Degradable Crystalline Polyperoxides from Fatty Acid Containing

Oct 31, 2018 - ... of Chemical Sciences, Indian Institute of Science Education and Research Kolkata , Mohanpur - 741246 , Nadia, West Bengal , India...
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Degradable Crystalline Polyperoxides from Fatty Acid Containing Styrenic Monomers Sourav Mete,†,‡ Piyali Mukherjee,‡ Binoy Maiti,†,‡ Sunirmal Pal,†,‡ Pradip Kr. Ghorai,‡ and Priyadarsi De*,†,‡ Polymer Research Centre and Centre for Advanced Functional Materials, ‡Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur - 741246, Nadia, West Bengal, India

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S Supporting Information *

ABSTRACT: Vinyl polyperoxides, alternating copolymers of vinyl monomers and molecular oxygen, are highly viscous amorphous materials because of the flexible peroxy (−O− O−) bonds in the main chain. In this study, crystalline polyperoxides have been synthesized by oxidative radical polymerization of styrenic monomers having fatty acid moieties attached to the phenyl ring using molecular oxygen at 100 psi pressure. Determination of active oxygen contents in polyperoxides, 13C NMR spectroscopy, and electron impact mass spectroscopy (EI-MS) confirmed alternating placement of −O−O− bonds after every styrenic monomer unit in the copolymer main chain. Thermal stability was studied by thermogravimetric analysis (TGA). Exothermic degradation of these polyperoxides was observed by differential scanning calorimetry (DSC), and degradation products have been identified from EI-MS study. DSC and powder X-ray diffraction (PXRD) studies revealed crystallinity in the polyperoxides with fatty acids having 12 carbon atoms or longer. This crystalline behavior was further supported by polarized optical microscopy (POM), where a birefringence texture which is characteristic of semicrystalline polymer was formed for polyperoxides with C ≥ 12 of the side-chain alkyl carbons. Transmission electron microscopy (TEM) was used to define the thickness and crystal structure of the polymers. Theoretical studies have been performed using density functional theory (DFT) to support the experimental interlamellar distance from X-ray diffraction studies.

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materials.18,19 Liquid marbles were also used as an efficient microreactor for the synthesis of polyperoxides in a good yield by the radical alternating copolymerization of 1,3-diene monomers with oxygen.20,21 Very recently, polystyrenes with various contents of peroxide units (a small content of peroxy groups to an alternating copolymer of styrene with oxygen) have been synthesized.22 Although fatty acids (FA) and their derivatives are important biorenewable resources with numerous applications in diverse fields,23 they have never been used in polyperoxide synthesis. Their easy availability, inexpensiveness, and biocompatible nature prompted us to use various FAs to the polyperoxide synthesis. Most of the reported polyperoxides are either gummy liquid or semisolid in nature, and thus difficult to handle quantitatively.24 To date, there is only one report by Jayaseharan et al. where crystallinity was found in poly(αphenyl styrene peroxide) (PAPSP).24 The detailed mechanism of PAPSP crystallization was never studied. Generally, polyperoxides do not form crystalline structures owing to

INTRODUCTION Polyperoxides belong to a narrow but important class of degradable polymers, with alternating placement of peroxy (−O−O−) bonds in the polymer main-chain repeating unit,1 which are prone to degrade in the presence of heat,2 photo,3 acid,4 base,5 enzyme,6 etc. They also show unique properties like highly exothermic degradation,7 in contrast to common polymers which commonly undergo endothermic degradation. Polyperoxides find academic and technological importance as polymeric initiators during the free radical polymerization of vinyl monomers,8 autocombustible polymeric fuel,9 curators in coating and molding compositions,10 dismantlable adhesive,11 branched polymer synthesis precursor,12,13 drug carrier,14 etc. Vinyl polyperoxides are generally prepared by the oxidation of vinyl monomers in the presence of high oxygen pressure.15 Solid state copolymerizations of dibenzofulvene,16 7,7,8,8tetrakis(ethoxycarbonyl)quinodimethane, and related monomers17 with oxygen were used to prepare alternating copolymers having repeating peroxy units in the polymer main chain. Polymerization of 1,3-diene monomers has been extensively studied, and various polyperoxides were readily synthesized by the radical copolymerization process using sorbic derivatives and molecular oxygen as the starting © XXXX American Chemical Society

Received: September 13, 2018 Revised: October 15, 2018

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DOI: 10.1021/acs.macromol.8b01981 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Synthetic Scheme for the Preparation of Fatty Acid Based Styrenic Monomers (VBFE) and Corresponding Polyperoxides

their low molecular weight and highly flexible backbone because of the main-chain peroxy links. Cais and Bovey demonstrated the effect of heteroatom unit on the transmission of configurational information in poly(styrene peroxide) by chain dynamics study using 13C NMR spectroscopy, compared to the corresponding hydrocarbon-backbone counterpart poly(styrene).25 In general, polymer chains form a crystalline domain when certain symmetry requirements are met in addition to regularity in structure, strong intermolecular forces, and an optimum flexibility. Greenberg and Alfrey demonstrated side-chain crystallization from long n-alkyl side chains of polymers based on acrylic and methacrylic acids.26 Hempel et al. also found crystalline behavior from poly(n-alkyl methacrylate) systems with C ≥ 12 of the side-chain alkyl carbons.27 Recently, we reported that melting temperature (Tm) increases with the increase of aliphatic chain length in fatty acid containing polymers, where polymers of fatty acid based methacrylate monomers were synthesized following the reversible addition−fragmentation chain transfer (RAFT) polymerization technique.28 In all of these examples, crystallinity was observed because of the side-chain crystallization of FA chains with C ≥ 12 of the side-chain alkyl carbons. Thus, to take advantage of side-chain crystallization of FA chains, herein we have polymerized styrenic monomers having fatty acid moieties with different chain lengths at the para-position of the phenyl group via oxidative polymerization in the presence of a free radical initiator 2,2′-azobis(isobutyronitrile) (AIBN) at 50 °C in toluene (Scheme 1). We used styrenic monomers because of their higher reactivity toward oxidative polymerization compared to the corresponding methacrylate derivatives.29 We demonstrated exothermic degradation of these polyperoxides with activation energies of degradations comparable to the dissociation energy of the O− O bond in dialkyl peroxides.30 Polyperoxides exhibit crystallinity depending on the chain lengths of the fatty acids, which were studied by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD) studies supported the formation of ordered lamellar organization.

