Yuewen
Xu
,
Weiwei L.
Xu
,
Mark D.
Smith
and
Linda S.
Shimizu
*
Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter St, Columbia, SC 29208, United States. E-mail: shimizls@mailbox.sc.edu; Fax: +1 803-777-9521; Tel: +1 803-777-2066
First published on 14th November 2013
A carbonate–stilbene bifunctional macrocycle was readily synthesized, and its assembly was studied by crystallization from several solvents. The macrocycle displayed columnar assembly from less polar solvents (THF and CH2Cl2), while a more compact structure was observed from 9:
1 CH2Cl2–acetone. All structures displayed organization through the C–H⋯O hydrogen bonding motif as well as through aryl stacking interactions. Upon dissolution and treatment with Grubbs’ II catalyst, this 30-membered ring underwent entropy-driven ring-opening metathesis polymerization (ED-ROMP) to give a precisely linear alternating A–B–A–B copolymer. This design of a single macrocyclic building block allows the formation of supramolecular self-assembly in the solid-state while affording a linear alternating polymer from solution.
Aromatic polycarbonates are important engineering plastics, especially those manufactured from bisphenol A, which are transparent and have relatively high glass transition temperatures and good processability.8,9 Aliphatic polycarbonates are attracting attention for medical applications due to their biocompatibility and degradability.10 Typically, aromatic polycarbonates, such as those derived from bisphenol A, are made from the corresponding phenol and phosgene or its derivatives. Aliphatic derivatives are synthesized by ring-opening polymerization of cyclic carbonates or spiroorthocarbonate monomers using anionic, cationic, coordination–insertion, organocatalytic or enzymatic methods.11 These cyclic monomers are often strained 3–8 membered rings, and the polymerization is driven by enthalpy that is associated with the release of ring strain.12 The polymerization of large cyclic carbonates with relatively low ring strain is thought to be entropy driven.13 Entropy driven ring-opening metathesis (ED-ROMP) is relatively rare,14–18 and polymers are favoured at high monomer concentrations. Macrocyclic alkenes and esters have been polymerized by entropy driven polymerizations.12 For example, Hodge and Kamau observed ED-ROMP of macrocyclic olefins with 21-, 28-, and 38-membered rings.19 Gautrat and Zhu reported the ROP of cyclic bile acids (35- and 38-membered rings) to afford degradable elastomers.20 This manuscript reports the ED-ROMP of a 30-membered dicarbonate macrocycle to give an alternating copolymer.
Copolymers are of great interests to polymer chemists.21 A block copolymer can introduce phase separation of two blocks and maintain its individual function. Random copolymers allow readily tuning of material properties, such as crystallinity and glass transition temperature, by varying the composition of the two monomers along the polymer backbones. In comparison, precisely alternating copolymers could render optimal positioning of two monomers and introduce extra fine microstructure. Thus, they could be useful in a variety of applications, such as organic light emitting diodes.
Our symmetrical macrocycle is bifunctional and contains both carbonates and stilbenes. Upon ring-opening metathesis polymerization using Grubbs’ II catalyst, macrocycle 1 produces a precisely alternating copolymer, where carbonate and stilbene groups were aligned next to each other. We used computations to estimate the ring strain present in 1 and investigated the subsequent ring-opening polymerization of the macrocycle. The polymerization was clearly concentration dependent with lower concentration (0.2 M) affording relatively low yield of polymer while higher concentrations (1 M) showed significant improvement in both yield and molecular weight. We observed significant isomerization of the cis-stilbene under our polymerization conditions to give a trans:
cis ratio of ∼8
:
