevant peaks in 1H and 13C NMR spectra are listed below: Poly(MY50DBI50): 1H NMR (CDCl3, 400 MHz): δ- 5.03 (4H, D, J, D' and J'- H), 3.97 (4H, e and f-H), 2.45 (2H, d-H), 1.94 (12H, F, G, F', G' and g-H), 1.58-1.30 (14H, A, E, A', E', a, b and j-H), 0.86 (18H, M, L, M', L' and k-H). Poly(MY50DBI50): 13C NMR (CDCl3, 400 MHz): δ- 175.7 (4-C), 171.8 (5-C), 131.5 (B, K, B' and K'-C), 124.9 (D, J, D' and J'-C), 64.6 (6-C), 48.5 (2-C), 43.0 (3-C), 37.1 (F and F'-C), 31.3 (A, A' and 1-C), 26.5 (E, G, E', G' and 7-C), 19.9 (8-C), 14.4 (M, L, M', L' and 9-C). Figure 4a illustrates 1H NMR spectrum of poly(MY50DBI50) copolymer. After polymerization, the peak for two olefinic protons at δ = 6.30 and 5.68 ppm of DBI monomer disappears completely. The two distinct triplet peaks of -OCH2 protons of DBI monomer (e and f-H) merge to a single signal at δ = 3.97 ppm after the polymerization, indicating the consumption of the double bond. In the 13C NMR spectra of DBI, the peaks at δ = 134.1 and 127.7 ppm (Figure S7, Supporting Information) represent the two olefinic carbon centers, which fade away after polymerization (Figure S8, Supporting Information). Absence of signature peaks at around δ = 4.5 and 5.5 ppm suggests that 1,2 vinyl and 3,4 addition units are not present in the copolymer microstructure (Figure 4a). Poly(MY50DBI50) predominately comprises of 1,4 cis and 1,4 trans



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Figure 4a. 1H NMR spectrum of poly(MY50DBI50).

Figure 4b. 13C NMR spectrum of poly(MY50DBI50).


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microstructure of the polymyrcene unit. In our previous investigation on polymyrcene microstructure,14 we could find the formation of both 1,2 vinyl and 3,4 addition units along with the 1,4 cis and 1,4 trans microstructure. The reason for the absence of 1,2 vinyl and 3,4 addition units can be attributed to the presence of sterically hindered di-butyl itaconate moiety as a comonomer which prevents the formation of such adduct. The signature peak for the -OCH2 proton at 3.97 ppm indicates the incorporation of itaconate unit into the copolymer. All the other methylene protons of both β-myrcene and di-butyl itaconate moiety appears as a broad conglomerate signature in the range of δ = 2.5 to 1.5 ppm. Figure 4b depicts the

13

C NMR

spectrum of poly(MY50DBI50) copolymer. The characteristics chemical shift values at δ ~155 and 116 ppm for 3,4 addition unit and 1,2 vinyl were absent. The signal for the two C=O groups (δ = 175.7 and 171.8 ppm) and -OCH2 units (δ = 64.6 ppm) points to the successful incorporation of the di-butyl itaconate component into the copolymer. The spectral region between δ = 37 to 14 ppm comprises of 13C signals due to methylene and methyl carbons of both β-myrcene and di-butyl itaconate moieties. Reactivity ratios of β-myrcene and dibutyl itaconate. The reactivity ratios of β-myrcene (MY) and dibutyl itaconate (DBI) in the persulfate initiated emulsion copolymerization were determined by two classical linear methods: Fineman-Ross36 (FR) and Kelen-Tüdös37 (KT). The KT method refines the FR method to equally weigh all data points and estimates the reactivity ratios through linear regression of the experimental data fit. The copolymer composition was determined from the area under the respective peaks in the 1H NMR spectrum. The FR and KT plots are shown in Figure 5 and the relevant parameters are presented in Table S3 (Supporting Information). Copolymerization of the two monomers leads to two types of propagating species – one having MY as the propagating end and the other having DBI. The asterisk (eqn. 1-4) 21 ACS Paragon Plus Environment