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(mixture of isomer), ethyl acetate, anhydrous N,N-dimethylformamide (DMF), methanol, tetrahydrofuran (THF), and chloroform were purified by following standard literature procedures.31 Instrumentations. The 1H and 13C NMR spectroscopic measurements were performed at 25 °C using a 500 MHz Bruker AvanceIII NMR spectrometer using tetramethylsilane (TMS) as an internal standard. Solid state FT-IR spectra were recorded on KBr pellets using a PerkinElmer Spectrum 100 FT-IR spectrometer. The molecular weights of polyperoxides were obtained from a Waters gel permeation chromatography (GPC) instrument in THF at 30 °C with a flow rate of 1.0 mL/min. The system consisted of a Waters model 515 HPLC pump, Waters model 2414 refractive index (RI) detector, and two columns (Styragel HT4 and Styragel HT3). The GPC instrument was calibrated with narrow molecular weight polystyrene standards. Positive mode electrospray ionization mass spectrometry (ESI-MS) was performed on a Q-Tof Micro YA263 high resolution (Waters Corporation) mass spectrometer. Thermal degradation studies were performed using a Mettler Toledo DSC1 STARe differential scanning calorimeter (DSC) at various heating rates (5, 10, 15, 20, and 25 °C/min) in a N2 atmosphere with sample weights of ∼4−5 mg. Thermogravimetric analysis (TGA) was done at 10 °C/min heating rate with a sample weight of ∼5−7 mg in a N2 atmosphere using a Mettler Toledo TGA/SDTA 851e instrument. Electron-impact mass spectrometry (EI-MS) spectra of polyperoxides in the positive mode were recorded at 70 eV in a Shimadzu QP5050A gas chromatograph−mass spectrometer with a direct inlet. The TEM images were recorded using a JEOL JEM-2100F instrument at 200 kV operational mode to determine the crystal morphology and thickness. TEM samples were prepared by drop-casting a dilute THF solution (0.5 mg/mL) on a carbon coated Cu-grid and then dried at room temperature. Powder X-ray diffraction (PXRD) experiments were performed in a Seifert XRD 3000P diffractometer operating at 45 kV and 30 mA, using Cu Kα radiation (λ = 1.54059 Å). Polarized Optical Microscopy (POM) Study. The crystalline morphology of polyperoxides was observed in thin films. The thin film was prepared by drop casting a THF solution of polymer (0.5 mg/ mL) on a glass coverslip. Then, the coverslips were heated to 70 or 75 °C and again cooled down to room temperature. This heating and cooling process was continued for four times. The glass slide containing the sample film was then positioned under an Olympus polarized optical microscope (model BX51) equipped with a camera system. This film was heated on a microscopic heating stage (Linkam THMS 600 equipped with a T-95 temperature programmer) to 70− 75 °C and slowly cooled at a rate of 5 °C/min. Computational Studies. We performed density functional theory (DFT) calculations by using the Gaussian 09 package32 to gain a microscopic understanding about the pattern formation by the polymer chains through molecular self-assembly. All of the calculations are performed through the unrestricted Becke’s threeparameter hybrid exchange functional33 combined with the Lee− Yang−Parr correlation function,34 abbreviated as B3LYP. We have used the 6-311G basis set in all cases with the polarizable continuum model (PCM) for the solvent. In this study, we have used toluene as the solvent. All of the structures are optimized to their minimum energy states without any symmetry constraint, and optimized structures are confirmed by the harmonic vibrational frequencies having no imaginary node.

EXPERIMENTAL SECTION

Materials. Fatty acids such as caprylic acid (CLA, ≥98%), capric acid (CRA, ≥98%), lauric acid (LA, ≥98%), myristic acid (MA, ≥99%), palmitic acid (PA, ≥99%), and stearic acid (SA, ≥95%) were purchased from Sigma-Aldrich and used as received. The 4vinylbenzyl chloride (VBC, Sigma, 99%), potassium carbonate (K2CO3, Merck), anhydrous toluene (Sigma, 99%), CDCl3 (Cambridge Isotope Laboratories, Inc., USA, 99.8% D), and highly pure oxygen (BOC, >99.99%) were used without any further purifications. The 2,2′-azobis(isobutyronitrile) (AIBN, Sigma, 98%) was purified by recrystallization from methanol. The solvents such as hexanes B

