ORIGINAL ARTICLE
Ecofriendly Autoxidation of Castor Oil/Ricinoleic Acid.
Multifunctional Macroperoxide Initiators for Multi Block/Graft
Copolymers
Baki Hazer1,2,3,4· Melike Eren2
Received: 2 May 2018 / Revised: 14 November 2018 / Accepted: 14 November 2018 © 2019 AOCS
Abstract Ecofriendly autoxidation is a reaction of air oxygen with unsaturated organic molecules at room tem-perature. Castor oil and ricinoleic acid were ecofriendly autoxidized for 5 months to obtain castor oil macroperoxide with a Mn of 1935 g mol−1 (Pcast5m) and the ricinoleic acid macroperoxide initiator (Prici5m) with a Mn of 1169 g mol−1. Peroxide groups thermally initiated the free radical polymerization of methyl methacrylate (MMA), n-butyl methacrylate (nBMA), and styrene (S). Peroxide formation in the oxidized castor oil and ricinoleic acid was confirmed using iodometric analysis, elemental analysis, and differen-tial scanning calorimetry technique. Peroxide decomposi-tion in both macroperoxide initiators was observed at 166
C for Prici5m and 170C for Pcast5m. Hydroxyl groups
of Pcast5m were reacted with methacryloyl chloride to obtain methacrylated castor oil macroperoxide (PcastMA). The polymerization rates of the obtained macroinitiators were compared. The polymerization rate order is Pcast5m > Prici5m > PcastMA. Polymerization of styrene by PcastMA resulted in an increase in molar masses and an increase in the polymerization time while those of the sty-rene polymerization by Pcast5m and Prici5m remained
constant. Carboxylic acid groups were reacted with amine-terminated polyethylene glycol (PEG), polydimethyl silox-ane (PDMS), and polytetrahydrofuran (PTHF) while the hydroxyl functionality initiated the ring-opening polymeri-zation of ε-caprolactone (CL). PEG-PMMA, Prici-PS-PDMS, Prici-PS-PTHF, Pcast-PS-PCL, Pcast-PCL-PMMA, and Pcast-PS-PnBMA multiblock copolymers were prepared and characterized using spectrometric, ther-mal, and stress–strain measurement techniques.
Keywords Ecofriendly autoxidation Ricinoleic acid Castor oil Multiblock copolymer Ring-opening polymerization
J Am Oil Chem Soc (2019) 96: 421–432.
Introduction
Environmental issue and limited petroleum resources directed polymer scientists to use renewable resources for chemical and plastic industries. Plant oils have been widely used in the chemical industry due to their renewable nature and relatively low price (Biermann et al., 2010; Metzger, 2009; Miao et al., 2014; Shimada et al., 1992; Wang et al., 2012; Zhang et al., 2014). There are a variety of reviews and severalfine methods reported on the preparation of the polymers from plant oils (Coates and Hillmyer, 2009; Guner et al., 2006; Köckritz and Martin, 2008; Lligadas et al., 2013; Meier et al., 2007). Autoxidation is a useful tool that is a reaction of unsaturated fatty acids with oxy-gen. Oxidation of unsaturated fatty acids and triacylglycer-ols in nature proceeds as autoxidation with the influence of light and air and results in off-flavor compounds and decreases the oil quality (Chen et al., 2010; McClements
* Baki Hazer
[email protected]; [email protected]
1 Department of Aircraft Airframe Engine Maintenance,
Kapadokya University, Mustafapasa kasabası, Üniversite Meydanı, 50420, Ürgüp, Nevsehir, Turkey
2 Department of Chemistry, Bülent Ecevit University, Üniversite
Caddesi, 67100, Zonguldak, Turkey
3 Department of Metallurgical and Materials Engineering, Bülent
Ecevit University, Üniversite Caddesi, 67100, Zonguldak, Turkey
4 Department of Nano Technology Engineering, Bülent Ecevit
University, Üniversite Caddesi, 67100, Zonguldak, Turkey J Am Oil Chem Soc (2019) 96: 421–432
and Decker, 2000). Air oxidation involves free radical intermediates due to the abstraction of allylic hydrogens and the subsequent formation of a delocalized free radical. These free radicals react with ambient oxygen to produce a peroxy radical that is converted into a peroxide or hydro-peroxide via H-abstraction. To accelerate the autoxidation process of the unsaturated plant oils, heating (Jiang and Hammond, 2002), oxygenflow (Fornof et al., 2006), ozo-nolysis (Narine et al., 2007), ultraviolet (UV)-light (Choe and Min, 2006a), and catalysts (Hazer and Akyol, 2016) were applied. Despite it takes longer oxidation time, our motivation is ecofriendly autoxidation of the unsaturated vegetable oils and fatty acids, which is carried out at room temperature under daylight and atmospheric conditions, without using any catalyst. In this manner, macroperoxide initiators were obtained from linseed oil (Cakmakli et al., 2004), soybean oil (Hazer and Kalaycı, 2017), linoleic acid (Allı et al., 2014), linolenic acid (Allı and Hazer, 2011), and oleic acid (Hazer et al., 2017) by this way.
