Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Can Chain-Reaction Polymerization of Octadecyl Acrylate Occur in Crystal? Miao Yao, Jun Nie, and Yong He* College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China S Supporting Information *
ABSTRACT: Octadecyl acrylate was proven to exist in rotator phases, and the mechanism of its chain-reaction photopolymerization was revealed. The polymorphic behavior of octadecyl acrylate was studied by differential scanning calorimeter (DSC) and X-ray diffraction, which concluded that octadecyl acrylate exhibits two rotator phases (RII and RI), one orthorhombic crystal phase (Cort), and one triclinic crystal phase (Ctri) phase. The chain-reaction photopolymerization of four phases of octadecyl acrylate were studied by photo-DSC, and the theoretical possibilities of one-dimension chain propagation in RII, RI, and Cort phases were analyzed by using the molecular dynamics simulation results. Combining the experimental and calculation results, the chain-reaction polymerization mechanism either intralayer or interlayer was discussed and disclosed. The question of whether the chain-reaction polymerization of octadecyl acrylate can occur in crystal was answered, and the reason was explained.
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irradiation. They found that the octadecyl acrylate in β-form showed little tendency to polymerize while the octadecyl acrylate in α-form polymerized pretty well and achieved high conversion. The X-ray diffraction pattern showed that the αform and β-form of octadecyl acrylate possessed a single and three strong diffraction peaks, respectively, upon which Shibasaki and Fukuda pointed out that α-form is in hexagonal packing and β-form is in triclinic packing. Thus, they ensured that the octadecyl acrylate polymerized in the crystalline state. Moreover, they explained that the difference in the polymerization reactivity between α-form and β-form is due to the molecular arrangement. Octadecyl acrylate in hexagonal form is similar in packing style to its polymer, so that it is easy to polymerize. The triclinic packing octadecyl acrylate has to perform an expansion to reach hexagonal packing as polymer, which is extremely difficult in a real situation; thus, the polymerization in triclinic form is almost impossible. We19 also reported the crystalline-state photopolymerization of octadecyl acrylate irradiated by UV light, in which the photopolymerization reactivities of both α-form and β-form were similar to Shibasaki and Fukuda’s work. For crystalline-state cycloaddition, just two double bonds need to move close enough for impact, but for chain-reaction polymerization, a much larger number of double bonds need to move close to form the main chain of polymer. However, the octadecyl groups of crystallized octadecyl acrylate are closepacked, which restrict the deformation of molecule skeleton and the rotation of single bonds. The carbon−carbon double
INTRODUCTION Crystalline-state photopolymerization was investigated more than 60 years ago and exhibits the potential applications in photoswitching, optical recording, and sensing.1−6 Because of the tight and regular molecular arrangement, regio- and stereospecific polymers can be easily obtained from crystalline-state photopolymerization.7 Moreover, solid-state free radical photopolymerization could possess low oxygen inhibition by hindering oxygen diffusion and low polymerization shrinkage due to the preshrinkage during solidification and crystallization. Among all crystalline-state photopolymerization systems, photoinitiated [2 + 2]8−10 and [4 + 4]11,12 cycloaddition reactions were mostly investigated in diene,15 diyne,16 heterocyclic derivatives,17 metal−organic framework,10 and phenanthrene-containing12 systems. The criteria for crystalline-state [2 + 2] cycloaddition was reported by Schmidt and co-workers.13,14 They proposed that the center-to-center distance d between the nearest-neighbor double bonds is of the order of 4 Å through topochemical rules, and the experimental observed limit is 3.5 Å < d < 4.2 Å. During the crystalline-state cycloaddition reactions, the molecule skeleton deforms to enable the two double bonds to move closer and form rings with two nearest double bonds of adjacent molecules. By this way, multifunctional molecules can form polymer based on the step-reaction mechanism. The research about chain-reaction crystalline-state photopolymerization seems much less than step-reaction polymerization but is obviously more important due to achieving the change from small molecule to macromolecule easily and rapidly. Shibasaki and Fukuda18 first reported the crystallinestate photopolymerization of octadecyl acrylate upon γ-ray © XXXX American Chemical Society
Received: March 27, 2018 Revised: May 2, 2018
A
DOI: 10.1021/acs.macromol.8b00652 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules bonds of adjacent octadecyl acrylates may not move to be close enough to encounter, which causes that octadecyl acrylate cannot polymerize in the crystalline state. However, the X-ray diffraction patterns of the α-form and β-form of octadecyl acrylate indicate that molecules are ordered arranged like that in a crystal. Therefore, the α-form of octadecyl acrylate should be a special state that possesses both high degree of freedom and relatively high ordered arrangement. In this work, we first studied the polymorphic behavior of octadecyl acrylate, verified the existence of the special phase in octadecyl acrylate, and then investigated the photopolymerization in each phase. Through molecular dynamics simulation, we confirmed that the special phases of octadecyl acrylate possess enough potential in rotation of bonds to perform photopolymerization, while the crystal phase does not. Moreover, we studied the way of chain propagation in a special phase and proposed a series of possible paths for intralayer and interlayer propagation.