DOI: 10.1021/acs.macromol.8b01981 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Oxygen consumption (Δp) as a function of time for the polymerization of VBFE monomers at 100 psi of oxygen pressure at 50 °C in dry toluene. COCH2CH2, 2H, m), 2.34 (COCH2, 2H, t), 5.08 (CH2O, 2H, s), 5.27 (CH2CH, 1H, d), 5.75 (CH2CH, 1H, d), 6.71 (CH2CH, 1H, q), 7.31 (CHCH, 2H, d), 7.39 (CHCH, 2H, d). ESI-MS of VBM (Figure S11): calculated m/z for [M + Na]+: 366.99, observed m/z = 367.27. VBP. Yield = 82%; 1H NMR (Figure S6, CDCl3, 500 MHz, TMS): δ (ppm) 0.87 (CH2CH3, 3H, t), 1.27 ( CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2, 24H, m), 1.63 (COCH2CH2, 2H, m), 2.34 (COCH2, 2H, t), 5.08 ( CH2O, 2H, s), 5.27 (CH2CH, 1H, d), 5.75 (CH2CH, 1H, d), 6.71 (CH2CH, 1H, q), 7.30 (CHCH, 2H, d), 7.40 (CHCH, 2H, d). ESI-MS of VBP (Figure S12): calculated m/z for [M + Na]+: 395.04, observed m/z = 395.28. Synthesis of Polyperoxides via Oxidative Polymerization. Polyperoxides were prepared using 0.5 mol/L monomer solution in anhydrous toluene (total volume 15 mL) in the presence of AIBN (0.05 mol/L) initiator in a 100 mL Parr reactor (Parr Instrument Co., USA). The Parr reactor is equipped with a digital pressure transducer, a temperature controller, and a mechanical stirrer. The monomer solution and AIBN were added to the preheated reactor, sealed tightly, and pressurized with oxygen to 100 psi and stirred (100 rpm) at 50 ± 1 °C. The decrease of O2 pressure was measured as a function of time by monitoring the pressure transducer. The polymerization was terminated by discharging the oxygen pressure. The obtained polymeric peroxides, poly(4-vinylbenzyl caprylate peroxide) (PVBCLP), poly(4-vinylbenzyl caprate peroxide) (PVBCRP), poly(4-vinylbenzyl palmitate peroxide) (PVBPP), poly(4-vinylbenzyl myristate peroxide) (PVBMP), poly(4-vinylbenzyl laurate peroxide) (PVBLP), and poly(4-vinylbenzyl stearate peroxide) (PVBSP), were precipitated in cold hexanes. The polymers were purified by repeated precipitation from chloroform solutions using hexanes as nonsolvent, followed by the removal of solvent by vacuum drying at room temperature for 12 h. Caution!!! Since polyperoxides degrade highly exothermically, they should be handled with extreme care and preferably stored in a dark vial in the refrigerator. Determination of Active Oxygen in Polyperoxides. The peroxide content as active oxygen was determined following a literature procedure with a slight modification.35 Standard I2 solutions (25, 50, 100, 200, and 400 μg/mL) were prepared in 20 mL glass vials in acetic acid and chloroform solvent mixture (2:1, v/v) and purged with dry nitrogen for 3 min. To these five vials, 40 μL of freshly prepared KI (200 mg/mL) solution was added and again purged with dry nitrogen for 3 min. For these five solutions, absorbance values were recorded at 475 nm in a UV−visible spectrometer using acetic acid and chloroform mixture (2:1, v/v) as reference. The absorbance values at particular I2 concentrations were used to construct a calibration curve.36 Next, polyperoxides were dissolved in chloroform (0.5 mg/mL) and purged with nitrogen for 3 min. A 40 μL portion of freshly prepared KI solution was added and again purged with nitrogen for 3 min. Acetic acid was added to the polymer solution to maintain the acetic acid to chloroform ratio at 2:1 (v/v). After 1 h, absorbance values at 475 nm were determined in a UV−visible spectrometer. The concentration of I2 (produced from the reaction

