Author Manuscript
Title: Efficient Synthesis of Cyclic Block Copolymer by Rotaxane Protocol via Linear- Cyclic Topology Transformation
Authors: Stephanie Valentina; Takahiro Ogawa; Kazuko Nakazono; Daisuke Aoki;
Toshikazu Takata, Prof. Dr.
This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofrea- ding process, which may lead to differences between this version and the Version of Record.
To be cited as: 10.1002/chem.201601266
Link to VoR: https://doi.org/10.1002/chem.201601266
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COMMUNICATION
Efficient Synthesis of Cyclic Block Copolymer by Rotaxane Protocol via Linear-Cyclic Topology Transformation**
Stephanie Valentina, Takahiro Ogawa, Kazuko Nakazono, Daisuke Aoki, and Toshikazu Takata*
Abstract: High yielding synthesis of cyclic block copolymer (CBC) using the rotaxane protocol via linear-cyclic polymer topology transformation was first demonstrated. Initial complexation of OH-terminated sec-ammonium salt and a crown ether was followed by the successive living ring-opening polymerizations of two lactones to a linear block copolymer having a rotaxane structure via the final capping of the propagation end. CBC was obtained in a high yield by an exploitation of the mechanical linkage via the translational movement of the rotaxane component to transform polymer structure from linear to cyclic. Furthermore, the change of the polymer topology was translated into a macroscopic change in crystallinity of the block copolymer.
Cyclic polymers have been receiving vast attention in recent years[1] as the absence of free chain ends leads to distinctive properties compared with their linear and branched counterparts of the same molecular weight and composition[2], stretching further the boundaries of polymer field. On the other hand, block copolymers widely known as an important material with scientific and industrial significance becomes a central objective in recent polymer science owing to their ability to self-assemble in bulk[3]
and also in solution[4]. Hence, cyclic block copolymers (CBC) should have the potential to merge both chemical incompatibility and the unique properties based on the cyclic architecture, opening doors to fascinating prospects of unprecedented properties not given by linear block copolymers or cyclic homopolymers alone. In fact, several studies have already suggested that CBCs could play a unique role in a variety of applications including lithography and drug delivery carriers[5]. However, the study of CBCs is significantly less developed mainly due to technical difficulties in the synthesis and purification method, while the synthesis and properties of cyclic homopolymers have been vastly studied in recent years.
Although the common method of cyclization from linear precursors under high dilution has proved to yield a variety of CBCs[6], poor yields and tedious purification steps remain a formidable challenge. While ring-expansion methods are useful in generating large cyclic polymers[7], these are limited to the
kind of monomers, and thereby the generation of CBCs is even more challenging than that of cyclic polymer.
Inspired by the dynamic nature of rotaxane[8] and its successful exploitation in polymeric system[9], we recently reported a novel synthetic method of cyclic polymer utilizing the mobility of the rotaxane components[10]. To enhance the applicability of this rotaxane protocol, the extension to the synthesis of CBC should be required. Herein we report the effective synthesis of CBC from its linear precursor via the rotaxane protocol, along with the change in polymer property brought about by the topological change (Scheme 1).
Scheme 1. Synthetic strategy of cyclic block copolymer via topology
transformation of macromolecular [1]rotaxane from linear to cyclic.
In this synthetic strategy, a stable pseudo[2]rotaxane was used to initiate a sequential polymerization of monomers A and B, followed by end-capping reaction to obtain macromolecular [2]rotaxane with block copolymer as its axle component.
Cyclization was then performed by linking the axle and the wheel, resulting in a lasso-shaped macromolecular [1]rotaxane.
Since the cyclization process must be incorporated in cyclic polymer synthesis, i.e. in initiator, monomer, or polymer level, we chose the cyclization in polymer level but it did not involve unfavorable cyclization between two polymer termini, characterizing the “rotaxane protocol”. Lastly, the removal of interaction between the wheel and the station moiety at the polymer terminus rendered the wheel to translate from one end to another end where the second station is located, causing the polymer structure to change from linear to cyclic.