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represents a radical. Assuming the reactivity of the propagating species is dependent only on the monomer unit present at the end of the chain (i.e. the penultimate unit in the radical chain end does not influence the reactivity of the radical),38 the plausible propagation reactions are as follows: MY* + MY → MY-MY*, rate =

(1)

MY* + DBI → MY-DBI*, rate =

(2)

DBI* + DBI → DBI-DBI*, rate =

(3) (4)

DBI* + MY → DBI-MY*, rate = The reactivity ratios of the two monomers can be represented as:

) and

=

=

The instantaneous copolymer composition equation can be written as: (5) where [MY] and [DBI] refer to the monomer composition in the feed and 'my' and 'dbi' refer to the respective composition in the copolymer. Upon rearrangement, the copolymer composition equation can be rewritten as:

-

(f-1) = or, Y1 =

X1 -

(6)

where F = [MY]/[DBI] and f = my/dbi. A plot of Y1 as an ordinate and X1 as abscissa generates a straight line, whose slope and intercept provide

and

respectively.

The Kelen-Tüdös method is represented as: Y2 = (

) X2 -

+ 22


(7)

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where Y2 =

; X2 =

and α = (X1max × X1min)1/2

Thus from the plot of Y2 and X2, we obtain a straight line. Extrapolation to X2 = 0 and X2 = 1 gives the value of both the reactivity ratios.

Figure 5. Determination of reactivity ratios by (a) Fineman-Ross and (b) Kelen-Tüdös methods.

Table 4. Reactivity Ratios of β-myrcene (MY) and Di-butyl Itaconate (DBI) as Determined by Persulfate Emulsion Polymerization Method. methods Fineman-Ross

0.795

1.528

1.215

Kelen-Tüdös

0.731

1.342

1.145

Table 4 summarizes the reactivity ratios as obtained from the two linear methods. Although the values obtained from the two methods are quite similar, it is evident that, the reactivity ratio of βmyrcene is lower than that of di-butyl itaconate. For both the methods, rMY 1 and (rMY × rDBI) is close to 1. This suggests that the DBI monomer is more reactive than the MY monomer towards both the propagating species (MY* and DBI*). Thus, initially the copolymer will contain slightly higher proportion of the more reactive DBI unit in random placement, while with gradual progress of the reaction, the copolymer composition will contain relatively higher 23 ACS Paragon Plus Environment


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proportion of MY unit. The range of the co-monomer composition varies from 42:58 in the initial stage to 57:43 in the final stage for a 50:50 MY:DBI copolymer, as determined from NMR (shown earlier). This kind of inherited ‘composition drift’ is well known in batch emulsion polymerization.38 The product of the reactivity ratios is a measure of the tendency for alternating behavior. Since the value of this product and the reactivity ratio of DBI is greater than one (and that of MY is less than one), emulsion copolymerization of MY and DBI do not belong to an azeotropic polymerization; rather it reveals a quasi ideal behavior (Figure S10, Supporting Information). However, despite this reactivity of DBI monomer, the copolymers exhibited single glass transition temperature as discussed later. Wide-angle X-ray diffraction analysis. X-ray diffractrograms of various synthesised copolymers are presented in Figure S11 (Supporting Information). It is evident that for each composition a broad hump at 2θ = 19° appears, which resembles the diffraction pattern of natural rubber.39 Thus, it can be attributed that all the synthesised copolymers are amorphous in nature. Presence of branched structure and pendent alkyl side chain renders the copolymers as well as the homo-polymers amorphous. Glass transition temperature. The rubbery property of a polymer is greatly governed by its glass transition temperature, Tg. For a rubber, the glass transition temperature should be below the ambient temperature. The DSC run from -80 to +20 °C for the copolymer specimens indicates complete amorphous nature of the synthesised materials (Figure S12, Supporting Information). The Tg value of the polymers lies in between -69.0 and -5.8 °C. For example, -60.3 and -45.6 °C are observed for poly(MY90DBI10) and poly(MY50DBI50) respectively. In order to compare the experimentally obtained Tg values with the theoretical ones, the Flory-Fox


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equation40 was employed. For a copolymer, having a single transition point, the glass transition temperature was calculated as: (8) where w1, w2 are weight fractions and Tg1, Tg2 are the glass transition temperatures of components 1 and 2 (in K) respectively. The experimental transition temperatures and those obtained from Flory-Fox equation are tabulated in Table S4 (Supporting Information).