Synthesis of 4-Vinylbenzyl Fatty Esters (VBFE). Styrenic monomers (Scheme 1) from various fatty acids such as CLA (VBCL), CRA (VBCR), LA (VBL), MA (VBM), PA (VBP), and SA (VBS) were synthesized (see 1H NMR characterizations in Figures S1−S6 and ESI-MS spectra in Figures S7−S12). In a typical example, SA (5.0 g, 0.0175 mol) and K2CO3 (2.43 g, 0.0175 mol) were taken in anhydrous DMF (20 mL) followed by the addition of VBC (2.6 g, 0.017 mol). The reaction mixture was heated at 85 °C overnight under magnetic stirring. After completion of the reaction (examined by thin-layer chromatography), the reaction mixture was cooled to room temperature and poured into cold brine solution. Then, the aqueous phase was extracted with ethyl acetate four times (4 × 50 mL). The organic phase was combined in a 500 mL beaker and dried over anhydrous Na2SO4. The organic solvent was removed by rotavap and purified further by silica gel column chromatography using ethyl acetate/hexanes (1:50, v/v) as a mobile phase; yield = 5.9 g (84%). 1 H NMR (Figure S1, CDCl3, 500 MHz, TMS): δ (ppm) = 0.87 ( CH2CH3, 3H, t), 1.22 ( CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2, 28H, m), 1.56 (COCH2CH2, 2H, m), 2.32 (COCH2, 2H, t), 5.08 (CH2O, 2H, s), 5.23 (CH2CH, 1H, d), 5.72 (CH2CH, 1H, d), 6.70 (CH2CH, 1H, q), 7.28 (CHCH, 2H, d), 7.40 (CHCH, 2H, d). ESI-MS of VBS (Figure S7): calculated m/z for [M + Na]+: 423.31, observed m/z = 423.25. Similarly, VBCL, VBCR, VBL, VBM, and VBP were prepared by following the above procedure. VBCL (yield = 78%); 1H NMR (Figure S2, CDCl3, 500 MHz, TMS): δ (ppm) 0.90 (CH2CH3, 3H, t), 1.30 (CH2CH2CH2CH2, 8H, m), 1.65 (COCH2CH2, 2H, m), 2.36 (COCH2, 2H, t), 5.11 (CH2O, 2H, s), 5.26 (CH2 CH, 1H, d), 5.74 (CH2CH, 1H, d), 6.71 (CH2CH, 1H, q), 7.32 (CHCH, 2H, d), 7.40 (CHCH, 2H, d). ESI-MS of VBCL (Figure S8): calculated m/z for [M + Na]+: 283.22, observed m/z = 283.17. VBCR. Yield = 82%; 1H NMR (Figure S3, CDCl3, 500 MHz, TMS): δ (ppm) 0.86 (CH2CH3, 3H, t), 1.19−1.31 ( CH2CH2CH2CH2CH2CH2, 12H, m), 1.61 (COCH2CH2, 2H, m), 2.34 (COCH2, 2H, t), 5.07 (CH2O, 2H, s), 5.23 (CH2 CH, 1H, d), 5.73 (CH2CH, 1H, d), 6.70 (CH2CH, 1H, q), 7.29 (CHCH, 2H, d), 7.37 (CHCH, 2H, d). ESI-MS of VBCR (Figure S9): calculated m/z for [M + Na]+: 311.27, observed m/z = 311.20. VBL. Yield = 80%; 1H NMR (Figure S4, CDCl3, 500 MHz, TMS): δ (ppm) 0.86 (CH2CH3, 3H, t), 1.23 ( CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 , 16H, m), 1.62 ( COCH2CH2, 2H, m), 2.33 (COCH2, 2H, t), 5.07 (CH2O, 2H, s), 5.25 (CH2CH, 1H, d), 5.75 (CH2CH, 1H, d), 6.71 (CH2CH, 1H, q), 7.28 (CHCH, 2H, d), 7.38 (CHCH, 2H, d). ESI-MS of VBL (Figure S10): calculated m/z for [M + Na]+: 339.32, observed m/z = 339.23. VBM. Yield = 82%; 1H NMR (Figure S5, CDCl3, 500 MHz, TMS): δ (ppm) 0.87 (CH2CH3, 3H, t), 1.24 ( CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2, 20H, m), 1.63 ( C

DOI: 10.1021/acs.macromol.8b01981 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Synthesis Details and Characterization of Various 4-Vinylbenzyl Fatty Ester Based Polyperoxidesa polymer

time (h)

conv.b (%)

Rp × 108 (mol L−1 s−1)

Mnc (g/mol)

D̵ c

Edd (kcal/mol)

active O2e (μg)

active O2f (μg)

PVBCLP PVBCRP PVBLP PVBMP PVBPP PVBSP

44.0 45.5 46.5 48.0 48.5 50.0

9.0 10.2 11.0 12.6 12.8 14.0

2.30 2.26 2.24 2.20 2.19 2.15

3960 4220 4380 4490 5280 5530

1.6 1.9 2.1 1.5 2.0 1.7

n.d. n.d. n.d. 40.3 41.4 34.6

n.d. 24.6 n.d. 21.3 19.6 18.8

n.d. 23.6 n.d. 20.8 19.7 18.7

Polymerization was carried out at 100 psi of oxygen pressure in dry toluene at 50 °C. [AIBN] = 0.05 mol/L, [monomer] = 0.5 mol/L. Conversion was calculated gravimetrically on the basis of monomer feed. cDetermined by GPC in THF. dCalculated from DSC study using Kissinger’s method. eTheoretical values. fExperimental values. n.d.: not determined. a

b

Figure 2. 1H NMR (left side) and 13C NMR (right side) spectra of (A) PVBSP, (B) PVBPP, and (C) PVBMP in CDCl3.

psi.38 The oxidative polymerization reactions usually proceed through a free radical method, and the rate of polymerization (Rp) can be determined from the slope of Δp against time plot.39 The Rp values of various VBFE polymerization reactions are listed in Table 1, where the Rp values decrease very little with the increasing bulkiness in the side chain and are almost constant for different VBFE monomers. Among the various poly(4-vinylbenzyl fatty ester peroxide) (PVBFEP) polymers, PVBCLP and PVBCRP are a viscous gummy material and the rest of the polyperoxides were found to be powdery in nature. Number-average molecular weights (Mn) and molecular weight distributions (dispersity, Đ) of the resulting polyperoxides were determined from GPC study (Figure S13), and results are shown in Table 1. The Mn values range from 3900 to 5600 g/mol, because this category of polymers undergo facile degradation during the oxidative polymerization, generating chain transfer agents such as aldehydes, alcohols, ketones, etc., which react with the macro-growing radicals, resulting in low molecular weight polyperoxides.40 The presence of peroxide (OO) bonds in the main chain was proved by FT-IR spectroscopy. Figure S14 displays the FT-IR spectra of PVBPP and PVBLP, where the band near 1014 cm−1 is due to the OO bond stretching.41 The absorption band that appeared at 1171 cm−1 is attributed to the CO stretching. Peaks coming in the region 2840−2961 cm−1 are due to the aliphatic and aromatic CH stretching modes. The peak at 1745 cm−1 is assigned to the carbonyl groups of both the fatty ester and terminal aldehyde moieties, whereas the peak at 1469 cm−1 denotes the CC bonds of the phenyl rings. The broad absorption in the region 3480−3570 cm−1 is attributed to the hydroxyl and

between KI and polyperoxide) was calculated from the above calibration curve. The active oxygen levels were measured by the modified ASTM E299-08 method37 considering 4.0 μg of active oxygen/mL is equivalent to 63.4 μg of iodine/mL.