In our recent article, we also studied the ecofriendly autoxidation conditions of soya oil in detail. The oil thick-ness spreads in Petri dish and the autoxidation time in flu-enced the molar mass of the obtained soybean oil macroperoxide initiator (Acar et al., 2013). According to the optimization results, a lower oil thickness and a higher autoxidation time lead to a higher molar mass of the soy-bean oil macroperoxide initiator.In our studies, oleic acid showed the slowest autoxidation (ca. 4 months) compared to both linolenic acid (ca. 1 month) and linoleic acid (ca. 2 months), which agrees well with the previously reported results (Choe and Min, 2006a). Oxygen uptake is typically related to the number of double bonds in the fatty acid molecules. This suggests that the amount of oxygen uptake follows the order linolenic acid > linoleic acid > oleic acid.
The hydroxyl functionality of castor oil and ricinoleic acid attracted much attention of the polymer scientists. Castor oil contains approximately 85–90 wt.% of triacyl-glycerols of ricinoleic acid. Because of hydroxyl function-ality, castor oil and ricinoleic acid are open to produce plant oil derivatives. The self-polycondensation of ricino-leic acid was carried out to obtain ricinoricino-leic acid polymer (Totaro et al., 2014). The polyols obtained by the transes-terification of methyl esters of ricinoleic acid with tri-methylol propane were reacted with diphenyl methane diisocyanate (MDI) to obtain elastomers (Petrovic et al., 2008). Mutlu and Meier reported an overview of recent developments in the preparation and characterization of castor oil-derived polyurethanes, polyesters, and polyam-ides (Mutlu and Meier, 2010).
In this work, we for thefirst time carried out autoxida-tion of ricinoleic acid and castor oil to accomplish the autoxidation in the series of the unsaturated vegetable oils.
Ricinoleic acid and castor oil were autoxidized under atmo-spheric conditions at room temperature to prepare ricinoleic acid and castor oil macroperoxides containing hydroxyl functionality. Likely, oleic acid, the autoxidation of these mono unsaturated natural products took a longer time (ca. 5 months) than poly unsaturated ones. The obtained macroperoxide initiators were used in the free radical poly-merization of styrene [or methyl methacrylate (MMA)/n-butyl methacrylate (nBMA)], ring opening polymerization of ε-caprolactone (CL), and condensation reactions of the amine-terminated polydimethyl siloxane [or amine termi-nated polyethylene glycol (PEG)/polytetrahydrofuran (PTHF)] to obtain multiblock copolymers. A multiple methacrylated castor oil (PcastMA) macroperoxide initiator was also newly synthesized. The products were analyzed using gel permeation chromatography (GPC), nuclear mag-netic resonance (NMR), and Fourier transfom infrared (FTIR) techniques. Mechanical properties of the multi-block/graft copolymers were also evaluated.
Experimental
Materials
Castor oil was supplied by a local stock market in Turkey (originally from India, purity: 86–90 wt.%). Castor oil was hydrolyzed in basic alcoholic solution to obtain pure ricino-leic acid. Briefly, 138 g of KOH was dissolved in 800 mL of ethanol (purity: 96 wt.%) in a 2 L beaker. To this solu-tion, 680 g of castor oil was added. The solution was con-tinuously stirred at 65 C for 3 hours. Then, the solution was acidified by adding slowly 1.5 L of aqueous H2SO4
(30 wt.%) and stirring continuously. The upper ricinoleic phase was separated using a separatory funnel and washed twice with warm distilled water. The obtained ricinoleic acid was dried on anhydrous Na2SO4.