Figure 6. Variation of glass transition temperature with β-myrcene percentage.

Figure 6 depicts plot of experimental and theoretical glass transition temperature with β-myrcene percentage in various copolymers. Polymyrcene possesses a Tg of -69.0 °C and the glass transition temperature of poly(di-butyl itaconate) is observed at -5.8 °C. All the synthesised copolymer exhibited a single transition point based on their composition, indicating random nature. With increase in the weight fraction of the β-myrcene moiety, the Tg value of the copolymers approaches towards more sub-ambient temperature, indicating rubbery nature of the synthesized materials. Tg was also measured from dynamic mechanical analyzer for selected 25 ACS Paragon Plus Environment


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samples. The value for poly(MY50DBI50) copolymer, as shown in Figure S13 (Supporting Information) is observed at -42 °C. Molecular dynamics simulation. In order to have an idea about the spatial disposition of the copolymer chains, molecular dynamics (MD) simulation was performed. All the theoretical calculations herein were executed by the ab initio Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies (COMPASS) force field.41 In this method, the total potential energy (ET) is presented by the following equation: ET = Ebond + Enonbond + Ecross = Eb + Eɵ +Eϕ+ Evdw + Ecoulomb + Ecross

(9)

where Eb is the bond stretching energy, Eɵ is the angle bending energy and Eϕ is the dihedral torsion energy. The sum of these three terms is the bonded energy or Ebond. Evdw is the van der Waals energy, Ecoulomb is the Coulombic energy and the sum of these two contributions is the nonbonded energy Enonbond. Ecross is the energy of cross terms between any two of the bonded items, such as the bond-angle cross term and the bond-bond cross term. This force field method has been widely used to optimize and predict the structural, conformational, and thermo-physical condensed phase properties of macromolecules.42-44 The geometry of the structures was optimized by steepest descent method. To minimize the unwanted contacts, five configurations of minimum energy were selected after energy optimization to get the cell with the lowest energy. The Ewald summation was adopted for the electrostatic interactions with an accuracy of 0.001 kcal/mol, and the atom-based summation was applied for the van der Waals interactions with a cut off distance of 12.5 Å, a spline width of 1 Å, and a buffer width of 0.5 Å. Thereafter, the systems were equilibrated in the NVT ensemble (where N is number of particles, V is volume and T is temperature) followed by NPT dynamics (where N is number of particles, P is


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pressure and T is temperature) at 298 K. Thereafter, equilibrium MD simulation was carried out for 1.25 ps at 1bar. Then, the system was cooled down to 100 K with 20 K interval. The thermostat and barostat were maintained by Andersen method.45 At each temperature, 1.25 ps MD simulation was performed at a constant pressure of 1 bar and with a time step of 0.25 fs. Figure 7a depicts the poly(MY50DBI50) copolymer chains within a simulated cubic amorphous cell and the corresponding polymer backbone (highlighted by magenta colour) is presented in Figure 7b.

Figure 7. Amorphous unit cell of poly(MY50DBI50) at 300 K, (a) Display by repeat unit; MY: orange, DBI: violet, (b) Display by polymer backbone; carbon: grey, hydrogen: white, oxygen: red, backbone: magenta, Cell parameters: a: 28.56 Å, b: 28.42 Å, c: 28.65 Å. (c) Change in specific volume with temperature for poly(MY0DBI100) (Graphical points were obtained during NPT dynamics at 20 K temperature interval). 27 ACS Paragon Plus Environment


