Redox Emulsion Polymerization of Terpenes: Mapping the Effect of the

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Redox Emulsion Polymerization of Terpenes: Mapping the Effect of the System, Structure and Reactivity Pranabesh Sahu, and Anil K. Bhowmick Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02001 • Publication Date (Web): 09 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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Industrial & Engineering Chemistry Research

Redox Emulsion Polymerization of Terpenes: Mapping the Effect of the System, Structure and Reactivity Pranabesh Sahua and Anil K. Bhowmick*a,b

aRubber

Technology Centre, Indian Institute of Technology Kharagpur, Kharagpur -721302,

West Bengal, India bCurrent

address: International Center for Polymers and Soft Matter, Department of

Chemical and Biomolecular Engineering, The University of Houston, 4726 Calhoun Rd, Houston, TX 77204-4004, USA *Corresponding author; E-mail: [email protected]; [email protected]

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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

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ABSTRACT: Synthesis of biobased polymers from renewable feedstocks has been the focus of research on sustainability. The

family of acyclic terpenes, particularly the β-

myrcene, β-ocimene and β-farnesene from renewable feedstocks imparts a classical chemistry close to isoprene (unsaturated hydrocarbon from petroleum). In this work, redox emulsion polymerization of these terpenes was carried out. The polymerization of βmyrcene produced a high molecular weight rubbery polymer (Mn = 1.68×105 Da) with a maximum yield of 65%. The spectroscopic measurements showed that polymyrcene predominantly contained 1,4-microstructure. The polymyrcene from the best redox initiator system was characterized in detail. Its microstructure was established using 1D and 2D NMR spectroscopy. Polymyrcenes synthesized using different recipes displayed a glass transition temperature from −70 °C to −58 °C, indicating rubbery nature. However, βocimene and β-farnesene polymerization yielded low molecular weight polymers with preferably 1,4-addition products. Density functional theory provided the ground state optimized structure of terpenes.

KEYWORDS:

Terpene;

Biobased

polymers;

Emulsion

spectroscopy; Density functional theory.


polymerization;

2D

NMR

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1. INTRODUCTION Over the past few years, the production and utilization of synthetic polymers have exponentially increased. Biobased materials have gained attention in the recent years due to severe concerns on ecological and economic balance.1 Increased pollution and especially the dramatic increase of oil prices, have first driven the scientific and industrial communities to look at potential substitutes for traditional low biodegradable petroleumbased polymers.2,3 There are different ways of preparing biobased materials, one from the biosynthetic procedure and another from the treatment of biobased monomers.4,5 Polymers obtained from naturally occurring feed stocks are promising materials with novel applications and enhanced properties with attention to biocompatibility and biodegradability. Hence, the development and production of sustainable materials from biomass constitute a steadily growing field of attention.4-6 Of the vast renewable feedstocks, the current researches are mainly focused on the transformation of vegetable oils, lignin and carbohydrates to new biobased useful products.3,4,6 In this circumstances, the wide family of terpenes present in leaves, fruits and flowers of different plants was not taken into attention, although they have been known since past hundreds years.7,8 They have been regularly used as flavors, fragrances, and in different medical formulations from the ancient past.9,10 Despite their structural diversity, terpenes are particularly a large and assorted class of hydrocarbons that provide a unique chemistry (particularly monoterpenes) to the unsaturated hydrocarbons, known from crude oil and gases.11,12 The building block of terpenes is derived from isoprene and may be composed of one, two, three, four, or more isoprene units. The common acyclic monoterpenes similar to classical petrochemical based monomers are myrcene, ocimene, alloocimene, and farnesene.13-15 While a number of sustainable monomers like γ-butyrolactone, lactide, and cyclic terpenoids are having the 3 ACS Paragon Plus Environment


ability of responding towards ring-opening polymerizations, only terpenes are known to be polymerized by free radical mechanism.15-17 In this context, emulsion polymerization, along with its myriad of advantages, like solvent less, green technique, faster rate and high molecular weight products, is a major industrial process for the production of different polymers.18-20 Polymer microstructure is strongly affected by the temperature of polymerization. In the study, redox initiated emulsion polymerization was employed for the synthesis of polymers at room temperature. At low temperature, redox initiators show much higher radical generation rate, and higher molecular weight polymer with desired polymer architecture.21,22 The other points of this study was also to see the variation of yield and molecular weight simultaneously while keeping the microstructure of the produced polymer regioselective. In this study, we aim to investigate the detailed synthesis, characterization and effects of ingredients of the recipe for the free radical polymerization of few acyclic terpenes. The effect of surfactants, namely potassium oleate (anionic), sodium dodecyl sulfate (anionic), cetrimonium bromide (cationic), Triton X-405 (non-ionic) surfactants, on polymerization reaction systems were evaluated. A few investigations were reported in the literature regarding the understanding of how these surfactants influence the nucleation mechanisms in this polymerization.23-25 The redox initiator couples used were based on two types of oxidising agents, peroxide and persulfate. These oxidants were united with reductant of different nature such as, sodium formaldehyde sulfoxylate (SFS) and sodium metabisulfite. The best redox pairs (those providing faster rate of polymerization, higher final conversion and molecular weight) were then evaluated and employed in a typical redox emulsion polymerization of β-myrcene and other terpenes. With our quest to see the reactivity of various terpenes by employing a green polymerization process, we herein describe the 4 ACS Paragon Plus Environment