A novel CBC comprising blocks of crystalline poly(ε- caprolactone) (PCL) and amorphous polyhexanolactone (PHL) was successfully prepared according to Scheme 2. The combination of PCL and PHL was chosen as their identical molecular mass at each repeating unit enables the characterization by mass spectroscopy (MS). A pseudo[2]rotaxane initiator 3 which consists of a sec-ammonium axle having OH group 1 and a substituted dibenzo-24-crown-8- ether (DB24C8) wheel 2 was generated in situ by mixing them in dichloromethane. The living ring-opening polymerization (ROP) of ε-caprolactone (CL) initiated by the OH moiety of 3 was carried out at an ambient temperature using diphenyl phosphate (DPP) as a catalyst[9b, 9c, 11].
[*] S. Valentina, Dr. T. Ogawa, Dr. K. Nakazono, Dr. D. Aoki, Prof. Dr.
T. Takata
Department of Organic and Polymeric Materials Tokyo Institute of Technology
2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552 (Japan) Email: [email protected]
[**] This research was financially supported by ACT-C program of the Japan Science and Technology Agency (JST)
Supporting information for this article is given via a link at the end of the document.
rotaxane initiator
monomer A monomer B
Cyclic block copolymer Linear block copolymer
removal of interaction
linking of components
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Scheme 2. Synthetic pathways of linear block copolymer (6), cyclic block copolymer (6NAc), and its linear model (5NAc).
The resulting PCL 4 was then used as a macroinitiator to start the subsequent living ROP of δ-hexanolactone (HL) which was followed by the end-cap reaction with 3,5-bis(trifluoromethyl)- phenyl isocyanate according to the urethane end-cap method.[12]
The isolated yield of linear block copolymer (LBC) 5 containing [2]rotaxane structure having the PCL-PHL block axle was sufficiently high (84%).
The successful block copolymerization to LBC 5 having both the PCL and PHL segments was clearly confirmed by MS spectral change and SEC-based molecular weight change, as shown in Figure 1. As we expected, the MS spectral change could strongly evidenced the block copolymer structure of 5, because CL and HL possess an identical molecular weight, being well corresponding to the SEC profiles with an appropriate increase in molecular weight (from Mn 3,900 to Mn 8,300).
Figure 1. Left: MALDI-TOF MS spectra of PCL homopolymer 4 and PCL-PHL block copolymer 5 (matrix: dithranol, M – PF6-
). Right: SEC profiles of their neutralized form 4NAc and 5NAc (eluent CHCl3, PSt standards).
The subsequent cyclization of 5 to 6 (conversion of [2]rotaxane to [1]rotaxane structure) was carried out by ring-closing metathesis (RCM) reaction between the two olefinic moieties using the Grubbs II catalyst. The RCM reaction highly efficiently proceeded to give 6 in 83% yield. The high efficiency is attributed to the proximity effect of the two olefinic groups drawn
near each other by both the rotaxane linkage and hydrogen bonding at the polymer terminus, although the RCM formed a very big ring (ca. 25-membered ring). The efficient cyclization is one of the key steps of this method. Since the polymer terminus highly favors the intramolecular reaction but not intermolecular one, it allows us to perform the very high concentration reaction of 5 (1 mM) without any slow addition of reagents. Even at this concentration no oligomer was observed, remarkably cutting off tedious purification steps.
Final macrocyclization of LBC 6 to CBC 6NAc by carrying the crown ether wheel from one terminal to the other terminal of linear polymer also proceeded in a high isolated yield (86%) by the typical N-acetylation[13] with acetic anhydride/triethylamine.
The N-acetylation of 5 was similarly employed to obtain 89%
yield of 5NAc having the wheel component at the opposite terminal as the model LBC toward CBC 6NAc. The MS spectra (Figure 2) revealed the experimental values consistent with the theoretical values, offering a strong evidence for the N- acetylation along with the formation of CBC 6NAc.
Figure 2. MALDI-TOF MS spectra of observed (black) and calculated (blue) m/z values of LBC 5NAc and CBC 6NAc (matrix: dithranol, M + Na+).