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EXPERIMENTAL SECTION
Materials. Octadecyl acrylate was purchased from TCI (Tokyo, Japan), recrystallized twice from 2:1 n-hexane/acetone (v/v) mixture solvent, and vacuum-dried at 0 °C for 8 h. The purity was checked by element analysis (vario EL cube, Elementar, Germany). The errors of element C and H are 0.03% and 0.01%, respectively. The type I free radical photoinitiator ethyl (2,4,6-trimethylbenzoyl)phenylphosphinate (TPOL) was obtained from High-tech Insight Co. Ltd. (Beijing, China). Characterization. Thermal analysis was carried out by a differential scanning calorimeter (DSC) (Q2000, TA Instruments, New Castle, DE). The sample weight was about 5 mg, and the scanning rate was 2 °C/min. X-ray diffraction data were obtained by the powder method with an X-ray diffractometer (Bruker D8 Advanced, Bruker, Karlsruhe, Germany) for the sample crystallized from solvent. Temperature-dependent X-ray diffraction experiments were performed on an X-ray diffractometer (X’Pert Pro MPD, PANalytical, Holland) over a temperature range of −10 to 40 °C using Cu Kα radiation (1.54 Å), power of 40 mA/40 kV, and rotating angle 2θ = 4°−40°. The sample was first heated from room temperature to 40 °C and kept for 5 min, followed by cooling. The heating and cooling rates were the same as those in DSC tests (2 °C/min). Photopolymerization was studied by using photo-DSC (Q2000, TA Instruments, New Castle, DE). Approximately 4 mg of sample (octadecyl acrylate and 1 wt % TPOL) was placed as a thin layer in a standard aluminum pan. The sample was irradiated by a 385 nm lightemitting diode light source (UVEC-4II, Lamplic Technology, Shenzhen, China), and the intensity was 3.5 mW/cm2. The conversions of photopolymerization were calculated according to the equation C = ΔHt/ΔH0thero, where ΔHt is the reaction heat evolved at time t and ΔH0thero is the theoretical heat for complete conversion. For acrylate double bonds, the ΔH0thero is 80 256 J/mol.20 Molecular weight and molecular weight distribution of poly(octadecyl acrylate) were determined by gel permeation chromatography (Waters 5152410, Milford, MA). THF was used as solvent and mobile phase at a flow rate of 1.0 mL/min. The calibration of molecular weight was based on polystyrene standards.
Figure 1. Differential scanning calorimeter (DSC) analysis of octadecyl acrylate. Scanning rate: 2 °C/min.