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synthetic methodologies and the properties of the terpene-based polymers, classified by the molecular architecture via redox initiated emulsion polymerization. 2. EXPERIMENTAL SECTION Raw Materials. β-myrcene (technical grade, 99%) Sigma-Aldrich, Ocimene (Sigma-Aldrich, ≥90%) and trans-β-farnesene (Sigma-Aldrich, analytical standard) were purified using 2M NaOH to make these inhibitor free and used. Potassium chloride (KCl), Potassium phosphate (K3PO4), tert-butyl hydroperoxide (Luperox TBH70X), and sodium formaldehyde sulfoxylate (SFS) were obtained from Sigma-Aldrich. Ethylenediaminetetraacetic acid iron (III) monosodium salt (Fe-EDTA), Alfa Aesar, Thermo Fisher Scientific India Private Limited, and Potassium oleate (K-Oleate, 98%), Loba Chemie, India were purchased. Sodium dodecyl sulfate (SDS), cetrimonium bromide (CTAB) and Triton X-405 were procured from Thermo Fisher Scientific, India. Deionized water was used for the polymerization process. Polymerization Procedure. Polymerization of β-myrcene (MY) was carried out in a dry round bottom flask under N2 atmosphere at room temperature (25 °C) for 20 h using a typical redox-initiator emulsion polymerization procedure following the recipe in Table 1. At first, potassium oleate, potassium phosphate buffer, potassium chloride and distilled water were added and stirred at 300 rpm for 20-30 min. Then the monomer, β-myrcene was charged into the reaction mixture by a micropipette, stirred for another 30 min to achieve a balanced emulsion. Subsequently, Fe-EDTA and SFS mixture were fed into the reaction vessel and the round bottom flask was maintained to inert atmosphere by N2 gas flushing. Then, tert-butyl hydroperoxide (TBHP) initiator was added into the reactor. Aliquots from the reaction vessel were taken out to follow the polymerization process. The polymer was precipitated into ethanol solvent and the resulting rubbery type product was washed with



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deionized water and dried under vacuum oven at 50 °C for 1 day.The polymerization is shown in Scheme 1. Table 1. Recipe of Emulsion Polymerization via Redox-Initiation Ingredients

Amount (g)

terpenes (β-myrcene, β-ocimene, β-farnesene) DI water potassium phosphate tribasic (K3PO4) potassium oleate Fe-EDTA potassium chloride (KCl) sodium formaldehyde sulfoxylate (SFS) tert-butyl hydroperoxide (TBHP)

Variable 180 2.00 4.50 0.15 0.30 0.05 0.06

Scheme 1. Free radical emulsion polymerization of β-myrcene. Effect of Ingredients on the Recipe. We aimed to explore the consequence of surfactants, initiator and reducing agent variation on the polymerization of β-myrcene, mainly on the yield, molecular weight and microstructure of the polymer. Different types of surfactants (anionic, cationic, non-ionic) and ammonium persulfate (APS) initiator and sodium metabisulfite reducing agent, were used as ingredients for the synthesis. Different redox recipes are tabulated and given in the supporting information (Table S1) with specific designation. The effects of surfactants (anionic, cationic and non-ionic) on the emulsion polymerization of β-myrcene were studied at room temperature. The comparative rate studies in the presence 6 ACS Paragon Plus Environment

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of all surfactants revealed that the surfactants exerted a comprehensive influence on these systems of polymerization. Only a few investigations have been reported earlier regarding the understanding of how these surfactants influence the nucleation process in the emulsion polymerization. The action of distinct redox initiator couples to initiate the emulsion polymerization of β-myrcene was investigated. The kinetics of reaction and the final conversion achieved were used to evaluate the performance in all the experimental design of polymerization recipe. Effect of Structure and Reactivity. The reactivity of few selected acyclic terpenes towards redox emulsion polymerization was investigated depending on the chemical structure. The fundamental factor that affects the reactivity and rate of polymerization is the structure of monomer. The diene monomer polymerization relies on the arrangement of the double bonds and on the number and nature of their substituents present. The reactivity of most of the terpenes in free radical polymerization is generally low compared to that of petroleum derived vinyl monomers, due to the bulky groups surrounding the double bond of terpenes. Acylic monoterpenes like β-ocimene and β-farnesene (Figure 1) bearing reactive conjugated double bonds were also polymerized using the same recipe (Table 1) and the polymerization procedure was followed as earlier. The effects of monomer structure on polymerization are explained later in the specific section.

Figure 1. Chemical Structure of Acyclic Terpenes. 7 ACS Paragon Plus Environment


Polymerization of β-ocimene. The polymerization procedure was similar as discussed earlier. The white milky latex obtained was coagulated in aqueous calcium chloride solution and the resulting viscous oily type product was wiped with water and dried under vacuum for characterization. The possible structure of polyocimene via redox emulsion is shown in Scheme 2.

Scheme 2. Selected possible structures of polyocimene. Polymerization of β-farnesene. The white milky latex was coagulated in excess ethanol and the resulting viscous liquid type polymer was extracted and swabbed with water and dried under vacuum. The polymerization scheme is given in Scheme 3.

Scheme 3. Redox emulsion polymerization of β-farnesene. 8 ACS Paragon Plus Environment

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Characterization Methods. Monomer conversion was determined gravimetrically. Gel permeation chromatography was carried out with a SHIMADZU GPC instrument equipped with Phenogel 10µ Columns for molecular weight measurement of the polymers at 25 °C. THF solvent (HPLC grade) was used as eluent at a flow rate of 1.0 mL/min. The sample concentration was kept to 2−3 mg/mL and filtered through PTFE microfilters (pore size of 0.2 μm) for injection. Malvern Nano ZS dynamic light scattering (DLS) instrument at a scattering angle of 90° was used for particle size measurements furnished with a 4 mW He−Ne laser. Fourier transform infrared spectroscopy (FT-IR) analysis of the synthesized polymers was taken on a PerkinElmer Spectrum 400 machine over a wave number range of 400−4000 cm-1 with 4 cm-1 resolution. AVANCE III 400 Ascend Bruker 600 MHz instrument was used to record the 1H NMR and 13C

NMR of the polymers in CDCl3 solvent at 25 °C. Two-dimensional (2-D) NMR

spectroscopy were recorded with the standard pulse sequence to measure the magnetization transfer between different of types nuclei through bonds or space. The resulted contour diagram after Fourier transformation indicates couplings between the nuclei and provides structural information about the polymer. Differential scanning calorimetry (DSC) was performed with a NETZSCH DSC 200F3Maia instrument to obtain the glass transition temperature (Tg) of polymers. About 8-10 mg of sample was taken in a DSC pan with nitrogen flow at a heating rate of 10°C/min from -100 to +100 °C. The data were collected from the second heating scan.