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The glass transition temperature of the polymers was calculated from the MD simulation by plotting V/T against T (where V = molar volume; T = temperature in Kelvin, Figure 7c) and determining the inflexion point from the graph. These theoretical values were found to be in close proximity with both the experimental findings (Table S4, Supporting Information) as well as those from the empirical Flory-Fox equation. Morphology of the copolymer. Atomic force microscope (AFM) and high-resolution transmission electron microscope (HRTEM) were used to envisage the morphology of the synthesised copolymer. The representative three dimensional topography and the corresponding AFM phase image of poly(MY50DBI50) are shown in Figure 8a-b. In AFM 3D topography image (Figure 8a), the brighter region corresponds to the territories having projected surface, while in phase image (Figure 8b) the terrain of different colours refers to materials of different hardness. The harder one delineates from the softer one in terms of colour contrast (the yellow being harder than the brown). The intriguing features obtained from the AFM measurements can be explained as follows. A very thin film of the copolymer was formed on the glass surface. Between the two building units of the copolymer, polymyrcene is elastic (rubbery) in nature and thus upon forming a film, it tends to protrude out from the surface. These protrusions appear as peaks in the 3D topography image and as these are soft in nature, the regions were mapped as darker one in the AFM phase image (based on twenty-five measurements, the average size of the soft regions was found to be ~0.129 µm).


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Figure 8. Morphology of poly(MY50DBI50) sample, AFM: (a) 3D topography and (b) Phase trace; HRTEM: (c) Phase image and (d) SAED pattern.

Figure 8c represents the HRTEM phase image of the poly(MY50DBI50) copolymer. In line with the AFM image, the TEM micrograph unveils the presence of two distinct phases in random manner. The polymyrcene units having higher electron density (due to two residual unsaturations) restricts the passage of the electrons and thus appears as dark spots (the average size of these regions were found ~0.150 µm). The selected area electron diffraction pattern (SAED, Figure 8d) displays diffuse rings without the presence of any bright spot, indicating the amorphous nature of the synthesized copolymer. 29 ACS Paragon Plus Environment


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Rheological properties of the copolymers. The synthesised copolymers and the pristine polymyrcene was crumb like and had elastic feeling upon physical handling. Poly(di-butyl itaconate), on the other hand, had a feeling of sticky and jelly like plastic material. Figure 9a shows the frequency dependence of complex viscosity for various synthesised copolymers and reveals the pseudoplastic nature with substantial shear thinning response in the entire frequency regime studied. The flow curves also depict a tendency to converge at higher frequency. The inset of Figure 9a represents the behaviour of the pristine polymers. The nature of poly(di-butyl

Figure 9. Plot of (a) Complex viscosity versus angular frequency (b) Storage modulus versus temperature.

itaconate) is more towards thermoplastic, while the other polymers are inclined towards rubbery behaviour. Much steeper fall in complex viscosity was observed for the synthesised copolymers compared to poly(di-butyl itaconate). This sharp fall in viscosity at higher shear rate can be attributed to the disentanglement of the macromolecular chains of the copolymers.46 The flow behaviour index (n) of these copolymers was calculated (Figure 9a) from the slope of the linear curve using the Power Law model.47


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It is evident that poly(di-butyl itaconate) having higher ‘n’ value (0.742) is less shear thinning than the other copolymers and pristine polymyrcene. Inclusion of DBI unit into the copolymer reduces the elastic nature of the copolymers and hence an increase in Tan δ value at 25 and 60 °C (Figure S14, Supporting Information). The variation of storage modulus with temperature is plotted in Figure 9b. In the temperature range of 25 to 125 °C, the storage modulus of poly(dibutyl itaconate) falls off rapidly, whereas polymyrcene offers notable resistance. With increasing weight percentage of β-myrcene in the copolymer, the G' values were found to be shifted towards higher magnitude. Thus, it can be concluded that, all the synthesised copolymers herein are expected to bear adequate load in the reasonable temperature frame so that they can find their utility as a single elastomer. Thermal properties of the copolymers. The thermal properties of the pristine polymers and their copolymers were evaluated using thermogravimetric analysis (TGA). Figure S15 (Supporting Information) represents the thermal decomposition curves for pristine polymers and copolymers. For all the homo and copolymers, various thermal degradation parameters such as: temperature of 20% weight loss (T20), temperature corresponding to maximum rate of degradation (Tmax) and percent residue at 780 °C are collated in Table 5. Table 5. Characteristic Temperatures and Amount of Residue at 780 °C. sample

T20 (°C)