°C) due to the crystallization is found, and on the immediate heating (run 4) a compensative endothermic peak (peak c, 29.56 °C) appeared, which also represents the melting of sample. The melting point of sample that crystallized from molten is different from that of sample crystallized from solution. It could be deduced that the two samples, which crystallized from molten and solution, are in different crystal forms. When the molten sample is cooled to 5 °C (run 5) and heated immediately (run 6), a pair of small peaks (peak d, 10.88 °C and peak e, 11.97 °C) indicated a reversible phase transition. When the molten sample is cooled down to −10 °C (run 7), the main exothermic peak is followed by a small peak and a medium exothermic peak (peak f, −1.59 °C). In successive heating curve (run 8), a medium endothermic peak (peak g, 21.93 °C) is found followed by peak c. On the basis of the results, it can be deduced that octadecyl acrylate exhibits four solid phases, which are referred as α-, β-, γ-, and δ-phase. The α-, β-, and δ-phase in this paper are equal to the α-, sub-α-, and β-phase in Shibasaki and Fukuda’s18 work, respectively. They did not distinguish the γ-phase from the δ-phase while first reporting the crystalline-state photopolymerization in 1979. Although aware of the difference between these two phases in 1988,21 they did not reveal the crystal form of γ-phase. The phase transition behavior of octadecyl acrylate is shown in Scheme 1. The δ-phase was obtained by crystallizing from the 2:1 n-hexane/acetone (v/v) mixture solvent. It is the most stable phase, which does not transform into other solid phases no matter cooling or heating, and just melts (peak a) at about 33 °C. During cooling the molten sample to −10 °C, it crystallized in α-phase first (peak
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RESULTS AND DISCUSSION Polymorphic Behavior Analysis of Octadecyl Acrylate. The thermal analysis results of octadecyl acrylate are shown in Figure 1. When the sample, which was crystallized from solution, cooled from room temperature to −10 °C (run 1), no peak is observed. On the heating curve (run 2), one endothermic peak (peak a, 33.03 °C) appeared, which represents the melting of sample. On the cooling curve of the molten sample (run 3), an exothermic peak (peak b, 28.28
Scheme 1. Phase Transition of Octadecyl Acrylate
B
DOI: 10.1021/acs.macromol.8b00652 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules b, 28.28 °C) and then transformed into the β-phase (peak d, 10.88 °C) and γ-phase (peak f, −1.59 °C). By heating the γphase, it transformed into α-phase (peak g, 21.93 °C) and then melted (peak c, 29.56 °C). When the molten sample cooled to 5 °C and then was heated immediately, α-phase and β-phase transformed reversibly. Peak d (10.88 °C) and peak e (11.97 °C) represent the α → β and β → α process, respectively. X-ray diffraction (XRD) pattern for the α-, β-, γ-, and δ-phase of octadecyl acrylate are shown in Figure S1. In the α-phase, a single intense diffraction corresponds to the spacing 4.13 Å. In the β-phase, two strong diffraction peaks correspond to the spacing 3.84 and 4.19 Å, while two strong diffraction peaks correspond to the spacing 3.72 and 4.60 Å in the γ-phase. For α-, β-, and γ-phase, the distinct diffraction peaks correspond to the second order of spacing 29.7 Å, which is nearly equal to the length of fully extended molecules of octadecyl acrylate.18 Therefore, the molecules in α-, β-, and γ-phase are packed with long-chain axis vertical to the lamellar plane. In the δ-phase, three strong diffraction peaks correspond to spacing 3.47, 3.74, and 4.48 Å, and they are similar to Shibasaki and Fukuda’s (3.50, 3.79, and 4.56 Å)18 and our previous (3.5, 3.8, and 4.6 Å)19 work, too. To further characterize the crystal structure and solid-state phase transitions during the cooling and heating process of octadecyl acrylate, temperature-dependent XRD experiments were performed. The sample was heated from room temperature to 40 °C and kept for 5 min to ensure the sample melted entirely. While the molten sample was cooled to 25 °C, only a single peak (4.13 Å) emerged, corresponding to the α-phase (Figure 2a). With the temperature decreasing further, the peak (4.13 Å) shifted to 4.19 Å and a new peak (3.84 Å) appeared, indicating the appearance of the β-phase. While keeping the temperature at 5 °C, with the isothermic time increasing, the diffraction peaks of β-phase (3.84 and 4.19 Å) decreased and two new peaks (3.72 and 4.60 Å) appeared and enhanced, meaning the transition from the β-phase to γ-phase. After 50 min, the peaks corresponding to the spacings 3.84 and 4.19 Å disappeared, representing the β → γ process had finished, and all the β-phase transformed into the γ-phase. After keeping the temperature at 5 °C for 1.5 h, the sample was heated to 25 °C; then the peaks (3.72 and 4.60 Å) reduced, and a peak (4.13 Å) appeared at about 20 °C (Figure 2b). It means that the γ-phase transformed into α-phase directly. The XRD results are consistent with DSC results. Among all phases of octadecyl acrylate, the special phase mentioned above is the key point to further research, which may help to answer the question of whether the chain-reaction polymerization of octadecyl acrylate can occur in the crystalline state. The rotator phase first appeared in our mind because the rotator phase was certainly proved in similar long chain normal alkane. Rotator phases are a series of special condensed phases between fully ordered crystal and isotropic liquid, which was first reported by Muller during his investigation on normal alkanes.22 They consist of layered structures with threedimensional crystalline order in the positions of the molecular centers of mass, but no long-range orientation order in the rotation of the molecules about their long axis. The molecules in rotator phases could perform free or hindered rotations, and some gauche conformers exist at the end of the molecules.23 If the α-form of octadecyl acrylate is a kind of rotator phase, its high conversion during photopolymerization can be easily accepted. Up to present, there are totally discovered five different rotator phases: RI, RII, RIII, RIV, and RV, which are
Figure 2. Temperature-dependent XRD results of octadecyl acrylate (a) during the cooling and isothermal process and (b) during the heating process.