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3. RESULTS AND DISCUSSION Synthesis of polymyrcene (PMY) polymer. β-myrcene was polymerized via redox initiated emulsion polymerization at 20-25 °C in ambient pressure. In order to gain high molecular weight and less branched polymer, redox-initiated emulsion was performed at room temperature. In addition, chain transfer mechanism acts much less in redox mechanism,

which

yields

polymer

of

lower

crosslinks

and

branching.

Ethylenediaminetetraacetic acid iron (III) monosodium salt (Fe-EDTA) was used as an oxidant and chelating agent. Sodium formaldehyde sulfoxylate (SFS) and TBHP were utilized as a reductant and free radical initiator respectively. According to Kojima and Hisasue,26 the initiator first reacts with the ferrous ion to give a higher valence ferric (Fe+3) ion, followed by a reduction to ferrous (Fe+2) by the SFS. EDTA acts as a chelating agent, which will complex with Fe+2 to suppress more reduction to Fe+1 in the aqueous phase. The effect of ingredients and factors of polymerization on different recipe were also investigated. The optimised reaction conditions (polymerization time) were chosen based on the yield and polymer molecular weight and was followed for all other reactions. Effect of Reaction Time. The effect of time on the polymerization was investigated. For this, the samples from the reaction mixture were taken out at an interval of 4 h up to 24 hours, followed by calculation of yield and molecular weight. Figure 2 and Table 2 evidently show that with reaction time, the percentage yield increases and the molecular weight reaches a maximal of 1.68 ×105 Da at 20 h for a particular recipe (R.R-1). The same trend was also observed for all other recipes. This is because, at higher reaction time, there is a possibility of chain transfer reaction which produces oligomers i.e. breakage of molecular chain. Therefore, the formation of oligomers predominates due to polymer chain scission, thereby reducing the percent yield of the reaction and follows a decreasing trend of 10 ACS Paragon Plus Environment

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molecular weight with reaction time. From the collective study of time variation on yield and molecular weight, the optimum reaction condition was fixed at RT for 20 h of polymerization. The finalized recipe was chosen based on the yield and molecular weight of the obtained polymers. Table 2 summarizes also the molecular weight, particle diameter, gel fraction and yield values for the synthesized PMYusing different recipes. The recipe with K-oleate as surfactant and TBHP as an initiator (R.R-1) produces maximum yield and molecular weight of the polymer.

Figure 2. Effects of time on yield of PMY synthesis. Table 2. Molecular Weight, Yield, Particle Size, Gel content and PDI of Redox Initiated Polymyrcene using different recipe. Polymer (polymyrcene)

Yield (%)

Molar Mass (Da)

Mw/Mn (PDI)

Gel Content (%)

Z-Average diameter (nm)

R.R-1

65

1.68 ×105

3.41

5

92

R.R-2

55

8.83 ×104

1.45

3

64

R.R-3

38

9.13×104

1.52

5

50

R.R-4 R.R-5

20 54

2.87×104 1.27×105

2.01 1.88

0 4

912 76

R.R-6

50

1.03×105

2.40

8

247



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Effect of Surfactants. The effect of emulsifier on polymerization reaction systems was observed following the kinetic characteristics of reaction. The semi logarithmic kinetic plots of ln (1/1−X) versus time are displayed in Figure 3. It is evident that the rate of polymerization increases in the case of anionic detergents and decreases for cationic detergents. The polymerization reactions obeyed first order kinetics and produced good fits to linear kinetic plots until the time taken for polymerization (Figure 3). The presence of anionic detergent (K-Oleate, SDS) enhances the polymerization rate of PMY (Figure 3a) due to greater negative charge on the micelles that possibly apply a repelling force connecting the growing polymer chains and decrease the probability of bimolecular termination. The soap is adsorbed onto the particle surface as the growth or aggregate of polymer chains increases. The adsorption of an anionic soap increases the charge in particle surface, thus stabilizing the particles at a smaller size. The increase in charge also leads to a reduction in the amount of aggregation and hence enhanced latex stability. This increases the stability of emulsion, which affects the rate of acceleration and molecular weight of the polymer.22 However, the orientation of the growing polymer chain favours termination in the case of positively charged micelle. Therefore, the rate of polymerization (Figure 3a) decreases in the company of a cationic detergent (CTAB).22 The polymerization rate was lowest for non-ionic surfactants (Triton-X-405) in Fe2+-H2O2 initiated polymerization (Figure 3a). Lower polymerization rates with broad particle size distributions were obtained due to the greater length of the poly (ethylene oxide) unit (n=40) in the surfactants. Latex particle size was obtained larger in non-ionic systems than the particle size obtained using an ionic emulsifier. This was documented in the earlier studies.25 Effect of Redox Systems. The performance of different redox initiator couples to initiate the emulsion polymerization of β-myrcene at RT was investigated. The combinations of 12 ACS Paragon Plus Environment