Tmax (°C)

% residue at 780 °C

Poly(MY100DBI0)

278

416

4.17

Poly(MY90DBI10)

351

400

0.03

Poly(MY80DBI20)

356

398

0.04

Poly(MY70DBI30)

347

389

0.45

Poly(MY60DBI40)

355

387

0.52

Poly(MY50DBI50)

345

381

0.17

Poly(MY0DBI100)

292

348

1.85



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After onset of degradation, polymyrcene sample weight gradually falls, until it reaches Tmax ~ 416 °C. Thereafter, the polymer degrades off quickly. For all the random copolymers, the thermogram pattern lies in between the two pristine polymers. In comparison to the pristine polymyrcene, all the synthesised copolymer registered an increase in the temperature of 20% weight loss (T20). This initial increment in the onset degradation temperature of the copolymers can be related to the increased interchain interaction due to the incorporation of the polar di-butyl itaconate unit. Once the degradation starts, the possible formation of crosslinked structure increases the Tmax value for polymyrcene. At higher temperature, the interchain crosslinking reaction at the pendent –CH=C(CH3)2 site of the β-myrcene unit leads to an increased Tmax values of the copolymers compared to poly(di-butyl itaconate) homopolymer. CONCLUSIONS The present work demonstrates a green and robust route for the preparation of fully bio-based elastomers based on renewable synthons: β-myrcene and di-butyl itaconate, for the first time. The weight ratios of the monomers were chosen in such a manner that the resulting copolymer would display rubbery properties. For example, with 50 weight percent β-myrcene content, the Poly(MY50DBI50) copolymer displayed a number average molecular weight of 64,700 Da and a sub zero glass transition temperature of -45.6 °C. The monomer reactivity ratios calculated from linear methods suggest quasi-ideal copolymerization behaviour of the two monomers. Inclusion of polar di-butyl itaconate moiety not only dictates the microstructure of the resulting copolymer, but also improves the thermal stability over their pristine counterpart. The synthesised copolymers were also found to behave as a pseudoplastic material along with adequate load bearing capacity, resembling the behaviour of an elastomer. Molecular dynamics simulation was applied to depict the spatial distribution of the polymer chains. Due to the integration of polar group, the resulting copolymers are expected to manifest good interaction with various 32 ACS Paragon Plus Environment

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functional fillers used in rubber industry without any further post-polymerization modification. These rubbers could be a potential substitute for other synthetic elastomers, as these mimic the microstructure of butadiene based synthetic elastomers. Importantly, this facile methodology can also be translated to the synthesis of various other terpene-based elastomers and the microstructures of the same can be tailored by judicious choice of the co-monomer. This present investigation thereby constitutes an inexpensive and green platform for the synthesis of sustainable rubbers of the future. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS publication website at http://pubs.acs.org. Temperature and time dependence of Poly(MY0DBI100) synthesis on yield and molecular weight, Temperature and time dependence of Poly(MY100DBI0) synthesis on yield and molecular weight, Monitoring of the polymerization kinetics, Variation of Mn with Z-average diameter of latex particles for copolymers, DLS particle distribution profiles for copolymer latices, FESEM image of poly(MY50DBI50) latex, Calculation of theoretical solubility parameter, Group contributions for solubility parameter component, Assignments of FTIR peaks for various homo and copolymers, 1H and

13

C NMR spectra of DBI monomer,

1

H and

13

C NMR spectra of

poly(MY0DBI100), 1H and 13C NMR spectra of β-myrcene monomer, Parameters used in FR and KT method, Plot of DBI mole fraction in feed and copolymer, X-ray diffractograms of various polymers, DSC thermogram of various polymers, Glass transition temperature of various polymers, Temperature sweep plot for poly(MY50DBI50) copolymer, Variation of Tan δ with DBI content at 25 and 60 °C, TGA curves of pristine polymers and various copolymers (PDF).



