Research Article pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10084-10092
Controlled Radical Polymerization of Myrcene in Bulk: Mapping the Effect of Conditions on the System Nicole Bauer,† Jessica Brunke,†,‡ and Gergely Kali*,† †
Organic Macromolecular Chemistry, University of Saarland Campus C 4.2, 66123 Saarbrücken, Germany INM-Leibniz Institute for New Materials, Campus D2 2, 66123 Saarbrücken, Germany
‡
S Supporting Information *
ABSTRACT: Solvent-free reversible deactivation radical polymerization of myrcene, a naturally occurring terpenoid monomer, with high regioselectivity was developed recently. Here, this green polymerization system is further improved to reach increased yields and produce polymers with high molar mass but still low dispersity and regioregular microstructure. To this end, two initiators (dibenzoyl peroxide, DBPO; azobis(isobutyronitrile), AIBN) at 65, 90, and 130 °C were applied, and it was demonstrated that these varying conditions have a huge effect not only on the monomer conversion and the molar mass of the product, but also on the microstructure of the resulting polymyrcene. The polymerizations utilized two trithiocarbonate chaintransfer agents, and were similar in yields, molar masses, and dispersity of the produced polymyrcene, but progressed differently for the diverse initiator−temperature pairs. Generally, in all systems, pseudo-first-order kinetics, linear increase of molar mass with conversion, and low Đ values were found as a result of controlled polymerization. The systems using AIBN and DBPO initiators at 90 and 130 °C, respectively, have rate constants of propagation (kapp p ) lower than the decomposition rates (kd) of initiators, likewise important to control the polymerizations. At 130 °C, also branching occurred at the higher stage of the reaction, and lower regioregularity developed during the polymerization as a consequence of the favorable junction formation at elevated temperature and increased viscosity. Generally, compared to the previous study on the reversible deactivation radical polymerization of myrcene via reversible addition−fragmentation chain-transfer polymerization process, significantly higher conversions (30 → 65%) and increased chain length (9 → 40 kDa) were reached. The dispersity values for these polymerizations remained as low as 1.3−1.6, and also regioregular microstructures (up to 94%) were detected. KEYWORDS: RAFT polymerization, Terpene, Myrcene, Trithiocarbonates, Green polymers
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INTRODUCTION
from renewable resources is that the modification during the polymerization process, such as functionalization/copolymerization are also possible. For this reason, bio-renewable monomers became more attractive as feedstocks than polymers. While most sustainable monomers like lactide, γ-butyrolactone, or norbornene are capable of undergoing ring-opening polymerizations, only terpenes are known to be polymerized by radical initiators.4,7,8 The structure of these molecules is derived from isoprene, the building block of natural rubber. Terpenes may consist of one, two, three, four, or more isoprene units and
The global problem of plastics industry being dependent on fossil resources can be solved by the exploitation of natural polymers, such as cellulose and natural rubber.1−4 The main drawback of using these polymers from bio-renewable resources is the difficult isolation of the pure macromolecule. Delignification of cellulose is a costly and pollutive process.5 Also, removal of proteins from natural rubber is cumbersome, because residual proteins, lipids, and other bioactive materials can cause allergic reactions in humans, hampering food grade and medical applications.6 To overcome this problem, biobased monomers can be used for polymerization, since purification of monomers (e.g., by distillation) is more simple than that of polymers. Another advantage of using monomers © 2017 American Chemical Society
Received: June 26, 2017 Revised: September 11, 2017 Published: September 14, 2017 10084
DOI: 10.1021/acssuschemeng.7b02091 ACS Sustainable Chem. Eng. 2017, 5, 10084−10092
Research Article
ACS Sustainable Chemistry & Engineering
polymerization of myrcene. Namely, azobis(isobutyronitrile) (AIBN) and dibenzoyl peroxide (DBPO) initiators, and 2ethylsulfanylthiocarbonylsulfanylpropionic acid ethyl ester (ETSPE) and S-1-dodecyl-S′-(α,α′-dimethyl-α″-acetic acid)trithiocarbonate (DATC) CTAs, were used for the synthesis. We chose these CTAs since they had already successfully mediated the radical polymerization of isoprene, a diene with similar structure to myrcene.32,33 The main points of this study are to increase yield and molar mass simultaneously while keeping the dispersity low and the microstructure of the produced polymer regioregular.