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H2O2 and Fe (II) salts, known as "Fenton's reagent", in the polymerization of diene monomers have been extensively studied in polymer chemistry for many years. Therefore, different compositions of redox systems have been studied which are very effective and useful particularly for free radical polymerizations in aqueous media. Besides peroxides, other redox systems used in this study include iron salts, sodium formaldehyde sulfoxylate (SFS) as reducing agent with ammonium persulfate (APS) as the initiator. Earlier Sahoo and Mohapatra27 investigated the effect of various transition metal–complex catalyst systems with ammonium persulfate to polymerize acrylates via emulsion polymerization. Redox emulsion experiments showed that, tert-butyl hydroperoxide (TBHP/SFS) system provided the highest conversion and lowest induction period (Table 2, Figure 3b), whereas for the systems with persulfate oxidants, ammonium persulfate (APS/SFS) was also the best alternative. The working mechanism was documented earlier in the emulsion copolymerization

of

tetrafluoroethylene

and

propylene.26

The

mode

of

initiator

decomposition, involving the catalytic system Fe(II)/EDTA/APS can be elucidated as follows: 𝐹𝑒 (𝐼𝐼) + 𝐸𝐷𝑇𝐴 = 𝑭𝒆 ― 𝑬𝑫𝑻𝑨 𝐶𝑜𝑚𝑝𝑙𝑒𝑥 𝐹𝑒 ― 𝐸𝐷𝑇𝐴 𝐶𝑜𝑚𝑝𝑙𝑒𝑥 + 𝑀𝑌 = 𝑴𝒀 ― 𝑭𝒆 ― 𝑬𝑫𝑻𝑨 𝐶𝑜𝑚𝑝𝑙𝑒𝑥 𝑀𝑌 ― 𝐹𝑒 ― 𝐸𝐷𝑇𝐴 𝐶𝑜𝑚𝑝𝑙𝑒𝑥 + (𝑁𝐻4)2𝑆2𝑂8 = 𝑺𝑶𝟒 ―. (𝑅.) (𝑅 ― 𝐼𝑛𝑖𝑡𝑖𝑎𝑡𝑜𝑟) 𝑀𝑌 (𝑀) + 𝑆𝑂4 ―.(𝑅.) = 𝑹𝑴. ― ― ― ―𝐼𝑛𝑖𝑡𝑖𝑎𝑡𝑖𝑜𝑛 𝑅𝑀. + 𝑀 = 𝑹𝑴𝟏. ― ― ― ―𝑃𝑟𝑜𝑝𝑎𝑔𝑎𝑡𝑖𝑜𝑛 𝑅𝑀1. + 𝑅𝑀2. = 𝑃𝑜𝑙𝑦𝑚𝑒𝑟 ― ― ― ― ― 𝑇𝑒𝑟𝑚𝑖𝑛𝑎𝑡𝑖𝑜𝑛 The rate of polymerization was lower compared to Fe2+-H2O2 initiated polymerization. Therefore, the variation in redox system results in the alteration of redox potentials of the reaction, which consequently affects the generation of initiating radicals and the rate of 13 ACS Paragon Plus Environment


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propagation. This also results in the formation of low molecular weight polymyrcene compared to peroxide initiated polymer (Table 2, earlier). Reducing agent (sodium metabisulfite) with different electron donor capacity was also assessed

as

a

redox

couple

in

this

work.

For

systems

with

hydroperoxide

oxidants/metabisulfite reductant, the polymerization shows good molecular weight of PMY with comparatively low yield with respect to the main recipe (Table 2, earlier). All the investigations displayed a conversion lower than 100%, which recommend that the redox system was consumed totally.

Figure 3. Kinetics plot of (a) effects of surfactants (b) effects of initiator in polymerization. FTIR characterization of the synthesized polymyrcene. Figure 4a shows the β-myrcene and polymyrcene FT-IR spectra. The small peak at 3090 cm-1 represents the =C–H stretching of the conjugated diene in the monomer, which vanishes after polymerization. The peaks at 2858, 2920, and 2968 cm-1 in the monomer correspond to the –CH, –CH2, and –CH3 asymmetric stretching vibration, which widen up during formation of the polymer. The intense C-1=C-2 and C-3=C-4 stretching frequency at 1595 cm-1 is associated with the conjugated double bond, whereas the isolated double bond stretching vibration appears at 1642 cm-1. After polymerization, the complete removal of absorption peak at 1595 cm-1 14 ACS Paragon Plus Environment

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indicates the formation of the polymer via conjugated double bond and the peak due to isolated double bond at 1642 cm-1 remains unaltered. The broad absorption peaks at 1376, 1445 cm-1 are attributed to the –CH3 and –CH2 bending vibration respectively. The relative increase of the peak area at 1445 cm-1 reflects the generation of more methylene groups after polymerization. The intense absorption peaks of C1, C3 and C4 centres (=C–H bending vibration) appear around 990 and 891 cm-1 in the monomer. After polymerization, the sharp drop in the intensities of both the peaks is due to the utilization of these two double bonds (C-1=C-2 and C-3=C-4). Figure 4b represents the FTIR spectra of all polymyrcene polymers synthesized using different recipe or ingredients. The signature peaks appeared in the polymers after polymerization is identical compared to the main recipe in all the cases. This confirms the formation of the polymer from the monomer using different recipes.

Figure 4. FTIR spectra of (a) β-myrcene and polymyrcene (using main recipe) (b) poly-myrcene (using different recipe). NMR Characterization. To investigate the composition and microstructure of polymyrcene, NMR spectra were recorded. The 1H and

13C

5b. Figure 5c - 5d represents the 1H and

NMR spectra of MY are shown in Figure 5a -

13C

NMR spectra of PMY polymer respectively.