Alongside the synthesis of the CBC, we also synthesized the linear and cyclic homopolymer of PCL and PHL using the same rotaxane method (Table 1, Scheme S2-S3). As the SEC charts (Figure 3a, Figure S20-S21) show an increase of retention time
DPP (110 mol%) O
O
PF6- N H2
10 O
O O O O O O O
O O
OnH
O O
(excess) DPP (70 mol%)
OCN CF3
CF3 O
PF6- NH2 10 O
O O O O OO O O
O O
On O
O O
H N
CF3
CF3 m
N N Mes Mes
Ru PCy3Ph
(2500 mol%.)
CH2Cl2 r.t., 12 h 88%
(7000 mol%)
CH2Cl2 r.t., 12 h
CH2Cl2 r.t., 12 h 83%
O
PF6- NH2 10 OH O
O O O O O O O
O O O
O O O O O
O N
H2 OH PF6-
10 +
CH2Cl2
O O O
TEA (100 eq.) (50 eq.)
THF 60 °C, 24 h
89%
O N 10
O O O
O O O O O O
O O
On O
O O
HN CF3
CF3 m
CH2Cl2 r.t., 12 h 84%
O
Cl Cl
O O O
TEA (100 eq.) (50 eq.)
THF 60 °C, 24 h
86%
NH2 10 O
O O O O O O O
O O
On O
O O
H N
CF3
CF3 m PF6-
O
O
O O O
O O O O O O
O HN
CF3
CF3 O
O O N
O O
O
n m
11
O DPP (110 mol%)
O O
O
PF6- NH2
10 O
O O O O O O O O
O O
OnH
O O
(excess) DPP (70 mol%)
OCN CF3
CF3 O
PF6- N H2
10 O
O O O O O O O O
O O
On O
O O
H N
CF3
CF3 m
N N Mes Mes
Ru PCy3Ph
(2500 mol%.)
CH2Cl2 r.t., 12 h 88%
(7000 mol%)
CH2Cl2 r.t., 12 h
CH2Cl2 r.t., 12 h 83%
O
PF6- N
H2 10 OH
O O O
O O O O O O
O O O
O O O O O O
O N
H2 OH PF6-
10 +
CH2Cl2
O O O
TEA (100 eq.) (50 eq.)
THF 60 °C, 24 h
89%
O N 10
O O O
O O O O O O
O O
On O
O O
H N
CF3
CF3 m
CH2Cl2 r.t., 12 h 84%
O
Cl Cl
O O O
TEA (100 eq.) (50 eq.)
THF 60 °C, 24 h
86%
N H2
10 O
O O O O O O O
O O
On O
O O
HN CF3
CF3 m PF6-
O
O
O O O
O O O O O O
O H N
CF3
CF3 O
O O N
O O
O
n m
11
O
1 2 [3] 4
6
5NAc
6NAc
DPP (110 mol%) O O
O
PF6- N H2
10 O
O O O O O O O O
O O
OnH
O O
(excess) DPP (70 mol%)
OCN CF3
CF3 O
PF6- NH2 10 O
O O O O O O O O
O O
On O
O O
HN CF3
CF3 m
N N Mes Mes
Ru PCy3Ph
(2500 mol%.)
CH2Cl2 r.t., 12 h 88%
(7000 mol%)
CH2Cl2 r.t., 12 h
CH2Cl2 r.t., 12 h 83%
O
PF6- N H2
OH 10 O
O O O O O O O O
O O O
O O O O O O
O N
H2 OH PF6-
10 +
CH2Cl2
O
O O
TEA (100 eq.) (50 eq.)
THF 60 °C, 24 h
89%
O N 10 O
O O O O O O O O
O O
On O
O O
HN CF3
CF3 m
CH2Cl2 r.t., 12 h 84%
O
Cl Cl
O
O O
TEA (100 eq.) (50 eq.)
THF 60 °C, 24 h
86%
NH2 10 O
O O O O O O O
O O
On O
O O
HN CF3
CF3 m PF6-
O
O
O O O
O O O O O O
O H N
CF3
CF3 O
O O N
O O
O
n m
11
O DPP (110 mol%)
O
PF6- NH2
10 O
O O O O O O O
O O
OnH
O O
(excess) DPP (70 mol%)
OCN CF3
CF3 O
PF6- NH2
10 O
O O O O O O O O
O O
On O
O O
HN CF3
CF3 m
N N Mes Mes
Ru PCy3Ph
(2500 mol%.)