are called hemi-, mono-, sesqui-, and diterpenes. Myrcene is a readily available monoterpene isolated from renewable resources pine, hops, and bay leaves. The biocompatibility of this monomer is confirmed by the U.S. Food and Drug Administration (FDA) since myrcene is approved as a food additive.9 The polymerization of myrcene is necessary for various aspects. The excellent biological and medical properties,10−12 as well as the high elongation at break of polymyrcene13,14 in combination with the two double bonds per repeating units and its bioavailability, are all vital features for green material science.7,9 The post-modification or postpolymerization of the double bonds widen the application potential not only as hydrophobic but also as amphiphilic or even as hydrophilic polymers or cross-linked structures.15−17 Polymyrcene is already used in material science, in polyurethane synthesis,15−17 production of biodegradable articles for personal use18 or adhesives,19 or even in biomedicine.10−12 Myrcene was previously polymerized by radical, anionic, cationic, and coordinative initiators. Free radical polymerization does not work well leading to branched/cross-linked products due to chain transfer.15−17,20 The highest conversions (close to 100%) were indeed obtained by anionic initiator in THF and benzene with molecular weights ranging between 5 and 30 kDa, but control of the structure was unsatisfactory (content of 1,4product only 40−50%).21,22 On the other hand, coordinative initiators like Ziegler−Natta-type or lanthanide catalysts gave rise to very high regularity (>98%) but poor polydispersities (PDI ≥1.5 or even higher), and these polymerizations cannot be considered as controlled ones.23−27 Reversible deactivation radical, or so-called controlled radical polymerization of several dienes, including naturally occurring, especially terpenoid monomers became the focus of attention in polymer chemistry.28 Reversible addition−fragmentation chain-transfer polymerization (RAFT) and nitroxide-mediated radical polymerization (NMP) were already successfully applied in myrcene polymerization. Very recently, NMP of myrcene, reaching 80% conversion and molecular weights up to 13 kDa, was described. This solvent-free method resulted in controlled microstructure, around 80−91% of 1,4 product.29 We also described a new system for the polymerization of myrcene via RAFT approach using a trithiocarbonate CTA in bulk.30 RAFT is a controlled polymerization technique that applies dithioesters, dithiocarbamates, trithiocarbonates, or xanthates to mediate radical polymerization.31 The resulting product had a regioregular microstructure, as high as 96% 1,4-addition, and predictable molar mass. The dispersity was low, in the range of 1.1−1.4, as a result of the controlled radical polymerization. In contrast, no pure thermal polymerization in the absence of initiator occurred, while free radical polymerization resulted in polymyrcenes with dispersities higher than 4. Also, copolymerization of RAFT-polymerized myrcene with styrene resulted in well-defined AB block copolymer that foreshadows some application possibilities, such as thermoplastic elastomers. The polymerization of this naturally occurring terpene, without consumption of organic solvent, is an excellent addition to the field of green polymer chemistry. The main problems of the RAFT polymerization of myrcene are the relatively low yield and consequently the short chains formed during the polymerization. The monomer conversion could reach only 30−50%, while the molar mass remained below 10 kDa, which is not adequate for potential applications.30 In this study, we aimed to investigate the effects of initiator, temperature, and chain-transfer agent (CTA) for the controlled
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EXPERIMENTAL SECTION
Materials. Myrcene (Aldrich, technical grade) and styrene (Aldrich, ≥99%) monomers were distilled under reduced pressure. 1,1′-Thiocarbonyl diimidazole (90%), 2-mercaptopropionate (≥95%), and ethanethiol (97%) were all purchased from Aldrich and were used as received. Toluene (Molar Chem) was refluxed over and distilled from Na before use. AIBN (Aldrich, 98%) was recrystallized from methanol and stored under N2 at −18 °C before the reaction. Methanol (Molar Chem) was used as received. Methods. Preparation of the Chain-Transfer Agents. 2-Ethylsulfanylthiocarbonylsulfanyl-propionic acid ethyl ester was prepared by the reaction of 1,1′-thiocarbonyl diimidazole, 2-mercaptopropionate, and ethanethiol as described before.33 1 H NMR (CDCl3): 1.25−1.29 (t, 3H), 1.33−1.37 (t, 3H), 1.58− 1.60 (d, 3H), 3.33−3.39 (q, 2H), 4.16−4.21 (q, 2H), 4.77−4.83 (q, 1H) S-1-Dodecyl-S′-(α,α′-dimethyl-α″-acetic acid)trithiocarbonate was synthesized according to the literature,34 reacting 1-dodecanethiol, acetone, carbon disulfide, and chloroform in the presence of sodium hydroxide utilizing phase-transfer catalyst. 