Diamine-terminated polyethylene glycol with MW~2000 g mol−1 (Jeffamine D-2003; PEG2KNH2) (O, O0-bis (2-aminopropyl) polypropylene glycol–block poly-ethylene glycol–block-polypropylene glycol) was a gift from Huntsman Co. (Istanbul, Turkey). The molar ratio of ethylene glycol units to propylene glycol units was 36/9. Oleic acid (purity: 85–88 wt.%) was kindly gifted by “CHS Endüstriyel Ürünler San. Tic. A.S¸. Büyükdere Caddesi No: 122 A Blok Kat:2 Esentepe Istanbul, Tur-key”. Amine-terminated polyTHF with Mn 1000 g mol−1 (PTHFNH2), amine-terminated polydimethyl siloxanes (Poly[dimethyl siloxanes-co-(3-aminopropyl)methyl silox-anes] with Mn 2500, PDMS2.5K-NH2, and 4400 g mol−1, PDMS4.4K-NH2 were purchased from Sigma-Aldrich Germany and used without further purification.
Tetrahydrofuran (THF) with butylated hydroxytoluene (BHT), concentrated H2SO4, and the other solvents were
supplied by Sigma-Aldrich. All chemicals were used as supplied without further purification. ε-CL was purchased from Aldrich. It is dried on CaH2and distilled under
vac-uum before use. Stannous octanoate (Sn-oct) as a catalyst was purchased from Sigma-Aldrich and used as supplied.
Autoxidation of Castor Oil and Ricinoleic Acid
32 g of castor oil (or ricinoleic acid) was spread in a Petri dish (Φ = 20 cm, oil thickness ~1.0 mm). Then, the oil was exposed to daylight under atmospheric conditions at room temperature for 5 months. During this time, the oil/-fatty acid was oxidized by uptaking oxygen from the air and castor oil macroperoxide, and Pcast5m (or ricinoleic acid macroperoxide, Prici5m) was formed. Without any further process, macroperoxide initiators, Pcast5m and Pri-ci5m, were used in the polymerization process.
Synthesis of Multimethacryloyl Derivative of Pcast5m
For this purpose, the mixture of 42.7 g of Pcastor5m (0.045 mol) and 6.2 g of triethyl amine (0.061 mol) was dissolved in dry CH2Cl2at 0C. To this solution, 6.2 g of
methacryloyl chloride (0.060 mol) in 10 mL of CH2Cl2
was added slowly for 10 min and stirring continuously. The needle crystals of triethyl amine hydrochloride were formed in the beginning. The reaction solution was left overnight and allowed to rise to room temperature. Then, needle crystals were removed by filtration. The solution was leached with petroleum ether. The rest of the needle crystals were removed again. The solvent evaporated. The viscose pink liquid (PcastMA-5) was dried under vacuum for 24 hours at room temperature.
Synthesis of Prici-g-Polystyrene (PriciS), Pcast-g-Polystyrene (PcastS), and PcastMA-g-Pcast-g-Polystyrene (PcastMA-g-PS) Block Copolymers
Free radical polymerization of styrene was initiated by three macroperoxide initiators, separately, in toluene solu-tion. For example, the mixture of Pcast5m (or Prici5m or PcastMA-5) (0.050 g) and styrene (4.52 g) was dissolved in 5 mL of toluene in a reaction bottle. Argon was intro-duced through a needle into the tube for about 1 min to expel the air. The tightly capped bottle was then put into a water bath at 95 C for 6 hours. Then, the content of the tube was coagulated into methanol (200 mL) under vigor-ous stirring. The precipitated graft copolymer was dried overnight under vacuum at 40C. Polymerfilm (approxi-mately 0.5 mm thickness) was casted from their chloroform solution (approximately 0.50 g of the polymer sample in
15 mL chloroform) in a Petri dish (Φ = 5.0 cm) covered with a piece of paper.
Amidation Reaction between PriciS and PDMSNH2 to Obtain Prici-g-PS-g-PDMS
For a typical reaction, a mixture of PriciS (0.656 g) and PEGNH2 (0.624 g) was dissolved in 30 mL of THF. The solution wasfiltered into a Petri dish (Φ = 7 cm) that was covered with a piece of cardboard loosely to allow the sol-vent to evaporate slowly for 48 hours. Thefilm of polymer blend covered with aluminum folio was kept at 110 C under vacuum for 2 hours. Crude graft copolymer was dis-solved in 10 mL of chloroform and the polymer was pre-cipitated in 200 mL of methanol. Then, this purified graft copolymer was dried at 40C under vacuum for 48 hours.