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The signals of different protons and carbons in the polymer are assigned and listed below. The spectra show that the polymer mostly have 1,4-cis and 1,4-trans-microstructure of polymyrcene. 1H

NMR of β-myrcene (MY) (CDCl3, 600 MHz): δ-5.05-5.31 (4H, 3, 7, and 1- H), 4.75 &

4.68 (2H, 4a and 4b-H), 2.23-2.30 (4H, 5 and 6-H), 1.76 (3H, 10-H), 1.67 (3H, 9-H). 13C

NMR of β-myrcene (MY) (CDCl3, 600 MHz): δ-146.2 (2-C), 139.0 (3-C), 131.6 (8−C),

124.2 (7-C), 115.6 (1-C), 112.9 (4-C), 31.5 (5-C), 26.8 (6−C), 25.6 (10-C), 17.6 (9-C). (Please see figures for designation) 1H

NMR of polymyrcene (PMY) (CDCl3, 600 MHz): δ-5.14 (4H, 3, 7, 3’and 7’- H), 2.05

(16H, 1, 1’, 4, 4’, 5, 5’, 6 and 6’-H), 1.69 (6H, 10 and 10’-H), 1.62 (9 and 9’-H). 13C

NMR of polymyrcene (PMY) (CDCl3, 600 MHz): δ-131.41 [8,8’-C], 124.43 [3,7,3’,7’-C],

139.01 [2, 2’-C], 30.42 [1,1’-C], 37.09 [5, 5’-C], 25.70 [10,10’-C], 17.70 [9,9’-C], 26.97 [4,6,4’,6’-C].


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Figure 5. 1H and


13C

NMR spectra of (a) and (b) β-myrcene. (c) and (d) polymyrcene (main

recipe). The 1H NMR spectrum of polymyrcene via redox emulsion (Figure 5c) displays a single peak at 5.14 ppm due to the olefinic protons (3, 7, 3’, 7’-H) of =CH– units. Chemical shifts of 1,2-addtion and 3,4-addition product protons were not spotted intensely. Although a small 17 ACS Paragon Plus Environment


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hump, around 4.70 ppm was observed. The microstructure of polymyrcene predominantly contains 1,4-addition product. A distinct broad peak at δ=2.05 ppm for the methylene protons is observed, which indicates the generation of more methylene units in the polymer. The sharp signals for the methyl protons of 1,4 addition product were observed at 1.69 and 1.62 ppm, respectively. Figure 5d shows the

13C

NMR spectrum of redox initiated polymyrcene. The signals of all

the methylene carbons occur at 26.97, 30.42 and 37.09 ppm respectively. The polymyrcene polymer also contains the signature peak of 1,4-addition product at 139.01 and 124.43 ppm (2, 3, 2’, 3’-olefinic carbons). Chemical shifts for the isolated double bonded carbons appear at 131.41 and 124.43 ppm respectively. The characteristic peaks of 3,4 and 1,2 vinyl units were absent, suggesting that the microstructure is sans of any 3,4 and 1,2 vinyl defects. This is because the side reactions are inhibited at lower reaction temperature, thereby leading to the formation of 1,4-polymyrcene microstructure. Effect of ingredients/polymerization condition on the polymer microstructure. Redox emulsion polymerization has a marked effect on the microstructure of polymyrcene, synthesized using various surfactants, redox couple and initiators at room temperature. The ratio of the various microstructures in the synthesized polymyrcene was calculated using the area under the peaks of each 1H NMR spectrum (Figure S1-S6, Supporting Information). The six protons of methyl groups (chemical shift, δ = 1.62 and 1.69 ppm) present in both microstructures were used as a reference peak for the calculation of microstructures (Figure 6). The area under the six methyl protons (marked as region A in Figure 6), the methylene protons (8 from 1,4- units and 4 from 3,4-units) and a methine proton from 3,4-addition, marked as region B, and the vinyl protons (Region C) are considered for calculation.


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According to Cawse et al.28 the molar ratio of 1,4-addition is considered as X and molar ratio of 3,4-addition as Y. Then the ratio of A/C protons is given by, 𝑨 𝟔 = 𝑪 𝟐𝑿 + 𝒀

― ― ― ― ― ― ― ― (𝟏)

Similarly, the ratio of A/B protons is given by the expression, 𝑨 𝟔 = 𝑩 𝟖𝑿 + 𝟓𝒀

― ― ― ― ― ― ― ―(𝟐)

Taking the arithmetic mean of both equations (1) and (2) and since X + Y = 1, the percentage of 1,4-addition can be calculated as: 𝑿 (𝟏,𝟒 ― 𝒂𝒅𝒅𝒊𝒕𝒊𝒐𝒏) = 𝟏 ―

𝟕𝑨 ― 𝟑𝑩 ― 𝟗𝑪 𝟑𝑨

― ― ― ― ― ― ― ―(𝟑)

Figure 6. 1H NMR spectrum of polymyrcene (area under the curve used for calculation). The absence of characteristic signal at δ around 5.5-5.6 ppm indicates that the 1,2-content present is either negligible or below the limits of detection from NMR spectra. So, we have neglected to consider the 1,2-microstructure. But the presence of small hump at around δ= 4.80 ppm in each NMR spectra also reflects the probability of formation of 3,4-



Page 20 of 40

microstructure, which is obtained from the calculations used to analyze the microstructure (Table 3) of polymyrcene polymers. Table 3. Percentage of microstructure obtained using various redox recipes. Sample

1,4-polymyrcene

3,4-polymyrcene

(polymyrcene)

(%)

(%)

R.R-1 (Main Recipe) R.R-2 R.R-3 R.R-4 R.R-5 R.R-6

90 66 90 44 68 63

10 34 10 56 32 37

So, taking the yield and molecular weight into consideration among the entire selected recipes for polymerization of β-myrcene, we have chosen to proceed with the final base recipe (optimized condition) for detailed characterization study and properties of the polymer. Therefore, the polymyrcene polymer obtained from the finalized recipe (R.R-1) was selected for thorough investigations on molecular structure by high field 2D NMR spectroscopy and properties by DSC analysis. 2D NMR Characterization. The characterization of high molecular weight polymers is one of the areas to benefit from the introduction of 2D NMR analysis. The objective of twodimensional NMR was to provide correlations between proton and carbons signals or spatial coupling between protons to one another based on some interaction, mostly J-coupling (HH, H-C or even C-C). 2D NMR techniques are especially useful in determining the connectivity between different types of nuclei in the polymyrcene polymer. From the twodimensional spectroscopy, homonuclear correlation (COSY and NOESY) and heteronuclear correlation (HSQC and HMBC) spectra were mostly studied in this research. Both F1 and F2 axis of the spectrum are guided by proton and carbon resonance frequencies. 20 ACS Paragon Plus Environment

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COSY NMR.