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AUTHOR INFORMATION Corresponding Author *Email: [email protected], [email protected], (AKB) Tel.: +91 (3222) 283180; Fax: +91 (3222) 220312. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors would like to thank IIT Kharagpur (affiliating institute) for providing the facilities. PS would like to acknowledge IIT Kharagpur for awarding a research fellowship to carry out this work. The authors gratefully acknowledge Mr. Subhabrata Saha for carrying out molecular dynamics simulation. “Permission from Accelrys Software Inc., Materials Studio - MS Polymer, September 2015, Accelrys Software Inc., San Diego” is also acknowledged.

REFERENCES (1) Gandini, A. Polymers from Renewable Resources: A Challenge for the Future of Macromolecular Materials. Macromolecules 2008, 41, 9491-9504. (2) Silvestre, A. J. D.; Gandini, A. Terpenes: Major Sources, Properties and Applications. In Monomers, Polymers and Composites from Renewable Resources; Belgacem, M. N.; Gandini, A. 1st ed.; Elsevier: Amsterdam, 2008; ch. 2. (3) Holmberg, A. L.; Reno, K. H.; Wool, R. P.; Epps, T. H. Biobased Building Blocks for the Rational Design of Renewable Block Polymers. Soft Matt. 2014, 10, 7405-7425. (4) Miller, S. A. Sustainable Polymers: Opportunities for the Next Decade. ACS Macro Lett. 2013, 2, 550-554.


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(5) Wang, S.; Kesava, S. V.; Gomez, E. D.; Robertson, M. L. Sustainable Thermoplastic Elastomers Derived from Fatty Acids. Macromolecules 2013, 46, 7202-7212. (6) Tang, D.; Macosko, C. W.; Hillmyer, M. A. Thermoplastic Polyurethane Elastomers from Bio-based Poly(δ-decalactone) Diols. Polym. Chem. 2014, 5, 3231-3237. (7) Yao, K.; Tang, C. Controlled Polymerization of Next-Generation Renewable Monomers and Beyond. Macromolecules 2013, 46, 1689-1712. (8) Wilbon, P. A.; Chu, F.; Tang, C. Progress in Renewable Polymers from Natural Terpenes, Terpenoids and Rosin. Macromol. Rapid Commun. 2013, 34, 8-37. (9) Liu, B.; Li, L.; Sun, G.; Liu, D.; Li, S.; Cui, D. Isoselective 3,4-(co)polymerization of BioRenewable Myrcene using NSN-ligated Rare-Earth Metal Precursor: An Approach to a New Elastomer. Chem. Commun. 2015, 51, 1039-1041. (10) Bolton, J. M.; Hillmyer M. A.; Hoye, T. R. Sustainable Thermoplastic Elastomers from Terpene-Derived Monomers. ACS Macro Lett. 2014, 3, 717-720. (11) Loughmari, S.; Hafid, A.; Bouazza, A.; Bouadili, A. E.; Zinck, P.; Visseaux, M. Highly Stereoselective Co-ordination Polymerization of β-Myrcene from a Lanthanide-Based Catalyst: Access to Bio-Sourced Elastomers. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 2898-2905. (12) Georges, S.; Bria, M.; Zinck, P.; Visseaux, M. Polymyrcene Microstructure Revisited from Precise High-Field Nuclear Magnetic Resonance Analysis. Polymer 2014, 55, 3869-3878. (13) Georges, S.; Touré, A. O., Visseaux, M.; Zinck, P. Coordinative Chain Transfer Copolymerization and Terpolymerization of Conjugated Dienes. Macromolecules 2014, 47, 4538-4547. (14) Sarkar, P.; Bhowmick, A. K. Synthesis, Characterization and Properties of a Bio-based Elastomer: Polymyrcene. RSC Adv. 2014, 4, 61343-61354.