One-Pot Synthesis of Multiblock Copolymers
The combination of the free radical, ring-opening. and con-densation polymerization was carried out to prepare a mul-tiblock copolymer. For example, 2.0 g of styrene (0.02 mol), 2.0 g of Pcast5m, 2.0 g of ε-CL, and 0.1 g of stannous octanoate in 5 mL of toluene were mixed in a glass bottle. Argon was introduced through a needle into the solution for about 1 min to expel the air. The tightly capped bottle was then put into a water bath at 95 C for 5 hours. Then, the polymer solution was poured in 200 mL of methanol. The graft copolymer obtained was dried under vacuum at 40C for 48 hours.
Characterization
Peroxygen Analysis of the Oxidized Samples
Peroxide analysis of Prici and Pcast was carried out accord-ing to the procedure reported in cited reference (Hazer et al., 2017). For example, a mixture of Prici (or Pcast) (1.0 g), 2-propanol (30 mL),
acetic acid (5 mL), and saturated aqueous solution of KI (1 mL) was refluxed for 10 min. The released iodine was then titrated against 0.1 N sodium thiosulfate solution. The peroxygen percent of the Prici (and Pcast) was calculated using the following equation (Eq. 1):
Peroxide wtð :%Þ ¼ V × 0:1 × 16f½ =10 × mg, ð1Þ where V (mL) is the volume of 0.1 M sodium thiosulfate solution and m (g) is the amount of Prici (or Pcast) sample.
Molecular weights were determined by size exclusion chromatography instrument, Viscotek GPCmax Auto sam-pler system, consisting of a pump, three ViscoGEL GPC columns (G2000H HR, G3000H HR, and G4000H HR), and a Viscotek differential refractive index (RI) detector
with a THFflow rate of 1.0 mL min−1at 30C. A calibra-tion curve was generated with four polystyrene (PS) green standards: 2960, 50,400, and 696,500 Da, of low polydis-persity. The polymer sample solution containing 0.05 g in 10 mL of THF wasfiltered and injected automatically into the instrument. Data were analyzed using Viscotek Omni SEC Omni 01 software.
Proton NMR spectra in CDCl3solutions of the samples
were taken at a temperature of 25 C using an Agilent NMR 600 MHz NMR (Agilent, Santa Clara, CA, USA) spectrometer equipped with a 3 mm broadband probe.
FTIR spectra of the polymer samples were recorded using a Perkin-Elmer FTIR Spectrometer 100.
Thermal analysis of the polymers was carried out under nitrogen using TAQ2000 DSC and Q600 Simultaneous DSCTGA (SDT) series thermal analysis systems. These differential Scanning Calorimeters (DSC) measure tempera-tures and heatflows associated with thermal transitions in the obtained polymer samples. The dried sample was heated from−50 to 190C at a heating rate 10C min−1 under a nitrogen atmosphere with theflow rate of the inert gas (nitrogen) of 50 mL min−1.
C and H elemental analyses of the oleic acid macroper-oxide and the used pure oleic acid were carried out using a LECO, CHNS-932 Elemental analysis instrument. Each measurement was repeated two times using 10 mg of the sample and the results were obtained on average of two measurements for each sample.
Stress–Strain Measurements
A Zwick/Roell Tensile Testing Machine using a 50 kg load cell with a stretch speed of 100 mm min−1 was used for stress–strain measurements of the rectangular shape with a size of 0.16 × 10 × 50 mm of solvent cast film samples from CHCl3. At least three specimens from each sample
were measured. Samples were dried at room temperature under vacuum for 10 days prior to measurement.
Results and Discussion
Likely autoxidation of the polyunsaturated plant oils such as soybean oil, linseed oil and fatty acids such as linolenic acid, linoleic acid and oleic acid, and hydroxyl-functionalized castor oil and ricinoleic acid were autoxi-dized in this work. Castor oil contains ricinoleyl more than 90%. Therefore, castor oil and ricinoleic acid became the subject of this work in preparing macroperoxide initiators via autoxidation. In this manner, they were exposed to air oxygen for 5 months, to obtain higher macroperoxides of castor oil and ricinoleic acid. We followed the increase in
the viscosity by the autoxidation time for soybean oil in our recent article (Acar et al., 2013). Therefore, an increase in the viscosity shows us the oxygen uptake from the air. We also followed the autoxidation of the fatty acids using size exclusion chromatography. We stopped the autoxidation whenever the polymerization degree increased up to 5–6.