1H-1H

COSY (Correlation Spectroscopy) is a convenient method for

determining couplings arising from neighboring protons (usually up to four bonds). The COSY spectrum (Figure 7) for polymyrcene contains both diagonal and cross peaks. The magnetization transfer occurs through bonds to the same type of nucleus (protons) and that make up the cross peaks’ coordinates. The correlation spot appear when there is spin-spin coupling between the protons, but when there is no coupling, no correlation spot in the form of cross peak is expected to appear. The cross peaks indicate couplings between the two protons, up to three or sometimes four chemical bonds apart. The most apparent crosspeak in the spectrum indicates a coupling interaction between 1,4,5,6-H and 3,7-H at 2.05 and 5.14 ppm (Figure 7). A weakly four-bond correlation spot (see the figure below) appears between 9,9’-H and 10,10’-H with 3,7-H at 1.62, 1.69 ppm and 5.14 ppm.

Figure 7. COSY (1H-1H) NMR of PMY (Main recipe, R.R-1). 21 ACS Paragon Plus Environment


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NOESY NMR. 1H-1H long range NOESY spectrum is useful for determining the correlation arising from the protons that are spatially close enough. NOESY experiments correlate all protons which are close enough to each other. A NOESY spectrum produces through space correlations via spin-lattice relaxation (nuclear overhauser effect, NOE). Figure 8 presents the NOESY spectrum of PMY having cross-peaks connecting signals from identical nuclei which are spatially close enough. The space correlation between the magnetically different methylene protons (1, 4, 5, 6-H) with the olefinic protons (3, 7, 3’, 7’-H) indicates that the coupled nuclei are close to each other via spatial arrangement in the polymer. A much weaker correlation spot was observed, due to the interaction between the double bonded protons (3, 7, 3’, 7’-H) and the protons of the methyl groups (10 and 10’-H).

Figure 8. NOESY (1H-1H) NMR of PMY (Main recipe).


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Heteronuclear Single Quantum Correlation (HSQC) Spectroscopy. The heteronuclear spectra relate information about proton-carbon single bond correlations and provide chemical information in the polymer. The HSQC experiment correlates carbons (13C chemical shift, F1 axis) with their directly attached protons (displayed on the F2 axis) via the 1JCH coupling. It is rich in information since it equally specifies the list of directly bound 1H-13C pairs present in a molecule. Each proton signal is correlated with a carbon signal. From Figure 9, the vinyl protons (3,7,3’,7’-H) at 5.14 ppm correlate with vinyl carbons (3,7,3’,7’-C) at 124.43 ppm. The methylene protons (1,4,5,6-H) at 2.05 ppm are correlated with carbon signals (4,6-C, 1-C, 5-C) at 26.97, 30.42, and 37.09 ppm respectively. Correlations between the protons (9 and 10-H) at 1.62 and 1.69 ppm attached to the methyl carbon (9 and 10-C) at 17.70 and 25.70 ppm were also observed. All of the assignments of HSQC spectral analysis support the formation of polymer.

Figure 9. HSQC NMR of PMY (Main recipe). 23 ACS Paragon Plus Environment


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Heteronuclear Multiple Bond Correlation (HMBC) Spectroscopy. This is a long-range proton-carbon single bond (with two or more chemical bonds) heteronuclear correlation, where the protons and the carbons lie along the F2 (X-axis) and F1 (Y-axis) domains of the spectrum. Each proton signal may be correlated with one or more carbon signals. In the case of polymyrcene (Figure 10), all the three-bond correlations and two-bond correlations appear strongly. The methyl protons (9 and 10-H) at 1.61 and 1.69 ppm correlate with vinyl carbons (7, 7’-C), methyl carbons (9 and 10-C) and quaternary carbon (8-C) at 124.43, 17.70, 25.70 and 131.41 ppm respectively. The vinyl protons (3,7,3’,7’-H) at 5.14 ppm correlate with methyl carbons (9 and 10-C) and methylene carbons (1,4,5,6-C) and show cross peaks at the respective position. The methylene protons (1,4,5,6-H) show correlation between methylene carbons (1, 4, 5, 6-C) and vinyl carbons (2, 3, 7, 8-C) and the cross peaks are well observed.

Figure 10. HMBC NMR of PMY (Main recipe). 24 ACS Paragon Plus Environment

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Effect of polymerization condition on properties of polymyrcene. Thermal Properties of polymyrcene samples: Thermal properties such as glass transition (Tg) and crystallization temperatures are crucial for polymers. An ideal Tg for elastomers should be much lower than the room temperature. Polymyrcene obtained from the main recipe registered a Tg of 70 °C (Figure 11a). The DSC heating ramp from -100 to +100 °C for a variety of PMY polymers synthesized using different recipe indicates completely amorphous nature. In Figure 11b, the change in Tg of the polymers is observed with the variation of recipe. The difference is quite contrasting between the polymer formed by R.R-1 and R.R-6 recipe, due to difference in the percentage of microstructures (Table 3, earlier).