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(15) Yahiro, K.; Shibata, S.; Jia, J. Shi-R.; Park, Y.; Okabe, M. Efficient Itaconic Acid Production from Raw Corn Starch. J. Ferment. Bioeng. 1997, 84, 375-377. (16) Willke, T.; Vorlop, K. D. Biotechnological Production of Itaconic Acid. Appl. Microbiol. Biotechnol. 2001, 56, 289-295. (17) Bozell, J. J.; Petersen, G. R. Technology Development for the Production of Biobased Products from Biorefinery Carbohydrates - The US Department of Energy’s “Top 10” Revisited. Green Chem. 2010, 12, 539-554. (18) Bartlett, P. D.; Altschul, R. The Polymerization of Allyl Compounds. I. Factors Governing the Acyl Peroxide-Induced Polymerization of Allyl Acetate and the Fate of the Peroxide. J. Am. Chem. Soc. 1945, 67, 816-822. (19) Marvel, C. S.; Shepherd, T. H. Polymerization Reactions of Itaconic Acid and Some of Its Derivatives. J. Org. Chem. 1959, 24, 599-605. (20) Cowie, J. M. G.; Henshall, S. A. E.; McEwen, I. J.; Velickovic, J. Poly(alkyl itaconates): 4. Glass and Sub-glass Transitions in the Di-alkyl Ester Series, Methyl to Hexyl. Polymer 1977, 18, 612-616. (21) Cowie, J. M. G. Physical Properties of Polymers Based on Itaconic Acid. Pure & Appi. Chem. 1979, 51, 2331-2343. (22) Arrighi, V.; Holmes, P. F.; McEwen, I. J.; Terrill, N. J.; Qian, H. Nanophase-Separated Regions and Side-chain Relaxation in Dialkyl Itaconate Copolymers. J. Mater. Chem. 2004, 14, 3306-3307. (23) Arrighi, V.; Triolo, A.; McEwen, I. J.; Holmes, P.; Triolo, R.; Amenitsch, H. Observation of Local Order in Poly(di-n-alkyl itaconate)s. Macromolecules 2000, 33, 4989-4991.


Page 37 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(24) Otsu, T.; Watanabe, H. Radical Polymerization Reactivity of Dialkyl Itaconates and Characterization of Their Polymers. Eur. Polym. J. 1993, 29, 167-174. (25) Winkler, M.; Lacerda, T. M.; Mack, F.; Meier, M. A. R. Renewable Polymers from Itaconic Acid by Polycondensation and Ring-Opening-Metathesis Polymerization. Macromolecules 2015, 48, 1398-1403. (26) Dai, J.; Ma, S.; Wu, Y.; Han, L.; Zhang, L.; Zhu, J.; Liu, X. Polyesters Derived from Itaconic Acid for the Properties and Bio-based Content Enhancement of Soybean Oil-based Thermosets. Green Chem. 2015, 17, 2383-2392. (27) Qiao, H.; Wang, R.; Yao, H.; Wu, X.; Lei, W.; Zhou, X.; Hu, X.; Zhang L. Design and Preparation of Natural Layered Silicate/bio-based Elastomer Nanocomposites with Improved Dispersion and Interfacial Interaction. Polymer 2015, 79, 1-11. (28) Spasojevic', P.; Stamenkovic', D.; Pjanovic', R.; Vragolovic', N. B.; Dolic', J.; Grujic' S.; Velickovic', S. Diffusion and Solubility of Commercial Poly(methyl methacrylate) Denture Base Material Modified with Dimethyl Itaconate and Di-n-butyl Itaconate During Water Absorption / Desorption

Cycles.

Polym.

Int.

2012,

61,

1272–1278.

(29) Wang, R.; Ma, J.; Zhou, X.; Wang, Z.; Kang, H.; Zhang, L.; Hua, K. C.; Kulig, J. Design and Preparation of a Novel Cross-Linkable, High Molecular Weight, and Bio-Based Elastomer by Emulsion Polymerization. Macromolecules 2012, 45, 6830-6839. (30) Materials Studio version 8.0.100.21, Accelrys Software Inc., San Diego: 2014. (31) Viclavova, E.; Hrivik, A.; Chrastova, V. Emulsion Copolymerization of Styrene and Butyl acrylate with Itaconic Acid. Makromol. Chem. 1992, 193, 2243-2250. (32) Chern, C. S. Emulsion Polymerization Mechanisms and Kinetics. Prog. Polym. Sci. 2006, 31, 443-486.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 41