Autoxidation time of unsaturated plant oil/fatty acid with a lower double bond such as oleic acid is longer than that of fatty acids with a higher amount of double bonds. The order of the relative reaction rates of air oxygen is linoleni-c>linoleic>oleic acid> (Choe and Min, 2006a). We added the ricinoleic acid at the end of this row because of the lon-gest autoxidation time of ricinoleic acid.
The autoxidation starts from the allylic methylene groups through the peroxide linkages. Schemes 1 and 2 show the peroxide formation leading to the macroperoxides of castor oil and ricinoleic acid, respectively.
Iodometric analysis was applied to determine the peroxy-gen content of the Prici5m and Pcast5m macroperoxide ini-tiators. Peroxide contents calculated using Eq. 1 were found to be 0.67 wt.% for Prici and 1.45 wt.% for Pcast while the precursors, pure ricinoleic acid and castor oil, showed less than 0.1 wt.% of peroxide. The peroxide con-tent of Prici is nearly the same as that of oleic acid macro-peroxide initiator, which was recently reported (Hazer et al., 2017). One of the confirmations of the peroxide for-mation in the macroperoxide initiators was the elemental analysis indicating an increase in the O% of oxidized castor oil and ricinoleic acid. The results of the C, H, and O
Scheme 1 The expected reaction design of the autoxidation of castor oil
elemental analysis and iodometric analysis of the ricinoleic acid and castor oil macroperoxides are shown in Table 1.
In addition, the exact estimation of the peroxide groups in the autoxidized samples was carried out using the heat release study by DSC (Hazer and Kalaycı, 2017; Murthy et al., 1994; Ulkowski et al., 2005). DSC thermograms indicating the peroxide decomposition of the macroperox-ides (Prici5m and Pcast5m) can be seen in Fig. 1. Peroxide decomposition starts at 132C for Prici-5 m and at 107C
for Pcast-5 m with the maximum decomposition tempera-tures of 166 and 170C, respectively.
GPC measurements gave the molar masses of the obtained macroperoxides. The degree of polymerizations (D.P.) of the macroperoxides was also calculated from the GPC results. Molar masses of the Prici-5 m and Pcast-5 m were found to be 1169 and 1935 g mol−1. The degree of polymerizations changes from 2 to 4. Molar masses and the degree of polymerizations of the obtained macroperoxide initiators can be seen in Table 2.
Hydroxyl functionality of the autoxidized castor oil was reacted with methacryloyl chloride to obtain its methacry-late derivative, Pcast-MA-5. Three new macroperoxides, Prici5m, Pcast5m, and PcastMA-5, were characterized using the1H NMR technique. Characteristic signals of the macroperoxides were observed at 3.5 ppm ( CH OH, Prici5m, and Pcast5m), 4.0–4.2 and 5.1 ppm (triacylgly-cerol CH2 O and CH O , respectively, Pcast5m and
PcastMA-5), and 5.2–5.5 ppm (double bonds, Prici5m, Pcast5m, and PcastMA-5). Fig. 2 shows the comparative
1
H NMR spectra of the three macroperoxides.
Scheme 2 The expected reaction design of the autoxidation of ricinoleic acid
Table 1 The results of elemental and iodometric analysis of the macroperoxides and pure starting materials
Sample Found, (wt.%) Pure ricinoleic acid,(wt.%) Peroxidea C H O C H O (wt.%) Prici5m 65.90 10.42 23.68 72.48 11.41 16.11 0.67 Pure castor oil, wt.%
Pcast5m 69.75 10.97 19.28 73.31 11.15 15.43 1.45
aFound by the iodometric analysis.
These new macroperoxide initiators were used in the free radical solution polymerization of styrene in toluene. A series of 20 wt.% of macroperoxide initiators in styrene were polymerized in toluene solution with different poly-merization times at 95C under argon. Polymerization con-ditions and GPC results are shown in Table 3a, b, c. According to Eq. 2, the overall rate constant, K, was calcu-lated from the slopes of the obtained plots. Fig. 3 shows the plots of Ln[Mo]/[M] against polymerization times for Pri-ci5m, Pcast5m, and PcastMA-5.
The polymerization kinetics of styrene initiated by the obtained macroinitiators was investigated in toluene. The results and the polymerization conditions are listed in Table 3a,b,c.