1 H NMR (DMSO-d6): 0.84−0.87 (t, 3H), 1.24−1.33 (m, 20H), 1.62 (s, 6H), 3.28−3.31 (t, 2H), 12.9 (s, 1H). Polymerization Procedures. All polymerization reactions were carried out in dry double-neck round-bottom flasks under N2 atmosphere for 3 days. Oxygen/air was removed from all the reactions by several N2/evacuation cycles at −50 °C. The reaction conditions followed a typical RAFT polymerization procedure as described below. The initiator (0.032 mmol, 5.3 mg AIBN, or 7.75 mg DBPO), the chosen CTA (0.32 mmol, 76.28 mg of ETSPEor 116.68 mg of DATC), and myrcene (10 mL, 7.91 g, 58.1 mmol) were added to Schlenk tubes. After seven vacuum/N2 purification cycles at −78 °C, the tubes were placed in an oil bath for 3 days at a predetermined temperature. Samples were taken from the reaction mixture to follow the polymerization. The polymyrcene products were precipitated into cold methanol without the addition of any stabilizer. The yellow, oily product was dried under vacuum at room temperature for 24 h. Dark yellow oily products were obtained. Characterization Methods. Monomer conversions were determined by gravimetry. To gain information about the molecular structure and to calculate the molar masses of the polymers, 1H and 13 C NMR spectra were obtained by using a Bruker Avance Ultrashield 400 (400.2 MHz for proton and 100 MHz for carbon measurements) spectrometer. ETSPE CTA and all the dry polymer samples were dissolved in deuterated chloroform (15 mg sample in 0.6 mL solvent), and measurements with 256 scans were performed. The signal of the solvent (7.27 ppm) was used as a reference for chemical shifts. The CTA DATC was measured in DMSO-d6 to investigate the carboxyl protons. Molar masses and dispersities of the polymers were measured by gel permeation chromatography (GPC) at 25 °C. GPC system was equipped with a PSS SDV 103 Å column and a Waters 2410 refractive index detector. The mobile phase was THF, and the flow rate was maintained at 1 mL/min using a Viscotek VE1121 GPC pump. The SEC calibration curve was based on linear polyisoprene standards for all measurements from 1070 to 47 300 g mol−1 from PSS. Measurements were carried out at room temperature. 10085
DOI: 10.1021/acssuschemeng.7b02091 ACS Sustainable Chem. Eng. 2017, 5, 10084−10092
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Scheme 1. Reversible Addition−Fragmentation Chain-Transfer Polymerization of Myrcene and the Possible Microstructures
Scheme 2. Chain-Transfer Agents, (Left) 2-Ethylsulfanylthiocarbonylsulfanyl-propionic Acid Ethyl Ester and (Right) S-1Dodecyl-S′-(α,α′-dimethyl-α″-acetic acid)trithiocarbonate, Applied in This Study To Mediate Radical Polymerization of Myrcene
The thermal properties of the polymyrcenes of this work were investigated using thermogravimetric analyses. The measurements were performed by Simultane Thermoanalyse STA Netzsch 449 C instrument from 35 to 800 °C, under argon flow at a heating rate of 5 K/min.
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RESULTS AND DISCUSSION Polymyrcene with highly ordered structure but low conversion was recently synthesized via RAFT polymerization, using 2ethylsulfanylthiocarbonylsulfanyl-propionic acid ethyl ester (ETSPE) chain-transfer agent (Scheme 1).30 In that work, initiator/CTA/monomer ratio of 0.1/1/180 was found to produce polymers with low dispersity (≤1.25) and close to the theoretical molar mass. For this reason, this feed composition is chosen to apply for this work. Here, this ETSPE and S-1dodecyl-S′-(α,α′-dimethyl-α″-acetic acid)trithiocarbonate (DATC) CTAs were used in combination with AIBN and DBPO radical sources for 3-day reactions. The initiators were chosen to be suitable for a large range of temperature (65−130 °C) that could be covered for the reaction. The two trithiocarbonates, already used in RAFT polymerization of isoprene,32,33 have different reinitiating (R) groups with various stabilities of the formed radicals that could effect on the preequilibrium of the polymerization (Scheme 2). First, the systems initiated by AIBN have been investigated. The polymerization kinetics was studied by sampling the polymer at predetermined times, followed by gravimetric analysis of the purified samples. As Figure 1 demonstrates, the conversions are highly depending on the temperature. In our previous study, using the AIBN-ETSPE system at 65 °C, 35% conversion was reached, while utilizing the same RAFT agent and initiator at 90 °C, the conversion becomes notably higher (43%). On the other hand, same radical source at 65 °C in combination with DATC CTA results in low conversion
Figure 1. Conversion versus time plots for bulk RAFT polymerization of myrcene using ETSPE and DATC CTAs at various temperatures, utilizing AIBN radical source.