orthorhombic, hexagonal, triclinic, monoclinic, and monoclinic, respectively.24 The character of rotator phase for long chain normal alkanes is that the area per molecule (as viewed along the chain axis) is 19.65 and 19.45 Å2 for RII and RI, respectively, which can be calculated from the X-ray diffraction data.25 For octadecyl acrylate, the values for α-phase and β-phase are 19.7 Å2 and 19.2 Å2, both of which could be identified as rotator phases. As reported previously, molecules in α-phase and βphase are untilted with respect to lamellar plane, and the short spacing of them is characteristic of hexagonal packing and orthorhombic packing of long hydrocarbon chains, respectively.26 Therefore, the α-phase is RII phase, and the β-phase is RI phase. The short spacing of γ-phase is characteristic of orthorhombic packing of long hydrocarbon chains, too, and the area per molecule of γ-phase is 18.7 Å2, which is similar to the reported value of herringbone orthorhombic crystalline phase (∼18.5 Å2);25 thus, the γ-phase is herringbone orthorhombic crystal phase (Cort). The area per molecule of δ-phase is difficult to obtain, but fortunately, it was reported as triclinic crystal phases (Ctri) by Shibasaki and Fukuda.18 Photopolymerization. Shibasaki and Fukuda reported the conversion of photopolymerization of α (RII) phase and β (Ctri, the δ in this paper) phase, but they did not mention the photopolymerization of RI phase because this mesophase is metastable. We studied the relationship between the stability of RI phase and the temperature and found that the RI phase can be reserved at 9 °C for at least 1 h (Figure S2). Kinetics of photopolymerization in each form of octadecyl acrylate were studied by using photo-DSC. Octadecyl acrylate in RII and RI showed high conversion (71% and 58%, respectively), while the C
DOI: 10.1021/acs.macromol.8b00652 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules conversion of octadecyl acrylate in Cort and Ctri are both less than 10% (Figure 3). Up to now, the answer for whether the
Figure 4. Aggregation models of octadecyl acrylate. The red solid circle and the black straight line represent the acrylate and octadecyl groups in octadecyl acrylate, respectively.
listed in Table 1, and the simulated physical quantities were consistent with the experimental ones. Table 1. Simulated (Model (a) in Figure 4) and Experimental Cell Parameters (a, b, and c), the Area per Molecule, and the Density of RII Phase
Figure 3. Kinetics of photopolymerization in RII (black line), RI (red line), Cort (blue line), and Ctri (pink line) phases of octadecyl acrylate.
simulation experiment
chain-reaction polymerization of octadecyl acrylate can occur in crystal was given as NO, and the focus of our work transfers to the understanding of essence of polymerization in rotator phase. The high conversion of rotator phase should be beneficial from the specialty of rotator phases on molecule arrangement and mobility. In our previous work of HDDA, we established a method to judge the possibility of chain propagation in crystal region and concluded that if the distance between atoms C2 (i.e., the reaction site of acrylate group) of two molecules is smaller than or equal to 2.54 Å, the reaction between these two molecules can occur. For chain-reaction propagation, a threemolecule model in a triangle was defined as a basic unit, and the distance between two adjacent molecules should be shorter than 5.08 Å.27 This method is not only suitable for crystal but also applicable for the system possessing ordered arrangement (e.g., rotator phase). For the RII, RI, and Cort phase of octadecyl acrylate, the maximum distances between two adjacent molecules are 4.77, 5.00, and 5.03 Å, respectively, which were calculated according to the method in Muller’s22 work. These three phases are all satisfy the prerequisite mentioned above, but just two rotator phases can polymerize and the crystal phase cannot, which suggested that the most major factor that affects the polymerization reactivity is the motion ability of the acrylate group. Theoretical Analysis on the Possibility of Chain Propagation. The motion of acrylate group is decided by the rotation of C−C and C−O bonds in octadecyl acrylate. The all-atom MD simulation was performed by using the GAFF force field in GROMACS to understand the rotation ability of bonds in rotator phase. The detailed simulation information was concluded in the Supporting Information. During the aggregation from liquid state to polycrystalline state, the acrylate groups can be on the same or different sides of the lattice layer, which exist three aggregation models (Figure 4). The deformation of the skeleton and the rotation angles of the C−C bonds of octadecyl groups, influenced by the packing density and the interaction between the molecules, are the main factor that affect the motion of acrylate groups. Thus, it supposed that the motion abilities of acrylate groups in the three models are similar, and the simulation result of one model can represent that of the other two.28 The simulation results are
a (Å)
b (Å)
c (Å)
area (Å2)
density (g/cm3)
4.75 4.77
4.10 4.13
30.01 29.62
19.5 19.7
0.9225 0.916719
The distribution of rotation angle of each bond was studied from the simulation result during the last 1 ns, for which the atoms and bonds of octadecyl acrylate are numbered as shown in Figure 5a. All the distributions are unimodal, and the peak
Figure 5. (a) Molecule structure, atomic number, and bond number of octadecyl acrylate. (b) Rotation properties of bonds.