For each macroperoxide initiator, the kinetic variation of Ln[M]o/[M] vs. time could be plotted. Where [Mo] is the
initial molarity, [M] is the molarity of the unreacted styrene monomer. The polymerizations displayed linear
Table 2 Molar masses and the degree of polymerizations of the obtained macroperoxide initiators
Macroperoxide Mn (Da) Mw (Da) PDI D.P.a
Prici-5 m 1169 1185 1.01 4.5
Pcast-5 m 2136 2290 1.07 2.5
PcastMA-5 2450 2631 1.07 2.5
aD.P. degree of polymerization.
Fig. 2 1H NMR spectra of the macroperoxides: (a) Prici5m, (b) Pcast5m, and (c) PcastMA-5
Fig. 3 Plots of Ln[Mo]/[M] vs. polymerization time for Pcast5m, Pri-ci5m, and Pcast-MA-5
semilogarithmic curves for monomer consumption. Fig. 3 shows the evolution of Ln([M]0/[M]) vs. time
correspond-ing to the three macroperoxide initiators. The polymeriza-tion results suggest that the polymerizapolymeriza-tion rate order was
Pcast5m > Prici5m > Pcast-MA-5. The polymerization rates of Pcast5m and Prici5m were slightly greater than that of PcastMA-5. We can say that the additional methacryloyl
Table 3 Polymerization conditions and GPC results of styrene initiated by the macroperoxide initiators (in 5 mL of Toluene, 95C): (a) Prici, (b) Pcast, and (c) PcastMA
Code Prici-5 m S Pol.Time Yield LnMo/M Mn Mw PDI
(g) (g) (min) (g) (Da) (Da)
(a) Polymerization of styrene initiated by Prici5m
PSrici-1 2.03 8.04 35 0.61 0.079 23,149 35,618 1.54 PSrici-2 2.01 8.01 60 1.08 0.145 24,817 36,318 1.46 PSrici-3 2.06 8.02 90 1.74 0.174 24,526 36,376 1.48 PSrici-4 2.04 8.03 120 2.00 0.287 25,476 38,039 1.49 PSrici-5 2.05 8.04 155 2.66 0.401 24,432 37,304 1.53 PSrici-6 2.04 8.04 190 3.03 0.473 24,847 38,012 1.53
Code Pcast-5 m S Pol.Time Yield LnMo/M Mn Mw PDI
(g) (g) (min) (g) (Da) (Da)
(b) Polymerization of styrene initiated by Pcast5m
PSC-56 2.06 8.03 20 0.39 0.050 22,148 34,638 1.56 PSC-51 2.05 8.03 30 0.68 0.089 21,914 34,206 1.56 PSC-52 2.02 8.02 50 1.06 0.141 21,848 34,667 1.59 PSC-53 2.02 8.03 70 1.38 0.189 25,409 38,052 1.50 PSC-54 2.03 8.03 110 2.37 0.350 24,181 37,187 1.54 PSC-55 2.02 8.01 180 4.16 0.733 24,383 39,503 1.62
Code PcastMA-5 S Pol.Time Yield LnMo/M Mn Mw PDI
(g) (g) (min) (g) (Da) (Da)
(c) Polymerization of styrene initiated by PcastMA-5
PSC-MA-56 2.04 8.01 20 0.34 0.044 23,855 41,691 1.75 PSC-MA-51 2.01 8.21 30 0.46 0.057 26,391 44,910 1.70 PSC-MA-52 2.02 8.02 50 0.66 0.086 26,823 45,484 1.70 PSC-MA-53 2.01 8.01 75 1.00 0.131 28,851 49,659 1.72 PSC-MA-54 2.02 8.02 115 1.29 0.176 30,913 53,401 1.73 PSC-MA-55 2.02 8.02 180 1.80 0.254 32,302 55,597 1.72
Fig. 4 Plots of the molar masses of the obtained polymers against polymerization time
Fig. 5 Plots of conversion of styrene vs. feeding concentration of the macroperoxide initiators in the bulk polymerization of styrene initi-ated by Pcast5m (a) and PcastMA-5 (b)
groups increased the chain transfer to the initiator leading to a decrease in the reaction rate. The polymerization rate of Pcast5m is slightly higher than that of Prici5m. This can be attributed to the fact that the decomposition temperature of Pcast5m starts at a lower temperature compared to Pri-ci5m as shown in Fig. 1. Therefore, the castor oil macro-peroxide initiator starts to decompose at a lower temperature (ca. 107C) and the radicals formed increase causing a high polymerization rate.