(13%), but the reaction times for the final products were not equal. At 90 °C with this DATC-AIBN system, also high conversion could be reached (∼78%). The semilogarithmic plots of both CTAs with AIBN initiator at 65 and 90 °C are linear (Figure 2). This linear correlation between the negative natural logarithm of the conversions (−ln(1 − C)) and reaction times indicates that the monomer consumption follows pseudofirst-order kinetics, which is a mandatory requirement for controlled polymerization. For AIBN-initiated and DATCmediated polymerization at 90 °C, only conversions below 25% were taken into account for the semilogarithmic plot. The deviation of the point at 78% conversion from the linear in the kinetic plot suggests some side reactions, such as chain transfer to monomer, at this higher stage of polymerization. To get comparable results for the different CTAs at the two applied temperatures, the rate constants of propagation have to be investigated for all those systems described. This is also 10086
DOI: 10.1021/acssuschemeng.7b02091 ACS Sustainable Chem. Eng. 2017, 5, 10084−10092
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Figure 2. Semilogarithmic plots for the bulk RAFT polymerization of myrcene using ETSPE and DATC CTAs at various temperatures, utilizing AIBN radical source.
Figure 3. Conversion versus time plots for bulk RAFT polymerization of myrcene using ETSPE and DATC CTAs at various temperatures, utilizing DBPO radical source.
important, because in some cases, mostly at lower temperature, some induction period is detectable in the conversion plots that could caused by the lower initiation than propagation rate (solid and hollow squares in Figure 2). The apparent rates of propagations (kapp p ) are determined as the slope of the pseudofirst-order kinetic plots and displayed in Table 1, together with the decomposition rates of the initiators used.
linear in all cases, indicating pseudo-first-order monomer conversion, as expected for RAFT polymerizations.
Table 1. Apparent Rate Constants of Propagation (kapp p ) and Decomposition Rates of the Initiators (kd) for the Various Systems CTA
a
initiator
T (°C)
−1 kapp p (s ) −5
kd (s−1)a
ETSPE
AIBN AIBN DBPO DBPO
65 90 90 130
8.20 1.38 6.33 2.20
× × × ×
10 10−4 10−5 10−4
9.15 1.52 3.11 5.73
× × × ×
10−6 10−3 10−7 10−3
DATC
AIBN AIBN DBPO DBPO
65 90 90 130
4.31 3.65 9.71 3.52
× × × ×
10−5 10−4 10−5 10−4
9.15 1.52 3.11 5.73
× × × ×
10−6 10−3 10−7 10−3
Decomposition rates of AIBN in benzene and DBPO in toluene.
Figure 4. Semilogarithmic plots for the bulk RAFT polymerization of myrcene using ETSPE and DATC CTAs at various temperatures, utilizing DBPO radical source.