Figure 11. DSC thermograms of (a) redox-initiated polymyrcene (b) polymyrcene with different variation in recipe. Effect of chemical structure of monomer on the polymerization. The prediction of reactivity towards polymerization based on the chemical structure was also investigated. Two different acyclic terpenes (β-ocimene and β-farnesene) were selected for the study, as they resemble β-myrcene molecular framework. The optimised set of reaction conditions and recipe was also followed for all the polymer synthesis from β-ocimene and β-farnesene. 25 ACS Paragon Plus Environment


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In the case of β-ocimene, the molecular structure contains one conjugated diene like that of isoprene. Due to the high polymerizability of the isoprene (presence of conjugated diene unit), β-ocimene can also be polymerized via a free-radical process. Zhao et al.15 reported the polymerization of α-ocimene via free-radical mechanism using hydrogen peroxide as the initiator. However, due to the presence of hindered double bond, the yield of polymerization was low and very low molecular weight oligomers are obtained. Similar is the result of our study on the polymerization of β-ocimene. The monomer conversion is just 15-20 % and only oligomers (Mn ~1.5 kg/mol) are produced. It is significant that the growing radicals cannot propagate through the hindered vinyl double bonds bearing substituent methyl group. Therefore, it is quite notable that the free radical polymerization mechanism is less suited for the polymerization of β-ocimene with sterically crowded diene group. The obtained viscous product was characterized by

1H

NMR

(discussed later) for microstructure determination. 1H

NMR characterization of the synthesised polyocimene. Figure 12a - 12b represents

the 1H NMR spectra of polyocimene polymer. From the NMR spectra of the monomer, the methyl groups protons attached to specific carbon appear at upfield values (δ =1.68-1.85 ppm). The olefinic protons (=CH–) appears at relatively downfield region, around δ= 4.956.38 ppm. After the polymerization, the magnitude of intensity of the olefinic signals decreases. The methylene and methyl protons of polyocimene appear as broad peaks relative to the monomer suggesting the formation of macromolecular chain. The presence of residual olefinic signals in the polymer indicates the non-consumption of unsaturated double bond completely during the polymerization process. This leaves the monomer unreacted. As no signature peaks for 1,4 and 1,2-addition product was obtained, exact information relating


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to formation of microstructures could not be determined. The chemical shifts assigned to magnetically different protons are given below. 1H

NMR of β-ocimene (OC) (CDCl3, 600 MHz): δ-6.38 (1H, 2-H), 4.95 & 5.15 (2H, 1-H),

5.40 (1H, 6-H), 5.50 (1H, 4-H), 2.88 (2H, 5-H), 1.68-1.85 (9H, 3’, 8, 9-H). 1H

NMR of polyocimene (POC) (CDCl3, 600 MHz): δ-6.38 (1H, 2-H), 5.40 (1H, 6-H), 2.88

(2H, 5-H), 2.00-2.30 (1, 4-H), 1.68-1.85 (9H, 3’, 8, 9-H), 5.50 (1H, 4-H, 1,2-microstructure)

Figure 12. (a) 1H NMR of β-ocimene and (b) polyocimene. A similar approach was used to investigate the polymerization of β-farnesene. Previously, the coordination polymerization of β-farnesene with iron-iminopyridine catalysts was reported.29 Later, anionic polymerization of the same was summarized giving 1,4-trans and 27 ACS Paragon Plus Environment


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1,4-cis selective product.30 The presence of conjugated diene unit (structure similar to that of isoprene) allows us to explore the polymerization process via free radical mechanism. The yield of polymerization is about 20-24 % and a molecular weight of (Mn ~ 3200 Da, PDI2.4) was obtained. As the growing radicals propagate, the presence of long pendant side chain in the monomer makes the oligomeric radicals more bulky. After a critical chain length is attained, it will be less prone to perforate into the micelles; thereby the chain propagation process slows down. This circumstance prevents the growth of the polymer chain and therefore reduces the molecular weight and yield of the polymer. It is also quite evident that the presence of long pendant side chain compared to main chain leaves the polymer of low molecular weight. The microstructure of polyfarnesene is dominated mainly by 1,4-cis and 1,4-trans distribution, which is clear from the 1H and 13C NMR spectrum of polyfarnesene. 1H

NMR of β-farnesene (FA) (CDCl3, 600 MHz): δ-6.25 (1H, 3-H), 5.00& 5.15 (2H, 4-H),

4.80&4.85 (2H, 1-H), 5.20 (2H, 7 and 11-H), 2.00 (8H, 5, 6, 9 and 10-H), 1.70 (9H, 8’, 13 and 14-H). 1H

NMR of polyfarnesene (PFA) (CDCl3, 600 MHz): δ-5.12 (3H, 3, 7 and 11-H), 2.05 (12H,

1, 4, 5, 6, 9 and 10-H), 1.62-1.88 (9H, 8’, 13 and 14-H). 13C

NMR of polyfarnesene (PFA) (CDCl3, 600 MHz): δ-135.01 [2-C], 131.00 [3-C], 129.73

[8,12-C], 124.40 [7,11-C], 27.22-39.76 [1,4,5,6,9,10-C], 14.00-19.45 [8’, 13, 14-C]. Figure 13a-c represents the 1H and

13C

NMR of β-farnesene and polyfarnesene. The redox

initiated polyfarnesene (Figure 13b) shows a broad peak at around 5.12 ppm equivalent to olefinic protons (3, 7, 11-H) of =CH– units which indicates the formation of 1,4-addition product. The complete disappearance of olefinic signal at δ=6.25 ppm (3-H) after polymerization indicates that the polymerization took place at C-1=C-2 and C-3=C-4 28 ACS Paragon Plus Environment

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position. The methylene protons at 2.05 ppm arise as a broad peak respective to the monomer, due to the formation of long chain macromolecules after polymerization. No distinct peak for methyl protons is noticed, rather a bimodal peak (δ =1.62 ppm and 1.88 ppm) appeared in the 1H NMR spectrum. The

13C

NMR spectrum of polyfarnesene (Figure 13c) shows several sharp peaks. The

presence of peaks around 135.01 and 131.00 ppm in polymer indicates the formation of 1,4microstructure. The isolated unsaturation is undeniably preserved showing peaks at 124.40 and 129.73 ppm respectively. The chemical shift values of all the other methyl and methylene carbons are in conformity with those of previous NMR studies on polyisoprene31,32 and polymyrcene (discussed earlier).

.