We evaluate the molar masses of the polymerization series in Table 3a,b,c. The molar masses of the obtained polymers were plotted against polymerization time as shown in Fig. 4. It is observed that the molar masses of the mers obtained by the PcastMA-5 have increased by the poly-merization time while those of the polymers obtained by
Prici5m and Pcast5m did not show any increase in molar masses by the time. Presumably, molar mass increases by the growing polymer chains attached to the additional methacryl groups leading to branching. Interestingly, all copolymers obtained by the methacrylated castor oil macro-peroxide initiators were soluble in common organic solvents, it was not observed any cross-linked polymer.
Conversion of the styrene monomer against the macro-peroxide concentration shows differences in the bulk poly-merization of styrene initiated by Pcast5m and PcastMA-5. Styrene is consumed in the free radical polymerization initi-ated by Pcast5m more quickly than initiiniti-ated by PcastMA-5 (Fig. PcastMA-5).
Diblock Copolymers Contain Undecomposed Peroxide Residue
Thermal analysis of the Pcast, Prici, and PS-PcastMA diblock copolymers indicated that they have still undecomposed peroxide in the polymer chain. Per-oxide decompositions of the diblock copolymers were observed at 142, 120, and 137 C for PSrici-6, PSC-55, and PSC-MA-55, respectively. The DSC thermograms of the diblock copolymers can be seen in Fig. 6. Then, these diblock copolymers initiate the free radical polymeriza-tion of a vinyl monomer. Therefore, free radical poly-merization of nBMA was initiated by one of them, PSC-55 to obtain multiblock copolymer, Pcast-g-PS-g-PnBMA. Table 4 shows the results and conditions of the nBMA copolymerization by PSCB diblock copolymers.
Fig. 6Synthesis of multiblock copolymers by the multifunctional macroperoxides
Table 4Results and conditions of the nBMA copolymerization by PSCB diblock copolymers to obtain Pcast-PS-PnBMA Multiblock copolymer. (100C, 5 hours, 10 mL toluene)
Code PSC-55 Mn nBMA Yield Mn Mw PDI Mech. property
(g) (g mol−1) (g) (g) (g mol−1) (g mol−1) Stress Strain
(MPa) (%) PSCB-1 0.40 24,400 3.4 2.15 31,200 58,000 1.86 15 133 PSCB-2 0.40 24,400 10.0 5.90 57,500 114,300 1.99 13 110 PnBMA-5 (control) 67,900 115,400 1.70 11 182 PS-40 (control) 41,250 94,700 2.30 9.1 5
Table 5 Ring-opening polymerization ofε-CL initiated by the hydroxyl-functionalized PS-Pcast diblock copolymer at 100C Pol.
PCS-51 ε-CL Sn-oct Toluene Time Yield Blocks
Code (g) (g) (g) (mL) (min) (g) (mol%) Remarks
PcastS-CL-1 1.34 2.4 0.10 10 438 2.36 48 PCL very brittle polimer
Comparing Mn of the precursors, the increase in Mn of the multiblock copolymers confirms the multiblock copolymer formation.
The mechanical properties of the obtained multiblock copolymers were evaluated by stress–strain measurements.
The results showed that the mechanical properties of the multiblock copolymers were improved. Their mechanical strength was found to be higher than their related homopol-ymers, while elongation was lower than PnBMA but much higher than PS.
Ring-Opening Polymerization ofε-CL Initiated by Stannous Octanoate
Castor oil and ricinoleic acid multifunctional macroperox-ides were used in the polymerization reactions of one or more monomers in one pot or separately. Hydroxyl-functionalized polystyrene castor oil copolymer (PCS-51, in Table 3) in the presence of stannous octanoate initiated the ring-opening polymerization of ε-CL. The PS-Pcast-PCL multiblock copolymer was obtained in good yield. The reaction conditions were tabulated in Table 5.
Characteristic signals of the PS (6.4–7.1 ppm), PCL (4.0, 2.4, and 1.6 ppm), and Pcast (0.9 and 3.6 ppm) blocks are observed in the NMR spectrum shown in Fig. 7. According to the ε-CL feeding, the PCL contents in the obtained block copolymers were found to be 48 and 70 mol% of PCL units from their NMR spectra.
FTIR spectra of the block copolymers contained the characteristic signals at 3027 and 1602 cm−1 for PS; at 1721 and 1186 cm−1for PCL as shown in Fig. 8.