The apparent rate constants for propagation, presented in Table 1, were carefully examined again. First of all, a lower rate of initiation than the rate of propagation at 90 °C elucidates the induction periods for these reactions (solid and hollow squares in Figure 3). Again, the rate constants for propagation were higher at elevated temperature, as it was expected, and independent of the CTA. Higher decomposition than propagation rate at increased temperature indicates instant initiation followed by chain propagation. The kapp p values for DBPO at 130 °C and AIBN at 90 °C initiated systems could be compared due to the same order of magnitude kd values of these initiators at the given temperatures. For those systems with the two CTAs, no significant difference between propagation rates were found (kapp p = 1.38 × 10−4−3.65 × 10−4 s−1). For both mediators, in combination with the two initiators at the given temperatures, no any effects of these variables on the rate constants were found. These results forfeit that the applied system is versatile and the kinetics not depend on the radical source nor on CTA. The molar mass evolution was also investigated with the conversion for all the systems, using ETSPE and DATC CTAs in combination with AIBN and DBPO initiators at various temperatures. As it can be concluded from Figure 5, the number-average molar masses ( M n ) increased linearly with the conversion. The correlation coefficients were ≥0.94 with the
35
From the comparison of the kpapp of propagation and decomposition rates of AIBN at 65 and at 90 °C, it is clearly seen that at the lower temperature, the decomposition of the initiator is slower than the propagation of the polymer chain, and this causes the induction period. Since for reversible deactivation polymerizations faster initiation than propagation is essential, reaction at 65 °C is not preferred. At 90 °C, the decomposition of the initiator is faster than the propagation that makes this system more attractive for our purposes, i.e., controlled myrcene polymerization. At this elevated temperature also, higher rate constant for propagation was determined than that of for 65 °C, roughly independent from the CTA. DBPO initiator was also used in combination with both ETSPE and DATC CTAs at various temperatures, similar to and higher than those used for AIBN. The monomer conversions utilizing ETSPE at 90 and 130 °C after 3 days were 21 and 50%, respectively. Applying DATC mediator for the polymerization with DBPO initiator, 34% conversion at 90 °C and 64% at 130 °C could be reached (Figure 3). The semilogarithmic plots of the polymerizations with different CTAs at various temperatures, presented in Figure 4, were 10087
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exception of the DATC-mediated reaction with AIBN initiator at 65 °C (Figure 5a). This later polymerization developed also linearly, having a correlation coefficient 0.87, probably due to the experimental error in the low conversion/molar mass region. The dispersity (Đ) values remained in all cases below 1.5−1.6, mostly below 1.4. This development of M n and the low Đ values are also mandatory for reversible deactivation radical polymerizations, and, in combination with the linearity of the semilogaritmic plots, confirm that all the reactions were controlled, or fairly controlled. The graphs for other initiator/ CTA/temperature systems are presented in the Supporting Information (Figures S1−S3). The reached molar masses depended highly on the conversion, as expected and shown in Table 2. Using AIBN as an initiator in combination with ETSPE CTA, the molar mass after 3 days reaction was around 9 kDa at 65 °C, and 12.7 kDa at 90 °C. The same initiator in combination with DATC results in 7.1 kDa polymer at 65 °C. Identical reaction conditions but at 90 °C, molar mass increases linearly until 25% monomer conversion producing 9 kDa polymers. For this system, the conversion was further increased, but the kinetic results were not matched with the theory of controlled polymerization. At higher conversion, the molar mass decreased to 3.7 kDa, and broader molar mass distribution (1.9) was observed. The reason for the dropping M n and the broadening of the distribution is conventional chain transfer to monomer, as already assumed based on the semilogarithmic plot. In this case, the chain transfer to monomer results in newly initiated polymer chains, reducing the apparent molar mass and broadening the Đ. For all polymerizations applying DBPO initiator, independent of CTA and temperatures, a linear increase of molar mass with conversion was detected. At 130 °C, the 40 kDa region was reached, but in this case, the dispersity values increased around and above 1.6. The measured molar masses were also compared with the theoretical ones, calculated from the feed composition and determined conversion using eq 1, Mntheor =
MMon[M]0 C + MCTA [CTA]0
(1)
where C is conversion, and presented as dashed lines in Figure 5. In the case of controlled polymerization, no side reactions were taking place, and the theoretical values agree well with the measured M n s. This is the case for AIBN initiator at both temperatures and CTAs, and DBPO initiator at 90 °C, also independent of the mediator of the polymerization. In contrast, at elevated temperature, there is a huge deviation from the theoretical values. Using DBPO initiator at 130 °C, for both CTAs around 40 kDa molar mass was detected above 50% conversion, much higher than the theoretical 15−20 kDa value. This result is connected to the side reaction of branching through the remaining double bonds on the backbone (Scheme 3). This side reaction is already well known for various dienes, and mostly occurred in high-temperature polymerizations and also at high monomer conversions.28 The increased viscosity of this bulk polymerization system and the applied high temperature make possible this branching at higher conversions. To prove the branching or cross-linking, two independent methods were employed to determine the M n of the polymer, and consequently to study this side reaction. GPC establishes
Figure 5. Evolution of molar masses (■) and Đ (●) with conversion for the bulk RAFT polymerization of myrcene using the initiator-CTA system (a) AIBN-DATC at 65 °C, (b) AIBN-ETSPE at 90 °C, (c) DBPO-ETSPE at 90 °C, and (d) DBPO-DATC at 130 °C. The theoretical molar masses (dashed lines) are also included. 10088
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Table 2. Monomer Conversions (C), Number-Average Molar Masses ( M n ), Dispersities (Đ), and 1,4-Microstructures of Polymyrcenes via RAFT Polymerizations Utilizing ETSPE and DATC CTAs and AIBN and DBPO Initiators at Various Temperatures ETSPE
DATC
AIBN C (%) M n (kDa) Đ 1,4 content (mol %)
DBPO
AIBN
DBPO
65 °C
90 °C
90 °C
130 °C
65 °C
90 °C
90 °C
130 °C
35 8.9
43 12.7
21 5.7
50 43.7
13 7.1
25 9.8
34 16.1
64 39.0
1.3 95
1.6 91
1.7 87
1.4 75
1.4 94
1.4 90
1.5 90
1.6 75
Scheme 3. Schematic Representation of the Possible Branching at High Conversions during the Polymerization at 130 °C
Figure 6. Correlation between the molar masses determined by GPC and 1H NMR for the reactions utilizing DATC CTA in combination with (a) AIBN initiator at 65 °C and (b) DBPO radical source at 130 °C.