(33) Thickett, S. C.; Gilbert, R. G. Emulsion Polymerization: State of the Art in Kinetics and Mechanisms. Polymer 2007, 48, 6965-6991. (34) Krevelen, D. W. V.; Nijenhuis, K. T. Properties of Polymers: Their Correlation With Chemical Structure; Their Numerical Estimation And Prediction From Additive Group Contributions, 4th ed.; Elsevier: Amsterdam, 2009, ch. 7. (35) Brandrup, J.; Immergut, E. H.; Grulke, E. A. Polymer Handbook, 4th ed.; John Wiley & Sons: Canada, 1999, ch. VII. (36) Fineman, M.; Ross, S. D. Linear Method for Determining Monomer Reactivity Ratios in Copolymerization. J. Polym. Sci. 1950, 5, 259-262. (37) Kelen, T.; Tüdös, F. Analysis of the Linear Methods for Determining Copolymerization Reactivity Ratios. I. A New Improved Linear Graphic Method. J. Macromol. Sci. Chem. 1975, A9, 1-27. (38) Odian, G. Principles of Polymerizations, 4th ed.; Wiely Interscience: USA, 2004, ch. 6. (39) Potts, J. R.; Shankar, O.; Du, L.; Ruoff, R. S. Processing–Morphology–Property Relationships and Composite Theory Analysis of Reduced Graphene Oxide/Natural Rubber Nanocomposites. Macromolecules 2012, 45, 6045-6055. (40) Fox, T.G.; Flory, P.J. Second-order Transition Temperatures and Related Properties of Polystyrene. J. Appl. Phys. 1950, 21, 581-591. (41) Sun, H. COMPASS: An ab-initio Force-Field Optimized for Condensed-Phase Applications Overview with Details on Alkane and Benzene Compounds. J. Phys. Chem. B 1998, 102, 73387364. (42) Luo, Z.; Jiang, J. Molecular Dynamics and Dissipative Particle Dynamics Simulations for the Miscibility of Poly(ethylene oxide)/Poly(vinyl chloride) Blends. Polymer 2010, 51, 291-299.


Page 39 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(43) Jawalkar, S. S.; Adoor, S. G.; Sairam, M.; Nadagouda, M. N.; Aminabhavi, T. M. Molecular Modeling on the Binary Blend Compatibility of Poly(vinyl alcohol) and Poly(methyl methacrylate): An Atomistic Simulation and Thermodynamic Approach. J. Phys. Chem. B 2005, 109, 15611-15620. (44) Prathab, B.; Aminabhavi, T. M.; Parthasarathi, R.; Manikandan, P.; Subramanian, V. Molecular Modeling and Atomistic Simulation Strategies to Determine Surface Properties of Perfluorinated Homopolymers and Their Random Copolymers. Polymer 2006, 47, 6914-6924. (45) Andersen, H. C. Molecular Dynamics Simulations at Constant Pressure and/or Temperature. J. Chem. Phys. 1980, 72, 2384-2393. (46) Banerjee, S. S.; Kumar, K. D.; Bhowmick, A. K. Distinct Melt Viscoelastic Properties of Novel Nanostructured and Microstructured Thermoplastic Elastomeric Blends from Polyamide 6 and Fluoroelastomer. Macromol. Mater. Eng. 2015, 300, 283-290. (47) Sadhu, S.; Bhowmick, A. K. Unique Rheological Behavior of Rubber Based Nanocomposites. J. Polym. Sci. B Polym. Phys. 2005, 43, 1854-1864.



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for Table of Contents use only

A Green Approach Towards Sustainable Polymer: Synthesis and Characterization of Poly(myrcene-co-dibutyl itaconate) Preetom Sarkar and Anil K. Bhowmick*

Facile fabrication of sustainable rubbery materials by environmentally benign emulsion copolymerization of β-myrcene and dibutyl itaconate.


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Page 41 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


for Table of Contents use only

A Green Approach Towards Sustainable Polymer: Synthesis and Characterization of Poly(myrcene-co-dibutyl itaconate) Preetom Sarkar and Anil K. Bhowmick*

Facile fabrication of sustainable rubbery materials by environmentally benign emulsion copolymerization of β-myrcene and di-butyl itaconate.


Green Approach toward Sustainable Polymer - ACS Publications

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