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RESULTS AND DISCUSSION Monomer Synthesis. Fatty acids were reacted with VBC via esterification condensation reaction in the presence of K2CO3 in anhydrous DMF at 85 °C to obtain side-chain FA containing styrenic monomers (Scheme 1). These monomers were purified by column chromatography and obtained in high yields (72−85%). The structures of all monomers were confirmed by 1H NMR spectroscopy (Figures S1−S6) and ESI-MS mass spectroscopy (Figures S7−S12). All of the proton peaks have been assigned on the corresponding NMR spectrum of the monomer. The experimental molecular masses obtained from ESI-MS analysis matched nicely with the theoretical molecular mass values. These results indicate successful synthesis of various side-chain fatty acid containing styrenic monomers, VBFEs. Synthesis and Characterization of Polyperoxides. Next, VBFEs were polymerized under 100 psi of oxygen pressure in dry toluene at 50 °C in the presence of AIBN initiator (Scheme 1). Figure 1 displays oxygen uptake (Δp) as a function of time for various VBFE monomers. Note that all of the polymerizations proceed through an induction period of 4−6 h. We suspect that some trace adventitious impurities in the synthesized monomers and/or toluene caused the inhibition time. Polymerization reactions were carried out at a constant pressure of 100 psi because in our previous work we observed that the oxidative polymerization of styrenic monomers was independent of oxygen pressure above 2 D

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Figure 3. (A) TGA thermograms at a heating rate of 10 °C/min, (B) DSC curves of PVBSP at different heating rates (ϕ), and (C) Kissinger’s plot of ln(ϕ/Td2) against 1/Td for different polymers. Both TGA and DSC studies were carried out under a nitrogen atmosphere.

Figure 4. EI-MS spectrum of PVBSP. Chemical structures of various degradation products corresponding to the various molecular ion peaks have been identified on the figure.

resonance signal at 7.9 ppm appeared due to the ortho protons of the benzoyl (C6H5CO) group which is present at the chain end.43 Similar types of 1H NMR spectra were obtained for other polyperoxides such as PVBPP and PVBSP. In Figure 2 (right side), the peaks corresponding to all of the carbon atoms in the polyperoxides are clearly observed in the 13 C NMR spectra. Here too, significant downfield shifts of main-chain methylene and methine carbons are observed due to the two more electronegative oxygen atoms directly attached to it. For example, in the 13C NMR spectrum of PVBSP, the methylene and methine carbons adjacent to the peroxy group appear at 75.6 and 82.4 ppm, respectively. Resonance signals at 65.7, 126.5−136.3, and 173.5 ppm are assigned to benzyl −CH2, aromatic carbons, and the carbonyl group of the ester moiety, respectively. Aliphatic carbons from the fatty acid chain appeared in the region of 14.0−34.2 ppm. For polystyrenic polymers, the main-chain −CH2− and −CH− carbons show peaks between 39 and 44 ppm,44 which are absent in Figure 2 (right side). This observation confirms the absence of −M−M− sequences (M: monomer) in the

hydroperoxide chain end groups, which formed during the oxidative polymerization via various chain transfer reactions.42 Similar kinds of FT-IR spectra were obtained for other polyperoxides (data not shown here). The structures of polyperoxides were also analyzed by 1H and 13C NMR spectroscopy in CDCl3. The 1H NMR spectra of PVBMP, PVBPP, and PVBSP are shown in Figure 2, where backbone methylene (3.92−4.26 ppm) and methine (4.98− 5.17 ppm) protons appear at the downfield region due to the adjacent peroxide group. In Figure 2A (left side), the peaks at δ = 0.88 and 1.25 ppm are observed for CH3 and 14 CH2 units of the aliphatic side chain, respectively. In addition, the OCCH2 and OCCH2CH2 protons from the FA moiety give resonance signals at 1.63 and 2.3 ppm, respectively. The 7.17−7.35 ppm signal is due to the repeating unit aromatic protons (and also the residual CHCl3 peak from the CDCl3 solvent). The resonance signal at δ 5.2 ppm is attributed to the CH2 protons adjacent to the phenyl ring. In addition, we observed a small peak at 10.1 ppm which was attributed to the OCH protons in the polymer chain ends.29 A weak E

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It is reported that polymers with long alkyl groups in the side chain can exhibit side-chain crystallization, irrespective of main-chain conformation.48 In order to check crystalline behavior from PVBFEP polymers in the bulk, DSC measurements were performed at 10 °C/min heating rate under a N2 atmosphere. Figure 5 displays DSC thermograms of all