CH2Cl2 r.t., 12 h 88%
(7000 mol%)
CH2Cl2 r.t., 12 h
CH2Cl2 r.t., 12 h 83%
O
PF6- NH2
OH 10 O
O O O O O O O O
O O O O O O O
O N
H2 OH PF6-
10 +
CH2Cl2
O
O O
TEA (100 eq.) (50 eq.)
THF 60 °C, 24 h
89%
O N 10
O O O
O O O O O O
O O
On O
O O
HN CF3
CF3 m
CH2Cl2 r.t., 12 h 84%
O
Cl Cl
O
O O
TEA (100 eq.)(50 eq.) THF 60 °C, 24 h
86%
NH2 10 O
O O O O O O O
O O
On O
O O
HN CF3
CF3 m PF6-
O
O
O O O
O O O O O O
O HN
CF3
CF3 O
O O N
O O
O
n m
11
O
5
100
80
60
40
20
0
26 25 24 23 22
100
80
60
40
20
0 100
80
60
40
20
0
26 25 24 23 22
100
80
60
40
20
0
elution time (min)
Mn 8,300 DPPCL 25, DPPHL 40
PDI 1.2 Mn 3,900 DPPCL 25 PDI 1.1
m/z
4
5
2775.50 (2775.75)
n = 16 2890.25 (2889.82)
n = 17 3004.24 (3003.89)
n = 18
2802.13 (2802.62)m+n =14
2916.83 (2916.69)m+n =15
3029.91 (3030.76)
m+n =16 Experimental(Theoretical)
114.75 113.99
114.70 113.08
5034.03 (5033.97) m+n = 33
5147.99 (5148.04) m+n = 34
5262.55 (5262.11) m+n = 35
5004.15 (5005.94)m+n = 33
5118.56 (5120.01) m+n = 34
5233.02 (5234.08) m+n = 35 113.96 114.56
29.53 29.43 29.88
m/z
O N 10
O O O
O O O O O O
O O
On O
O O
HN CF3
CF3 m O
O O O
O O O O O O
O HN
CF3
CF3 O
O O N
O O
O
n m
11
O
5NAc
6NAc
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COMMUNICATION
in all cases of cyclic polymers as compared to their linear analogues, G values give a number smaller than 1, indicating a decrease in hydrodynamic volume as expected for cyclic structures. Especially in the case of cyclic PCL, the value of 0.78 shows a very good agreement with the reported value of covalently linked cyclic PCL of 0.78[2g]. There is no report yet on the synthesis and G values calculation for PHL and PCL-b-PHL to compare with, but the larger values in both cases are probably due to the inherent nature of PHL in SEC column, or due to the non-covalent mechanical linkage of the cyclic polymers. This explanation seems reasonable because of the value in the middle of two homopolymers.
Table 1. Synthesis of linear (L) and cyclic (C) homopolymers and block copolymers
[a] SEC: Eluent CHCl3, and PSt standards. [b] Peak top value ratio of cyclic to linear polymer Mp,C/ Mp,L.
Figure 3. (a) SEC profiles of linear and cyclic block copolymers 5NAc and 6NAc, and (b) Simulated structures of linear and cyclic block copolymers 6 and 6NAc (prepared by OPLS2005, Macromodel 9.9)
The removal of the hydrogen bonding between the crown ether wheel and the ammonium moieties in 6 forced the wheel to move to the urethane moiety at the other end which could form a new hydrogen bonding in 6NAc. In accordance with it, the urethane NH proton NMR signal z downfield-shifted (Figure S3).
The strength of these hydrogen bonds can be altered by changing the structure of the terminal phenyl carbamate group as covered in our previous work[11b]. By introducing strongly electron withdrawing trifluoromethyl groups into the phenyl
terminal, the urethane and the wheel form strong enough interaction to render 6NAc mostly cyclic (Figure 3b).