position and full width at half-maxima (FWHM) are summarized in Figure 5b. It can be deduced that all the bonds showed satisfied rotation ability due to FWHM of all the bonds exhibited are all larger than 10°. Adopting the rotation of each bond to investigate the movement of atom C2 is the most straightforward way, and the rotation of 19 bonds should be considered simultaneously. To study the chain propagation, three molecules in a triangle are considered at the same time.27 The distance of atoms C2 between molecule 1 and 2 (d12C2C2*) D
DOI: 10.1021/acs.macromol.8b00652 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 6. Proportion of (a) Δx, (b) Δy, and (c) Δz.
crystalline state is supposed to have the same ability with that in rotator phases for bond rotation, they can just possess a little polymerization possibility while the molecules aggregated in model 2. In fact, due to higher density of crystalline phase than that of rotator phases, the ability for bond rotation of molecules in crystalline phase is lower than that in the rotator phase. Furthermore, in polycrystalline state, just a portion of molecules will aggregate in model 2; thus, the possibility for octadecyl acrylate to perform chain-reaction polymerization in crystalline phase is very low. It is consistent with the experimental results of photopolymerization conversion discussed above. Chain Propagation Path. Theoretically, the octadecyl acrylate in rotator phases may polymerized in straight line or in curve intralayer. As shown in Figure 7a, the polymerization path can turn 60°, 120°, and 180° through a 4-, 5-, and 8-molecule center, respectively. Besides, the octadecyl acrylate in rotator
and the distance of atoms C2 between molecule 2 and 3 (d23C2C2*) are expressed in eqs S8 and S9, which are functions of displacement of atom C2 along x, y, and z directions (Δx, Δy, and Δz) after a series of simplifications (the detailed process is concluded in the Supporting Information). From the simulation results, the statistical results for moving ability of atom C2 along x, y, and z directions are included in Figure 6. Δx is mainly concentrated in −2.0 to 2.0 Å, and the proportion of Δx is symmetric about zero. Δy is mainly concentrated in −3.0 to 1.0 Å, and the value of Δy can be either greater or less than zero. Δz is mainly concentrated in −1.0 to 0 Å, and almost all the values of Δz are less than zero. Furthermore, rotator phases can flip different extents, as Sirota and co-workers25 reported that the long-chain normal alkanes flip 180° through displacing along its length by ±1.27 Å (a half of the repeat distance along the chain). Also, Wentzel and Milnera29 confirmed this kind of flip can just appear in rotator phases, but impossible for well-ordered crystalline phases from the simulation of normal alkanes. Thus, an extra variable quantity l should be added into eqs S8 and S9 to make the third part of these two equation change into (c ± l + dz)2. The l can be valued as 0, 1.27, and 2.54, which represent three conditions in the two-molecule system that participate in chain propagation: (1) two molecules either do not flip or both flip 180° and displace along the same direction, (2) just one molecule flips 180°, and (3) the two molecules flip 180° and displace along different directions. The chain propagation possibility of rotator phases under three conditions of three models (model 1, 2, and 3 in Figure 4) was calculated,30 and the result are summarized in Table 2. (The detailed calculation process is concluded in the Supporting Information.) From the result in Table 2, the octadecyl acrylate in rotator phase can have high chain-reaction polymerization possibility while l = 1.27 Å. For the octadecyl acrylate in crystalline state, molecules cannot flip 180° and displace along its long axis, which means that l can just be zero. Although the molecules in Table 2. Chain Propagation Possibility (CPP) of Rotator Phases under Different Conditions of Different Modelsa RII phase
RI phase
CPP
model 1
model 2
model 3
model 1
model 2
model 3
l=0 l = 1.27 l = 2.54
no high middle
middle middle low
no no no
no high middle
middle middle low
no no no
Figure 7. Chain propagation path in curve (a) intralayer and (b) interlayers. The black solid circles represent the atoms C2 of molecules that in the same lattice layer, and the yellow solid circles represent the atoms C2 of molecules that in the adjacent lattice layer in model 1.