polyperoxides and indicates a strictly alternating copolymer structure between the monomer and oxygen. Nevertheless, we determined active oxygen content in the polyperoxides by using the standard test method for peroxides.37 Experimental active oxygen content values are shown in Table 1, along with the theoretical active oxygen content values calculated on the basis of the amount of used polymer (0.5 mg) during the UV− visible spectrometric experiment. Excellent agreement between the experimental and theoretical active oxygen content values confirms a strictly alternating copolymer structure between the monomer and molecular oxygen. To determine the thermal stability of polyperoxides, TGA curves were recorded under a N2 atmosphere at 10 °C/min (Figure 3A), where we observed a three-step decomposition process. The first stage degradation starts at ∼120 °C, attributed to the decomposition of −O−O− bonds in the polymer backbone, and afterward, further decomposition takes place for the degradation products and side-chain ester bonds and fatty acid moieties. From Figure 3A, 5.2 and 5.4% weight losses are observed between 120 and 180 °C for the PVBSP and PVBPP, respectively, whereas the theoretical values of formaldehyde were calculated (Scheme 1) on the basis of repeating unit as 7.0 and 7.4%, respectively (see the Supporting Information). Thermal behavior of polyperoxides was further studied by DSC under a N2 atmosphere at different heating rates. Figure 3B shows DSC thermograms of PVBSP at different heating rates (see also Figure S15 for PVBPP), which explain highly exothermic degradation of PVBSP like other polyperoxides.1 It was already reported by Miller and Mayo that vinyl polyperoxides degrade via random scission of mainchain peroxy bonds followed by unzipping of the βperoxyalkoxy radicals to give the degradation products, formaldehyde and other carbonyl compounds (Scheme 1). The initial exotherm suggests the decomposition of main-chain O−O bonds. Note that the enthalpy of degradation (ΔHd) was not determined due to the very high heat of vaporization of the degradation products. Nevertheless, the activation energy for the thermal degradation (Ed) was calculated using Kissinger’s method from the slope of the ln(ϕ/Td2) against 1/Td plot (Figure 3C), where ϕ is the heating rate and Td (K) is the peak temperature.45 The Ed values listed in Table 1 were found to lie between 34.6 and 41.4 kcal/mol, which are comparable to the O−O bond dissociation energy,30 thus signifying degradations of these polymers are initiated by the scission of the peroxy bonds in the main chain. Thermal degradation products of synthesized polyperoxides were investigated by EI-MS analysis, a suitable technique to identify primary degradation products of polymers.46 The EIMS mass spectrum of PVBSP is shown in Figure 4, where the molecular ion peaks are formed due to electron impact. All of the important ion peaks are assigned in the EI-MS spectrum and presented in Figure 4 (also in Table S1). The peak at m/z = 403 is assigned to the 4-formylphenyl stearate, which is formed by the degradation of weak peroxy bonds (Scheme 1). Main-chain O−O bond breaking is considered as the major reason behind the formation of all of the molecular ion peaks, following the chain unzipping mechanism.47 Also, the PVBPP (Figure S16 and Table S2) and PVBCRP (Figure S17 and Table S3) displayed degradation similar to that of PVBSP. Thus, alternating placement of peroxide bonds in the copolymer chain is confirmed on the basis of the identification of degradation products from EI-MS study.

Figure 5. DSC thermograms of various side-chain fatty acid containing polyperoxides under a N2 atmosphere at a heating rate of 10 °C/min.

polyperoxides, indicating crystalline melting temperatures (Tm) of approximately 46, 49, 52, and 70 °C for the PVBLP, PVBMP, PVBPP, and PVBSP, respectively. Thus, despite the absence of any strong intermolecular force, crystallinity appeared in these polymers because of alkyl chain length on the basis of intermolecular/intramolecular van der Waals interactions between the pendant long alkyl chains. Sometimes, we got two peaks during the heating cycle (data not shown here), maybe because of side-chain transformation to various morphological forms during the crystallization process.49 The PVBCLP and PVBCRP did not show any Tm. Note that we could not determine the degree of crystallinity for these polyperoxides from DSC study because of unavailability of heat of fusion data for the perfect crystal. Nevertheless, the alternating −O−O− groups in the polymer main chain did not disturb the side-chain crystallization of FA chains at C ≥ 12 of the side-chain alkyl carbon. The crystalline behavior of these polymers was further investigated by polarized optical microscopy (POM). When PVBSP, PVBPP, and PVBMP were heated above their Tm, the polymers were melted and became isotropic in nature. On cooling down the polymer below their Tm, a birefringence texture which is characteristic of semicrystalline polymer was observed (Figure 6),50,51 whereas, in the case of PVBCLP, no birefringence texture was formed (Figure S18). The origin of crystalinity in PVBSP, PVBPP, and PVBMP is due to the intermolecular/intramolecular van der Waals interaction between the long alkyl chains. However, in the case of PVBCLP, the chain length is not sufficient enough to form ordered structure and thus unable to form crystallization induced birefringence texture. Fatty acid moieties in the side chains establish lamellar structures through the hydrophobic interaction among the side-chain fatty acid segments.52 To understand the formation of lamellar morphology through the ordering of long individual fatty acid chains, the small-angle X-ray scattering (SAXS) measurements were carried out for the polyperoxides. In the literature, to obtain the perfect orderness in the crystalline state of the sample, the PXRD studies were performed by annealing the sample above the melting transition observed in the DSC F

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Figure 6. POM images of PVBSP (top row), PVBPP (middle row), and PVBMP (bottom row) at different temperatures.

Figure 7. (A) SAXS and (B) WAXS profiles for PVBSP, PVBPP, PVBMP, and PVBLP.

thermogram.53 However, in our study, the PXRD measurements were carried out without annealing the samples because of the possibility of partial decomposition of the main-chain labile peroxy bonds upon heating. The SAXS profiles of the polymers are shown in Figure 7A, where only one broad scattering peak for PVBSP (2θ = 1.50−1.68°), PVBPP (2θ = 2.01−2.09°), PVBMP (2θ = 2.04−2.13°), and PVBLP (2θ = 2.14−2.20°) is observed, thus providing evidence for the formation of unique ordered lamellar structure. No higher order scattering peaks are detected from the SAXS study in Figure 7A. The interlamellar spacing (d) is determined from the SAXS study for the PVBSP, PVBPP, PVBMMP, and PVBLP as 26.91, 22.00, 21.01, and 20.06 Å, respectively. As expected, the d value decreases with the decreasing chain length of the fatty acid pendant in the side chain of the polyperoxides.54 To study the effect of FA chain length on crystallinity, the wide-angle X-ray scattering (WAXS) measurements are performed (Figure 7B). All of the samples exhibit two peaks (2θ = 20.7 and 21.57°), although the second peak is much sharper for the PVBSP and PVBPP, and it almost merges for the PVBMP and PVBLP. The peak at around 2θ = 20.7° (d =