Condensation Reactions of PS-g-Prici with the Amine-Terminated PTHF and PDMS
Carboxylic acids of PS-g-Prici diblock copolymers were reacted with the amine-terminated PTHF and PDMS via amidation reaction. The results and conditions of the ami-dation reactions are shown in Table 6. The characteristic signals of PS and PTHF blocks in the Prici-PS-PTHF tri-block copolymers (Prici3S-THF-1, and Prici3S-THF-2) were observed in the1H NMR spectrum shown in Fig. 9.
Fig. 7 1H NMR spectrum of PcastS-CL-2
Fig. 8 FTIR spectra of the PS-Pcast-PCL block copolymers (PcastS-CL-1, PcastS-CL-2)
Table 6 Condensation reactions of PS-g-Prici with amine-terminated PTHF and PDMS (in 10 mL of toluene, 0.1 g of Sn-oct, polym. Time 7 hours)
Code Prici3S PTHF Yield Blocks Mn Mw PDI Remarks
(g) -NH2(g) (g) (mol%) (g mol−1) (g mol−1)
Prici3S-THF-1 1.56 0.61 1.35 6 PTHF 20,573 33,101 1.61 very brittle polimer Prici3S-THF-2 1.62 1.41 1.38 7 PTHF 16,796 27,498 1.64 very brittle polimer
Code Prici3S ε-CL PDMS Yield Blocks Mn Mw
(g) (g) 4.4 K (g) (g) (mol%) (g mol−1) (g mol−1) PDI Remarks
PriciS-D-CL-1 1.02 1.55 0.63 1.07 7 PCL 16,694 26,913 1.61 very brittle
polimer 8 PDMS
Pcast-PCL-PMMA Triblock Copolymers obtained by One-Pot Polymerization
Castor oil macro peroxide initiator was used in the one-pot polymerization ofε-CL and MMA in bulk. The results and conditions of the one-pot polymerization can be seen in Table 7. The 1H NMR spectrum of the copolymer con-tained the characteristic signals of the related blocks (Fig. 10).
Prici-PEG-PMMA multiblock copolymers were also obtained by the one-pot polymerization of the mixture of Prici5m, PEG-ED, and MMA in toluene solution at 95C in the presence of the Sn-oct2catalyst. The obtained block
copolymers contained PEG between 6 and 12 mol%. The
results and conditions of the one-pot synthesis of the block copolymers are tabulated in Table 8. The PEG signal was observed in the1H NMR spectrum shown in Fig. 11.
Fig. 9 (a) The1H NMR spectrum of the Prici-PS-PTHF triblock copolymer (Prici3S-THF-2) and (b) the1H NMR spectrum of the Prici-PS-PDMS triblock copolymer (PriciS-D-CL-1)
Table 7 The results and conditions of the one-pot polymerization ofε-CL and MMA
Code Pcast5m ε-CL MMA Pol. Time Yield PCL
(g) (g) (g) (min) (g) (mol%)
bPmcaL-1 - 1.05 6.10 6.09 120 2.02 2.2
bPmcaL-2 - 3.03 6.21 6.04 120 3.12 4.5
Fig. 10 The1H NMR spectrum of the Pcast-PCL-PMMA triblock
Conclusion
Autoxidation of the unsaturated fatty acids/vegetable oils results in a macroperoxide initiator. Autoxidation of castor oil and ricinoleic acid resulted in macroperoxide initiators containing hydroxyl functionalities. The polymerization kinetics of styrene polymerization initiated by the hydroxyl functional macroperoxide initiators was evaluated. One-pot polymerization of the heterocyclic monomer, vinyl mono-mer, and/or amine-terminated oligomer was carried out. These hydroxyl, peroxide, and/or carboxylic acid func-tional macroperoxide initiators open up a new gate to pre-pare different multiblock copolymers containing biodegradable, hydrophilic, and hydrophobic blocks. This leads to synthesis of very versatile multiblock copolymers that could be promising materials for medical and industrial applications.
Acknowledgments This work was supported by the Bülent Ecevit University Research Funds (#BEU-2017-72118496-01 and #BEU– 2016 - 72118496– 10). The Authors thank Serdar Çoban and Sıdıka Saraç Tabaklı (Cilas Kauçuk, Devrek, Zonguldak, Turkey) for stress– strain measurements. Finally, the Authors thank Yücel Mercimek for his technical support.
Conflict of Interest The authors declare that they have no conflict of interest.
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