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Figure 13. (a) 1H NMR of β-farnesene. (b) and (c) 1H and 13C NMR of polyfarnesene. In the case of polymerization of β-ocimene, the reactivity through the isolated double bond (C6=C7) was not observed clearly due to overlapped peaks in the 1H NMR spectrum (Figure 12), whereas the presence of peaks due to isolated double bonds (C11=C12) in both the 1H and

13C

spectra (Figure 13) of polyfarnesene suggests nonparticipation of

pendant double bonds for reaction. Although very little reactivity on the pendant double bond is expected to have a small amount of crosslinking, but there is no gel fraction obtained.

This

was

also

verified

from

the

corresponding

NMR

spectra.

Explanation by Density Functional Theory (DFT). It is among the most popular and allround methods in computational chemistry to explore the electronic structure (principally the ground state) of molecular geometries. Specifically, DFT finds increasingly broad application for the interpretation and prediction of reactivity in a molecule. Figure 14a shows the ground state optimised chemical structure of β-myrcene molecule computed by B3LYP/6-311G (d, p) modelling. It is clear that C-1, C-2, C-3 and C-4 carbons are surrounded by a greater electron density, making the region more suitable towards polymerization. From Figure 14b, 30 ACS Paragon Plus Environment

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it is noticed that the HOMO is located over the entire molecule and the LUMO (Figure 14c) is mainly spread onto the conjugated double bond position. This suggests greater accumulation of charge density on the conjugated region, which makes the site very much susceptible to polymerization.

Figure 14. (a) Optimized ground state chemical structure of β-myrcene (b) and (c) the molecular orbitals (HOMO and LUMO) of β-myrcene in B3LYP/6-311G (d, p) mode. The formation of polyocimene addition product was also supported and explained from the theoretical calculation via Density Functional Theory (DFT). To get an idea about the relation between structural moiety and reactivity of the molecule, DFT modelling method was conducted with Becke's three-parameter hybrid functional (B3LYP) system using 6311G (d, p) as a basis set. Figure 15a shows the optimised structure of β-ocimene obtained by computational modelling. The dipole moment of ground state β-ocimene obtained is 31 ACS Paragon Plus Environment


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0.9449 Debye. The electronic charge around the central atoms has been considered for predicting the relative sites of reactivity towards polymerization. From the ground state molecular structure, it is clear that C-1, C-2 carbons are more electron rich, thus making this area more prone towards polymerization giving favourably 1,2-microstructure. Figure 15b and 15c represents the HOMO and LUMO of the molecule, which shows HOMO, is outspread over the entire molecule, and LUMO is mainly localized over the conjugated double bonds. Therefore, it is quite evident that polymerization of β-ocimene proceeds via the conjugated double bonds.

Figure 15. (a) Optimized ground state structure of β-ocimene (b) and (c) the molecular orbitals (HOMO and LUMO) of β-ocimene in B3LYP/6-311G (d, p) model.


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The low polymerization of β-farnesene was also verified from the DFT analysis (Figure 16). The ground state optimized structure shows that the C1, C3, C4 atoms are electronically rich bearing negative charge (Figure 16a). Therefore, the relative reactivity towards electrophilic attack is more in the conjugated double bonded region. Also from the HOMO and LUMO orbital picture (Figure 16b and 16c), LUMO is mainly confined onto the conjugated double bonds, i.e. C-1=C-2 and C-3=C-4 positions, thus making the area potentially more active towards polymerization.

Figure 16. (a) Optimized ground state structure of β-farnesene (b) and (c) the molecular orbitals (HOMO and LUMO) of β-farnesene in B3LYP/6-311G (d, p) model.



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4. CONCLUSIONS In this study, we investigated the solvent-free radical polymerization of terpenes, green monomer from renewable resources. Out of all the monomer chosen for polymerization study, β-myrcene comes out as a most suitable candidate for the successful polymerization. The polymerization of β-myrcene resulted in a rubbery polymer having Tg of -70 °C. From the 1H and

13C

NMR analysis, the redox initiated polymyrcene contained predominately 1,4-

microstructures.The components of redox polymerization were varied based on main recipe, such as different surfactants, initiators, and redox couples. These variations in components showed a significant effect on the kinetics, yield of the polymerization and structural properties of the polymyrcene. The effect of anionic surfactants on the polymerization rate was more as compared to other surfactants. Redox emulsion experiments also showed that the systems using the TBHP/SFS redox pair provided the lower induction period, higher conversion and highest molecular weight. Taking the best recipe (R.R-1) into consideration, the resulting polymyrcene was characterized in detail. Two-dimensional NMR showed the couplings between proton and carbons signals or spatial proximity of protons to one another, which supports the formation of polymyrcene. It also helps to see the connectivity between different types of nuclei in the polymyrcene polymer via homonuclear correlation (COSY and NOESY) and heteronuclear correlation (HSQC and HMBC) phenomenon. The glass transition temperature of the synthesized polymyrcene confirms the amorphous rubbery nature of the polymer. Spectroscopic measurements coupled with DFT calculation supported the participation of conjugated double bond in the polymerization process. The study describes the synthetic methodologies, process design and development of terpenebased polymers, classified by the position of their double bonds, nature of substituents and molecular architecture. 34 ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Different redox recipe used for synthesis of polymyrcene, 1H NMR spectra of polymyrcene using different recipe. AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; [email protected] ORCID Anil K. Bhowmick: 0000-0002-8229-5353 Pranabesh Sahu: 0000-0002-5010-3900 Notes The authors declare no competing financial interest toward any individual or organization. ACKNOWLEDGEMENTS The authors gratefully acknowledged IIT Kharagpur for all the supportand necessary facilities. Pranabesh Sahu deeply thanks CSIR (HRDG), New Delhi, for the senior research fellowship (Ref. No: 20/12/2015(ii) EU-V). AKB acknowledges the partial support of Uchhatar Avishkar Yojana (UAY), MHRD, New Delhi and INAE Chair Professorship.



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Redox Emulsion Polymerization of Terpenes: Mapping the Effect of the

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