2.17 Å) in the WAXD profile indicates the amorphous halo.55 The sharp peak for PVBSP and PVBPP is observed at 2θ = 21.57° (d = 2.09 Å) due to the crystallization of the long FA segments present in the polymer side chains. As the d spacings between the two lamellas are different with dissimilar alkyl chain lengths, as observed from the SAXS study, the higher peak intensity at 2θ = 21.57° (percentage of crystallinity) in PVBSP is noticed in the WAXS profile. Thus, the hydrophobic interaction among the alkyl chains increases with increasing fatty acid chain length to provide formation of crystalline domains. The WAXS profiles of polymers display another diffraction peak at 24.8°, which also indicates the crystallinity of the polyperoxides.56 However, we are unable to assign crystal planes. The presence of crystal packing in the polyperoxides having FA side-chain segments is directly visualized in TEM images. For TEM studies, samples were prepared by drop-casting and TEM images of PVBMP and PVBLP are shown in Figure 8. From the corresponding selected area electron diffraction (SAED) patterns, well-defined lamellae are observed and the lattice spacing values are determined as 2.05, 2.28, and 2.21 Å for the PVBLP, PVBMP, and PVBSP (Figure S19), G

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from their optimized structures, as shown in Figure S21. Interestingly, in all of the cases, the values obtained from the DFT calculations are in good agreement with the experimental value determined from SAXS study (Table S4). We have summarized the possible mechanism of crystalline domain formation in the polyperoxides due to the hydrophobic intraction among the alkyl chains with n ≥ 12 (Figure 10).

Figure 8. TEM micrographs of PVBMP (left side) and PVBLP (right side) along with the corresponding diffraction pattern.

respectively. These values match reasonably well with the lattice spacing value determined for these polyperoxides from the WAXS measurements as 2.09 Å. To verify the interlamellar distance, we computed the sidechain length of a single repeating unit of PVBPP from the optimized structure of a model repeating unit, HOO CH(C 6 H 4 CH 2 OC(O)(CH 2 ) 14 CH 3 )( CH3). We also computed the average side-chain lengths of the oligomers having three, four, and five repeating units. The optimized structures are shown in Figure 9 and Figure S20. In all of the cases, we observed similar side-chain lengths of repeating units from the PVBPP. This indicates that the number of side-chain hydrocarbon chains does not have any effect on the length of the hydrocarbon chain pendants. This is because the straight all trans conformation has the lowest energy. However, the side chains are going away in outward directions in the oligomers having three and four repeating units, whereas the chain in the middle remained in its original position (Figure 9b and c and Figure S20). Thus, the vacant space at the end of the polymer chains allows only the terminal chains to go away in the outward direction. Hence, increasing number of repeating units increases the compact packing of middle chains in the polymer matrix, except for the terminal chains (Figure 9c). Since the number of repeating units has no effect on the side-chain length, we have further calculated the side-chain lengths of PVBLP, PVBMP, and PVBSP considering the model repeating unit of the corresponding polymer chains

Figure 10. Crystalline domain formation in the side-chain fatty acid based polyperoxides.

Conclusions. In summary, we have successfully synthesized renewable resource derived fatty acid containing polyperoxides, PVBFEP. The PVBFEPs are strictly alternating copolymer (1:1) of VBFE and molecular oxygen and showed exothermic degradation comparable to the dissociation energy of the O−O bond in dialkyl peroxides and other polyperoxides. Although the −O−O− bonds in the polyperoxide main chain impart flexibility to the backbone compared to the only hydrocarbon-backbone analogue,25 PVBFEPs showed crystallinity when the FA moieties had 12 carbon atoms or longer. The origin of the crystalinity was found to be side-chain crystallization because of the intermolecular/intramolecular van der Waals interaction between the alkyl chains with C ≥ 12. Noncrystalline PVBCLP and PVBCRP are a viscous gummy material, due to the nonrestricted flexibility of their main chain. The crystalline polyperoxides are solid powdery material and are thus easy to handle quantitatively and show remarkable thermal stability up to ∼120 °C. Among the crystalline polyperoxides, the melting temperature Tm increases systematically with the increase of side-chain lengths. This microphase separation was supported by WAXS study, where we observed crystalline organization of alkyl chains with C ≥ 12. SAXS study confirmed that the d spacing between the two lamellae are different with dissimilar alkyl chain lengths, which

Figure 9. Optimized structures of PVBPP repeating units using the Gaussian 09 package in toluene as solvent: (a) single unit (side-chain-length distance = 25.9 Å), (b) three repeating units (average side-chain-length distance = 26.1 Å), and (c) four repeating units (side-chain-length distance = 25.9 Å). C, gray; O, red; H, cyan. H