5. These new polycarbonates may have interesting material properties and applications.
1·CH2Cl2 | 1·C4H8O | 1 | |
---|---|---|---|
Empirical formula | C35H30Cl2O6 | C38 H36O7 | C34H28O6 |
Formula weight | 617.49 | 604.67 | 532.56 |
T/K | 150(2) | 100(2) | 150(2) |
Crystal system | Triclinic | Monoclinic | Monoclinic |
Space group |
P![]() |
P21/n | P21/c |
a/Å | 10.9128(8) | 14.555(4) | 11.2731(8) |
b/Å | 11.6446(8) | 5.6690(16) | 10.6669(7) |
c/Å | 12.4210(9) | 18.310(5) | 22.7178(15) |
α/° | 97.081(1) | 90 | 90 |
β/° | 100.513(1) | 96.521(6) | 91.852(2) |
γ/° | 98.806(1) | 90.00 | 90 |
V/Å3 | 1514.93(19) | 1501.0(7) | 2730.4(3) |
Z | 2 | 2 | 4 |
D c/mg mm−3 | 1.354 | 1.338 | 1.296 |
μ/mm−1 | 0.260 | 0.092 | 0.088 |
F(000) | 644 | 640 | 1120 |
Refl. collected, unique | 21![]() |
16![]() |
30![]() |
R int | 0.0433 | 0.0517 | 0.0662 |
GOF on F2 | 1.065 | 1.022 | 0.912 |
R 1[I > 2σ(I)] | 0.0492 | 0.0416 | 0.0394 |
wR2 (all data) | 0.1449 | 0.1011 | 0.0840 |
Δρmax,min/e Å−3 | 0.588/−0.458 | 0.24/−0.21 | 0.387/−0.128 |
![]() | ||
Scheme 1 Synthesis of macrocycle 1. Reagents and conditions: (i) LiAlH4, THF, 95%; (ii) 1,1′-carbonyldiimidazole, THF, reflux, 41%. |
Macrocycle 1 crystallized from CH2Cl2 in the triclinic system with space group P. The asymmetric unit consists of one C34H28O6 molecule and one encapsulated CH2Cl2 molecule (Fig. 2a and b). Unexpectedly, the carbonate macrocycle assembled into columns assisted by four short C–H⋯O interactions.24,25 Blue lines in Fig. 2b indicate interactions with distances at least 0.2 Å shorter than the sum of the van der Waals radii of H and O (2.72 Å). Donor–acceptor (D(H)–A) distances range from 3.278(2) for C2(H2A)⋯O5′ to 3.467(2) for C17(H17B)⋯O1′. Dichloromethane molecules were encapsulated within the columnar host structures through contacts between the hydrogens of CH2Cl2 and the carbonate oxygen that give short donor–acceptor (D(H)–A) distances of (C(H)⋯OCO2 = 3.62 Å) as well as to the neighboring aryl group (H⋯centroid 3.18 Å).
Columnar assembly was also observed in the THF solvate C34H28O6·C4H8O. The compound crystallized as colorless needles in the monoclinic space group P21/n. The asymmetric unit consists of half of one macrocycle, which is located on a crystallographic inversion center, and a THF molecule (Fig. 3). The THF is disordered across the inversion center. The macrocycles stack into columns through CH⋯O interactions between the methylenes of 1 and the carbonyl oxygen of a neighboring macrocycle with a D⋯A distance of 3.492(2) from C(17)⋯O(1). Fig. 3b displays these short CH⋯O contacts as well as those between carbonate ester oxygen O2 and methylene adjacent to carbonate (H2) with D⋯A = 3.548(2) Å from C(2)⋯O(2). The cycles make an angle of 58.67(1)° with the column axis. The column is filled with THF solvates likely in contact with the column walls through CH⋯O and van der Waals interactions, though such analysis is less reliable because of the THF disorder.
A different, solvent-free crystal form was obtained by crystallization from 9:
1 CH2Cl2–acetone. The colorless block crystals are monoclinic with the space group P21/c. The macrocycles assemble primarily through CH–O interactions. Fig. 4b shows the short CH⋯O interactions between the carbonate oxygen (O4) and C–H donors on two neighboring macrocycles. The donor–acceptor (D(H)–A) distances are 3.250(2) Å from C(10) to O(4) and 3.247(2) Å from C34 to O4. In addition, we observed offset face-to-face stacking interactions (illustrated by pairs of like color) and edge-to-face (two molecules of unlike color) stacking interactions (Fig. 4c).
It appears that CH⋯O short contacts are an important organizational element in each of the three crystal forms. The small, low polarity solvents CH2Cl2 and THF were included in the channels of these columns. The self-assembly of macrocycles by non-covalent interactions has also been used to generate confined environments that can influence organic reactions.26 Thus, we were interested to see if the columnar structures would be porous after the solvent was evacuated.