the molar mass based on the hydrodynamic volume of the macromolecule. For branched polymers, this size is lower than that of for their linear analogs with the same molar mass, resulting in lower apparent than real molar masses from the measurement. Using the integral values of appropriate signals of the polymeric backbone and the CTA, 1H NMR is also a sufficient and size-independent method to determine the molar mass of polymers. In our case, the ratio of integrals of the signals of methine groups at the double bonds of polymyrcene between 4.5 and 6.0 ppm, and the methylene protons next to the sulfur at 3.3 ppm of the trithiocarbonate end groups were used for the calculation of the degree of polymerization.30 Comparison of the molar masses, determined by these two methods can reveal if branching during the reaction occurred. In Figure 6, we represent the relationship between the molar masses, determined by GPC and 1H NMR. It is clearly seen that, in the case of reactions achieved at low temperature (Figure 6a, DATC-AIBN at 65 °C), there is no significant difference between the two results, the GPC and the NMR measurements ending up with similar molar masses. Increasing the temperature, and also reaching higher monomer conversions, deviation of the 1H NMR-determined molar masses
from the GPC ones occurs (Figure 6b, DATC-DBPO at 130 °C). Also in this case of reaction at 130 °C, at low conversions, there is a linear correlation between the two molar masses, indicating no branching in this early stage of the reaction, but as the polymerization progressed the difference become more significant. Specifically, the molar mass, determined by GPC remained low, around 10 kDa, while the one determined by 1 H NMR increases even to 40 kDa. This deviation at higher conversions is a clear indication of branching at this late stage of polymerization. Also, interesting point that no insoluble fraction was detected even at high conversions, meaning that only branching occurs and no gelation takes place. Here should be noted, that even after branching, the Đ values remained quite low, around and below 1.6 for both CTAs used at this high temperature, in combination with DBPO initiator, and the increase of molar mass with conversion remained linear. These 10089
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In the case of AIBN-initiated polymerizations at 65 °C, high regioregularity in microstructure was found for both CTAs (94−96%). Increasing the temperature up to 90 °C, the amount of 1,4-addition drops to ∼90% for all the utilized CTAs and radical sources. Finally, for polymyrcenes, synthesized using DBPO initiator at 130 °C, lower regioregularity was found than in other cases. It is already known that radical polymerizations of dienes at lower temperatures result in higher 1,4 content, and as the temperature increases, the regioregularity of the microstructure decreases.36 Even using this elevated temperature for the polymerization, the mentioned low regioregularity, i.e., the amount of 1,4-addition, was around 75%. In contrast to temperature, the effect of monomer conversion on regioregularity was not investigated before. It is of particular importance for bulk polymerizations, where a significant increase in viscosity occurs during the reaction. For this reason, we also investigated the development of the microstructures over conversion for these polymers, produced at 130 °C. As it is demonstrated in Figure 8, 1,4 addition is highly favorable at the
results suggest some control over the reaction despite the branching. For all the above-described systems, it is seen that both CTAs are mediating well the polymerization, regardless of the initiators or temperatures applied herein. Without a doubt, the pseudo first order kinetics together with the linearity of molar mass as a function of conversion, as well as the low Đ values are all confirming the controlled character of these radical polymerizations of myrcene. Microstructure of the Polymyrcenes. One of the greatest addition of RAFT polymerization to polymyrcene production is the highly regioregular microstructure of the product with simple synthesis route, in bulk and using cheap starting materials. In our recent study, we reached around 96% 1,4-addition, that is close, although less selective, to the regioregularity of polymyrcene produced by the highly regio-/ stereoselective, but costly coordination polymerization (98%).23−25 Since it is already described that the RAFT polymerization has some effect on the microstructure of dienes,28 the structural units in polymyrcenes, synthesized using various CTAs and initiators on different temperatures, are compared. The ratio of the various microstructures in the polymyrcene was calculated based on the work of Cawse et al. using the appropriate chemical shifts of the polymer as presented in Figure 7.15 For the calculations we used the
Figure 8. Development of the different microstructures with the increasing monomer conversion in DATC mediated RAFT polymerization of myrcene utilizing DBPO initiator at 130 °C, as estimated from 1H NMR.