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(7) Kishore, K.; Ravindran, K. Thermal Reactivity of Poly(styrene peroxide): A Thermodynamic Approach. Macromolecules 1982, 15, 1638−1639. (8) Nanda, A. K.; Kishore, K. Catalytic Oxidative Polymerization of Vinyl Monomers Using Cobalt Phthalocyanine Complex and an Exploratory Investigation on the Polymerization of Vinyl Acetate. Macromolecules 2001, 34, 1558−1563. (9) Kishore, K.; Mukundan, T. Poly(styrene peroxide): an autocombustible polymer fuel. Nature 1986, 324, 130−131. (10) Subramanian, K.; Kishore, K. Application of polystyrene peroxide as a curative in coating and molding compositions. Eur. Polym. J. 1997, 33, 1365−1367. (11) Sato, E.; Tamura, H.; Matsumoto, A. Cohesive Force Change Induced by Polyperoxide Degradation for Application to Dismantlable Adhesion. ACS Appl. Mater. Interfaces 2010, 2, 2594−2601. (12) Kurochkin, S. A.; Silant’ev, M. A.; Perepelitsyna, E. O.; Grachev, V. P. Molecular oxygen as a regulator of primary chain length of branched polymers formed in 3D radical polymerization. Oxidative polymerization of styrene. Polymer 2013, 54, 31−42. (13) Kurochkin, S. A.; Silant’ev, M. A.; Perepelitsyna, E. O.; Berezin, M. P.; Baturina, A. A.; Grachev, V. P.; Korolev, G. V. Synthesis of highly branched polymers via three-dimensional radical polymerization in the presence of oxygen. Polym. Sci., Ser. B 2012, 54, 223− 233. (14) Fujioka, T.; Taketani, S.; Nagasaki, T.; Matsumoto, A. SelfAssembly and Cellular Uptake of Degradable and Water-Soluble Polyperoxides. Bioconjugate Chem. 2009, 20, 1879−1887. (15) Mukundan, T.; Kishore, K. Synthesis, characterization and reactivity of polymeric polyperoxides. Prog. Polym. Sci. 1990, 15, 475− 505. (16) Nakano, T.; Nakagawa, O.; Yade, T.; Okamoto, Y. Solid-State Polymerization of Dibenzofulvene Leading to a Copolymer with Oxygen. Macromolecules 2003, 36, 1433−1435. (17) Nomura, S.; Itoh, T.; Ohtake, M.; Uno, T.; Kubo, M.; Kajiwara, A.; Sada, K.; Miyata, M. Polymerization by Insertion of Molecular Oxygen into Crystals of 7,7,8,8-Tetrakis(ethoxycarbonyl)quinodimethane. Angew. Chem., Int. Ed. 2003, 42, 5468−5472. (18) Matsumoto, A.; Higashi, H. Convenient Synthesis of Polymers Containing Labile Bonds in the Main Chain by Radical Alternating Copolymerization of Alkyl Sorbates with Oxygen. Macromolecules 2000, 33, 1651−1655. (19) Matsumoto, A.; Taketani, S. Regiospecific Radical Polymerization of a Tetrasubstituted Ethylene Monomer with Molecular Oxygen for the Synthesis of a New Degradable Polymer. J. Am. Chem. Soc. 2006, 128, 4566−4567. (20) Sato, E.; Yuri, M.; Fujii, S.; Nishiyama, T.; Nakamura, Y.; Horibe, H. Liquid marbles as a micro-reactor for efficient radical alternating copolymerization of diene monomer and oxygen. Chem. Commun. 2015, 51, 17241−17244. (21) Sato, E.; Yuri, M.; Fujii, S.; Nishiyama, T.; Nakamura, Y.; Horibe, H. Liquid marble containing degradable polyperoxides for adhesion force-changeable pressure-sensitive adhesives. RSC Adv. 2016, 6, 56475−56481. (22) Silant’ev, M. A.; Perepelitsina, E. O.; Grachev, V. P.; Kurochkin, S. A. Irregular polystyrene peroxides−a promising macroinitiators synthesized by radical polymerization under oxygen inflow. Eur. Polym. J. 2017, 89, 67−77. (23) Biermann, U.; Friedt, W.; Lang, S.; Lühs, W.; Machmüller, G.; Metzger, J. O.; Rüsch gen Klaas, M.; Schäfer, H. J.; Schneider, M. P. New Syntheses with Oils and Fats as Renewable Raw Materials for the Chemical Industry. Angew. Chem., Int. Ed. 2000, 39, 2206−2224. (24) Jayaseharan, J.; Kishore, K.; Nalini, G.; Gururow, T. N. First Report on a Semicrystalline Vinyl Polyperoxide. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 4033−4036. (25) Cais, R. E.; Bovey, F. A. Carbon-13 Nuclear Magnetic Resonance Study of the Microstructure and Molecular Dynamics of Poly(styrene peroxide). Macromolecules 1977, 10, 169−178.

was confirmed by DFT calculations, as the interlamellar distance obtained from the DFT study nicely matches with the SAXS data. The lattice spacing value of 2.09 Å from the WAXS data is comparable to the interchian distances of 2.05−2.28 Å for the polyperoxides, observed from TEM analysis. These studies indicate that a minimum of 12 methylene groups in the long alkyl chain are needed to decouple the flexible backbone conformation to form the crystalline lattice of the side chains in the polyperoxides.57 The present study opens up the possibility of generating a variety of other crystalline polyperoxides by suitable design of the backbone and pendant segments.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01981. 1 H NMR and ESI-MS spectra of various monomers, GPC traces, FT-IR spectra, DSC thermograms, EI-MS spectra, molecular ion identification, POM images, TEM image and optimized structures of repeating units of various polyperoxides, and table for interlamellar spacing of polyperoxides obtained from SAXS and DFT study (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Priyadarsi De: 0000-0001-5486-3395 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.M. thanks Council of Scientific and Industrial Research (CSIR), Government of India, for their research fellowships. The authors thank Mr. Sandeep Chaudhary (Indian Institute of Chemical Biology, Kolkata, India) for recording EI-MS spectra and Professor Tarun Kumar Mandal (Indian Association for the Cultivation of Science, Kolkata, India) for assistance with POM study.

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REFERENCES

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