Removal of the dichloromethane under vacuum (48 h) caused a marked change in the morphology of the crystals, experimentally suggesting that the interactions that hold the carbonate macrocycles together are not as robust as the urea interactions that typically drive our bis-urea macrocycles into stable columnar structures.6,7 We next investigated the porosity of these crystals. Crystals of 1 (145 mg) obtained from CH2Cl2 after vacuum drying were subjected to N2 gas adsorption measurement at 77 K. We observed a type II isotherm (Fig. 5a), which is consistent with mesoporous materials.27 The BET method was applied to the isotherm at P/Po between 0.06 and 0.2 to give a calculated surface area of 314 m2 g−1. The pore volume and pore size were calculated as 0.23 cm3 g−1 and 3.32 nm respectively. The results of gas adsorption experiment indicate that, upon removal of the solvent, 1 is mesoporous and shows a pore size that is much larger than the cavity of a single macrocycle.
We turned to powder X-ray diffraction (PXRD) to compare the structure of the desolvated sample with two crystal forms of 1 (1·CH2Cl2 and the compact structure of 1 from 9:
1 CH2Cl2–acetone). The single crystal data from the crystal forms of 1 were used to generate simulated PXRD patterns using Mercury.28 Part of the desolvated sample (50 mg) was ground to a powder and examined by PXRD (Fig. 5b, blue line). The pattern is sharp and intense, indicating a highly ordered structure. Comparison of peak positions and intensities of the observed pattern with the two simulated patterns suggests that this structure is distinct from the other crystal forms.
Ring-opening metathesis polymerization (ROMP) is a powerful technique for synthesizing high molecular weight functional polymers.29 Typically, this type of polymerization takes place in small strained cycle with a size of 3–8 membered ring. The main driving force of polymerization is the release of ring strain, an enthalpically driven process. Larger macrocycles with low ring strain have minimal enthalpy change on polymerization and have less of an enthalpic driving force.18,30 Monomers with ring strain energy below a critical value of 5 kcal mol−1 should not readily polymerize under typical conditions.13 Thus, we next investigated the ring strain in macrocycle 1.
Using the crystallographic coordinates (.cif file), we generated a model of macrocycle 1 using Spartan 10 and deleted the coordinates of CH2Cl2 guest.31 The Monte Carlo search at ground state with Molecular Mechanics (MMFF) force field was carried out to investigate the lowest energy conformer of macrocycle 1, and the simulated annealing method was used by the program to examine 100000 conformers and the lowest 100 conformers were kept. We further examined the lowest energy conformer (Fig. 6) by the density functional calculation (DFT) at B3LYP level and using the 6-31+G* basis set, which afford an energy E1 of −1763.89456 a.u.
To investigate the geometry optimized model in Fig. 6, the C2–C3 and C19–C20 bonds were broken to obtain two identical open chains (fragment 5). Monte Carlo search was performed to study the conformer distributions of fragment 5, which has two hydrogens added to the ends of the fragment. A corresponding DFT energy calculation of the lowest energy conformer of fragment 5 was carried out using identical basis set and afforded an energy E5 of −883.146979 a.u. Because this energy also reflects the formation of two C–H bonds (EC2′–H and EC3′–H) and the breaking of C–C bonds (EC–C) as well as any ring strain inherent in 1. Thus, the ring strain energy ERS of macrocycle 1 should be given by following equation:
ERS = E1 − 2E5 + 2ΔE | (1) |
ΔE = EC2′–H + EC3′–H − EC–C. | (2) |
We estimated the C–C bond energy by breaking the fragment 5 into radical 6 and 7 and calculated the energy of these two radicals by DFT at B3LYP level with 6-31+G* basis set. The C–C bond energy (EC–C = −0.157355 a.u.) was determined to the energy difference between fragment 5 and E6, E7 of the corresponding radicals. Similarly, models 8 and 9 were designed to estimate the energy of two C–H bonds (EC2′–H and EC3′–H) which formed at C2′ and C3′ of fragment 5, the bond energy was obtained as a difference between the energy of model 8 or 9 and the energy of corresponding radicals, which gives EC2′–H of −0.668356 a.u. and EC3′–H of −0.687446 a.u. Taken together, ΔE(−1.198447 a.u.) was calculated based on eqn (2). This afforded a strain energy ERS of macrocycle 1 of 1.57 kcal mol−1, suggesting relatively little strain. Given this low strain, we were not expecting to observe a ROP under typical conditions.