The definitions of the level of possibility (“low”, “middle”, and “high”) are given in the Supporting Information.
a
E
DOI: 10.1021/acs.macromol.8b00652 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules phase can also polymerize between two adjacent lattice layers or across the interface of two crystalline grains. There are two modes for the polymerization between two adjacent lattice layers (Figure 7b). The confirmation for the possibility of polymerization in curve intralayer and interlayers is included in the Supporting Information. This variability enables octadecyl acrylate to polymerize more rapidly and completely. The traditional free radical photopolymerization in liquid state possess characteristics of obvious chain transfer and rapid chain termination because of high motion of monomers, chain radicals, and fragment radicals of initiators. In the rotator phase, the chain transfer and termination reaction will be strongly restricted by the interference of polymer chains.18 The molecular weight of poly(octadecyl acrylate) formed in rotator state was 1−2 times higher than that of polymer formed in liquid state (Table S3), and the molecular weight distribution of former is much lower than that of latter, which confirms that significantly less chain transfer and termination reactions exist in rotator phase polymerization. In rotator phases, the monomers orderly arranged before polymerization, which may play a positive role in enhancing the stereoregularity of polymer. However, the study on the stereoregularity of polymer formed in rotator phase is complex and immature, and we will work on it in the future.
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AUTHOR INFORMATION
Corresponding Author
*(Y.H.) E-mail:
[email protected]. ORCID
Jun Nie: 0000-0003-2698-1751 Yong He: 0000-0002-4689-966X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors kindly thank the Beijing Normal University for providing instrument for temperature-dependent X-ray diffraction experiment. The authors also thank the National Key Research and Development Program of China (2017YFB0307800) and National Natural Science Foundation of China (51373015 and 51573011) for their financial support.
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CONCLUSION Besides the previously reported and widely investigated long chain n-alkane, octadecyl acrylate was proved to exist in rotator phases. In its four solid forms, two are confirmed as rotator phases (RII and RI), and the other two are crystal phases (Cort and Ctri). The most stable phase (Ctri) can crystallize from solution or by transformation from RII and Cort phase. The RII phase could be obtained from molten and can transform into Cort phase via a metastable RI phase during cooling process. The octadecyl acrylate can perform photopolymerization in two rotator phases with high conversion but shows poor polymerization in two crystal phases. The bonds of octadecyl acrylate in rotator phase can rotate and lead to the motion of reactive site (i.e., the atoms C2 of acrylate groups). The flip of octadecyl acrylate, which is the distinguishing feature of rotator phases, enables two atoms C2 of adjacent molecules to move close enough for bonding more easily. The octadecyl acrylate in rotator phases can propagate in straight line or turn 60°, 120°, and 180° in photopolymerization. It can also perform photopolymerization between two adjacent lattice layers or across the interface of two crystalline grain. The molecular weight of polymer formed in rotator was 1−2 times higher than that of polymer formed in liquid state, which indicates that less chain transfer and termination reaction exist in rotator phase polymerization. The rotator phases of octadecyl acrylate can exist at relatively low temperature, which have potential application on low-temperature photopolymerization in the biological field.
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of atoms C2 between two molecules (dC2C2*), the calculation process of chain propagation possibility of rotator phases under three conditions of three models, and the detailed study on the polymerization paths (PDF)
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00652. Figures S1−S10 and Tables S1−S3, study on the stability of RI phase, the detailed simulation information on rotator phase, the simplification process of the distance F
DOI: 10.1021/acs.macromol.8b00652 Macromolecules XXXX, XXX, XXX−XXX
Article
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DOI: 10.1021/acs.macromol.8b00652 Macromolecules XXXX, XXX, XXX−XXX