The diffusion coefficient D of LBC and CBC was measured by DOSY NMR to obtain an additional evidence for the structural transformation (Figure S22-S25). In the previous reports, linear polymers present smaller D values than their cyclic counterparts, where the ratio DL/DC being about 0.85[14]. In the present case, the D values obtained for 6 and 6NAc were 2.21 x 10-10 m2/s and 2.57 x 10-10 m2/s respectively, giving rise to D6/D6NAc of 0.86, very similar to the reported value. Thus, the change in polymer topology clearly corresponded to the change in solution property in addition to the spectroscopic changes.
As mentioned above, sequential ROPs followed by close- proximity RCM reaction led to the synthesis of LBC having rotaxane terminus with high purity in high yield. These, combined with the 100% effective cyclization of LBC based on the wheel translation and fixation to the other terminus, made possible an effective synthesis of CBC.
Finally, the crystalline nature of PCL and the amorphous nature of PHL prompted an evaluation into the effect of topology on the thermal property of the linear and cyclic block copolymers.
As shown in the DSC thermograms (Figure 4), the linear polymer 5NAc exhibited both a glass transition peak of PHL and an endotherm melting peak of PCL. Interestingly, despite having a similar composition to 5NAc, the cyclic polymer 6NAc showed a higher melting point. This is probably due to the lowered entropy change for cyclic polymer, being in accordance with the Grayson’s report with cyclic polymer.[15]
Figure 4. DSC traces of linear and cyclic block copolymers 5NAc and 6NAc (heating rate: 20 °C/min, 2nd heating).
In summary, we have successfully developed the effective synthetic method of cyclic block copolymer from linear one that has proven the meaning of and enhanced the value of the rotaxane protocol. We could obtain more than 1 g of [1]rotaxane- type LBC 6 in the key cyclization reaction of 5 in a 100 mL flask making use of a selective intramolecular fashion. Since the final macrocyclization of 6 to CBC 6NAc does not depend on the concentration, large amount synthesis of CBC can be simply and easily attained. It is of significance that identical block copolymers with the only difference being their linear and cyclic structures showed a differing crystallization behavior.
Furthermore, it could readily attain reversible property change of CBC that accompanies reversible linear-cyclic topology change, as we have already reported reversible linear-cyclic topology change via the N-protection-deprotection protocol.[10] The present work will allow more accessibility to effective large-scale synthesis of various CBCs directed toward versatile applications.
Keywords: cyclic block copolymer・topology・rotaxane・
structural transformation・ring-opening polymerization Polymer Mn,NMR Mn,SECa
Mp,SECa
PDIa Gb PCL25-b-PHL40 (L) 5NAc 8,800 8,300 10,500 1.2
0.84 PCL25-b-PHL40 (C) 6NAc 8,800 7,600 8,800 1.2
PCL25 (L) 7NAc 4,200 3,900 4,400 1.2 0.78 PCL25 (C) 8NAc 4,200 3,400 3,400 1.2 PHL50 (L) 9NAc 7,100 6,800 7,700 1.2
0.91 PHL50 (C) 10NAc 7,100 6,000 7,000 1.2
6NAc 6
(a)
(b)
6NAc
elution time [min]
100
80
60
40
20
0
26 25 24 23 22
5NAc
Mp 8,800 Mn 7,600 PDI 1.2 Mp 10,500
Mn 8,300 PDI 1.2
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COMMUNICATION
Stephanie Valentina, Takahiro Ogawa, Kazuko Nakazono, Daisuke Aoki, Toshikazu Takata*
Page No. – Page No.
Efficient Synthesis of Cyclic Block Copolymer by Rotaxane Protocol via Linear-Cyclic Topology
Transformation
Linear to cyclic: A gram-scale synthesis of cyclic block copolymer was achieved by precise control on the movement of rotaxane components so as to change the polymer structure from linear to cyclic. This rotaxane protocol offers an alternative approach towards generating CBC in practical scales.
NH2 10 O
O O O O O O O
O O
On O
O O
H N
CF3
CF3 m PF6-
O
O
O O O
O O O O O O
O H N
CF3
CF3 O
O O N
O O
O
n m
11
O
Linear block copolymer Cyclic block copolymer movement of
rotaxane components PCL PHL