The ring-opening metathesis polymerization of macrocycle 1 was carried out using the highly efficient Grubbs' catalyst second generation and afforded reasonable high-molecular-weight polymers after quenching with ethyl vinyl ether.32 The polymer was precipitated by addition of methanol. The NMR spectrum of crude reaction mixture upon quenching indicated unreacted monomer and resulting polymer were observed as two major components, although minor fractions of cyclic oligomers were also possible but were not observed from NMR and SEC data. This suggests the cyclic oligomers concentration is very low, <5%.30 A range of reaction conditions was carried out to investigate the influences of monomer concentration and reaction time on polymerizations. In all cases, the monomer to catalyst ratios were set to 100:
1 for consistency; therefore, the highest degree of polymerization the polymer could possibly possess would be 100. The polymerization was clearly concentration dependent, since at a lower concentration of 0.2 M the polymerization produced polymer at relatively low yield and molecular weight.13 However, when the concentration increased to 1 M, which was the highest concentration before monomer started to crystallize from chloroform, the polymerization evidently showed significant improvement in both yield (60%) and molecular weight (Mn = 45 kg mol−1, entry 3 Table 2).20 Given the polymerization is clearly concentration dependent and that DFT calculations suggest low ring strain, our hypothesis is that the polymerization may be entropy driven. Indeed, Fogg et al. suggest that an entropy driving force can be created at appropriately high concentration of cyclic olefins.33 When ROMP was carried out at a shorter reaction time, we observed similar conversion and molecular weight, which suggests that the reaction equilibrium is likely established within a short period of time.
Entry | M/G2a | Conc. | t (h) | Conv. (%) | M n b (kg mol−1) | M w b (kg mol−1) | Đ b | D p c |
---|---|---|---|---|---|---|---|---|
a [Monomer]![]() ![]() |
||||||||
1 | ∼100 | 0.2 M | 18 | 31 | 13 | 20 | 1.5 | 24 |
2 | ∼100 | 0.7 M | 18 | 55 | 38 | 68 | 1.8 | 71 |
3 | ∼100 | 1 M | 18 | 60 | 45 | 86 | 1.9 | 85 |
4 | ∼100 | 1 M | 6 | 58 | 43 | 87 | 2.0 | 81 |
The ROP affords a copolymer with precise alternation of stilbene and carbonate groups along the macrocycle as illustrated in Scheme 2. The structure of resulting alternating polymer was confirmed from 1H NMR spectroscopy, which showed a clear distinction from the macrocycle monomer. The cis-stilbene is known to isomerize to the more stable trans-stilbene under UV-irradiation or thermal initiation.34–37 In ROMP, cis double bond is also known to undergo isomerization to reach toward an equilibration of backbone stereochemistry through secondary metathesis.19,38 The chemical shift of the methylene units of the stilbene are typically used to differentiate between the two isomers.39 The chemical shift at 7.1 and 6.6 ppm correspond to the vinylic protons (Ha in Fig. 7) on trans- and cis-stilbene, respectively and are baseline separated in our spectra. Thus, integration of these two gave a trans:
cis ratio of 8
:
5 for entry # 3. This ratio was also observed for the methylene units on trans- and cis-stilbene (Hb, Fig. 7). Other conditions gave similar ratios. Thus, we observe significant isomerization of the cis-stilbene under our polymerization conditions.
The molar mass of resulting polymer was analysed by size exclusion chromatography (SEC). The polymer 4 (entry 3 Table 2) displayed a single peak with a maximum retention time of 20.8 min as shown in Fig. 8a. The polydispersity of polymer 4 all fell in the range of 1.5–2.0, which is typically observed in the ROMP derived from monomers with a low ring strain.40–42 Furthermore, the secondary metathesis at extended reaction time (≥6 h) could also contribute to the slight broadening of the molecular weight dispersity.43,44 The polymer also displayed a single glass transition temperature (Tg) at 73 °C as analysed from differential scanning calorimetry (Fig. 8b) suggesting no microphase separation takes place.
UV-vis spectroscopy provides a further insight into the extended conjugation length of stilbene functionality upon polymerization. The absorption maxima of the solution of macrocycle 1 (2.5 × 10−5 M) originally was displayed at 284 nm, and shifted to a longer wavelength upon polymerization as shown in Fig. 8c. Two major absorption bands at 302 and 316 nm correspond to the cis and trans stilbene units, respectively and provide additional evidence that isomerization occurs under the reaction conditions. Indeed, the parent trans-stilbene should show a π → π* transition at a higher wavelength due to enhanced conjugation resulting from a more planar geometry of trans-stilbene.45,46
Footnote |
† Electronic supplementary information (ESI) available: NMR spectra, gas adsorption measurements, ring strain calculations, and crystallographic data. CCDC 957772–957774. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ra45055d |
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