beginning of the polymerization (∼95%), but as the conversion increases, the 1,4 content decreases and in parallel the amount of 3,4-addition increases. The 3,4 microstructure could also be identified from the 13 C NMR spectrum of this product (Supporting Information, Figure S4), due to the appearance of the chemical shifts of the new olefin group at 154 and 107 ppm and the saturated backbone at 40 ppm, which are not present the 1,4polymyrcene spectrum.30 IR spectroscopy also confirmed the appearance of 3,4 microstructure in the case of polymerization at elevated temperature, due to the absorption band at 890 cm−1 (Supporting Information, Figure S5). Here it should be noted that, besides the occurrence of the two microstructures, branching through the double bonds could not be confirmed or excluded based on the 1H, 13C NMR or IR spectra, due to the superposition of the signals of branched structures with the ones of linear polymyrcene. The branching side reaction could also apparently reduce the amount of 1,4addition that gives rise to 3,4 addition.28,33 Thermal Behavior. The thermal stabilities of the synthesized polymyrcenes, with the highest conversion each, were investigated by thermogravimetric analyses (TGA) under
1
Figure 7. 400 MHz H NMR spectrum of the polymyrcene, synthesized using AIBN/DBPO initiator-CTA system at 65 °C, after 3 days in CDCl3.
integral values of the six methyl protons in region A, the methylenes (8 from 1,4-, and 4 from 3,4-) and a methine from 3,4- in Region B, and the methine protons next to the double bond at Region C, using eqs 2 and 3. A 6 = C 2x(1,4) + x(3,4)
(2)
A 6 = B 8x(1,4) + 5x(3,4)
(3)
From eqs 2 and 3, the molar ratio of the 1,4-addition (x(1,4)) and 3,4-addition (x(3,4)) can be calculated. x(1,4) = 1 −
7A − 3B − 9C 3A
(4) 10090
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Ar flow (for TGA thermograms, see Supporting Information, Figure S6). All the polymers synthesized at 65 and 90 °C show good thermal stability; only moderate weight loss occurs below 300 °C as an onset of degradation. In this first step, chemical modifications, such as cross-linking or cyclization takes place. After this onset, the weights of all the polymers decreasing rapidly. The decomposition temperatures of 50 (±0.47) % weight loss were around 385 °C, as shown in Table 3. The
weight loss (%)
T (°C)
DATC_AIBN_65 DATC_AIBN_90 ETSPE_AIBN_90 ETSPE_DBPO_130 ETSPE_DBPO_90 DATC_DBPO_130 DATC_DBPO_90
50.20 50.29 49.90 50.17 50.19 50.02 50.47
398 387 389 353 380 363 389
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02091. Experimental procedures for the preparation of CTAs; molar mass/dispersity vs conversion graphs; and characterization of some polymers by 13C NMR, FTIR, and thermogravimetry (PDF)
Table 3. Results of Thermogravimetric Analyses of the Produced Polymyrcenes sample
Research Article
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Gergely Kali: 0000-0002-8538-6971 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Prof. Gerhard Wenz is gratefully acknowledged for all of his support. The authors would like to thank Blandine Boßmann for the GPC analyses and Robert Drumm for the help with the TGA measurements.
degradation was complete at 460 °C, with low amount of residue (0.41−3.80%), which is an indication of the high purity of the polymers. In the case of polymers produced at 130 °C, a higher onset (∼30% instead of
