Sequence-Regulated Polymers via Combination of Orthogonal

Chapter 15

Downloaded by STANFORD UNIV on February 18, 2015 | http://pubs.acs.org Publication Date (Web): September 22, 2014 | doi: 10.1021/bk-2014-1170.ch015

Sequence-Regulated Polymers via Combination of Orthogonal Passerini Three-Component Reaction and Thiol-ene Reaction Jian Zhang, Xin-Xing Deng, Fu-Sheng Du, and Zi-Chen Li* Key Laboratory of Polymer Chemistry & Physics of Ministry of Education, Department of Polymer Science & Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China *E-mail: [email protected]

The present work describes a facile approach to sequenceregulated polymers via a combination of orthogonal Passerini three-component reaction and thiol-ene reaction. The first step was the synthesis of an α,ω-diene compound via the Passerini three-component reaction of 1, 6-diisocyanohexane with 10-undecylenic acid and a monoaldehyde. In the second step, thiol-ene reaction of this diene compound with 3-mercaptopropionic acid converted the dienes to dicarboxylic acids. Finally, these dicarboxylic acid monomers were copolymerized with 1, 6-diisocyanohexane and another monoaldehyde to get polymers, each repeating unit of which contains four different side groups in an AABB sequence.

Introduction The past two decades have witnessed a great progress in precise synthesis of polymers with controlled molecular weights, end groups and defined complex architectures (1, 2). This was largely enabled by the development of controlled living radical polymerization and orthogonal click reactions (1, 2). One of the remaining challenges for polymer chemists is to develop simple synthetic methods for polymers with well-defined monomer sequence (3–6). It is expected that these polymers will exhibit unique properties and functions as biomacromolecules do, for example chain folding and catalytic properties of proteins (3–6). A number of techniques have been developed to address this © 2014 American Chemical Society In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


challenge. Sequence-defined oligomers by the classical Merrifield solid-phase synthetic method is perhaps the most well-established approach (7). Recently, this approach has been extended to the simple synthesis of other types of sequence-defined oligomers by using orthogonal efficient organic reactions (8–11). Step-wise polymerization of monomers with encoded monomer sequence is another approach to get sequence-defined copolymers, but the sequence is only limited to periodic microstructure (12–14). Chain polymerization of special monomer pairs can produce sequence-regulated polymers in a more simple way (15–22). Controlled polymerization can tune the side groups in a programmed manner along a defined polymer chain. In our previous paper, we developed a facile synthetic method that can simultaneously construct the backbone and side group sequence of segmented multi-block copolymers (23). The technique involves the polymer supported liquid phase synthesis of PEG diacid macromonomers via stepwise Passerini three-component reaction (P-3CR) with tert-butyl isocyanoacetate and a functional aldehyde followed by selective hydrolysis, and the final multicomponent polymerization of these diacid monomers with phenylacetaldehyde and 1, 6-diisocyanohexane. One drawback of the procedure is that the carboxylic groups for P-3CR and the final multi-component polymerization needs to be protected and deprotected, thus making the synthesis not so straightforward. In this report, we developed a more straightforward approach to sequence-regulated polymers via combination of orthogonal P-3CR and thiol-ene reaction. As shown in Scheme 1, starting from 1, 6-diisocyanohexane, the P-3CR of which with an aldehyde and 10-undecylenic acid leads to an α, ω-diene with two side chain groups originated from the aldehyde. Then, the dienes were transformed into dicarboxylic acids by the photo-catalyzed thiol-ene reactions with 3-mercaptopropionic acid. This step does not add more side chains, but increase the chain length with end carboxylic acid groups which can be used for another circle of P-3CR and thiol-ene reaction to add more side groups. P-3CR polymerization of these dicarboxylic acids with 1, 6-diisocyanohexane and another monoaldehyde generates final polymers with four side groups in a defined AABB sequence. Noteworthy, in the course of our work, Meier et al reported a similar approach for the synthesis of sequence-defined tetramer and block copolymer via iterative application of the P-3CR and the thiol-ene reaction (11).

Experimental Materials 1, 6-Diisocyanohexane (Sigma-Aldrich; >98%), 10-undecylenic acid (Alfa Aesar; 98%), 3-mercaptopropionic acid (Alfa Aesar; 99%), 2, 2-dimethoxy-2-phenylacetophenone (DMPA, Alfa Aesar; >98%), and o-nitrobenzaldehyde (Beijing Chem. Works) were used as received. Phenylacetaldehyde (Alfa Aesar; >95%) and 2-methylpropanal (Beijing Chem. Works) and all the solvents were redistilled before use. 224 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Scheme 1. Synthesis of sequence-regulated polymers by combination of Passerini reaction, thiol-ene reaction and Passerini polymerization. Measurements Average molecular weights and the polydispersities (PDI) of the polymers were measured by gel permeation chromatography (GPC). The GPC instrument was equipped with a Waters 1525 binary HPLC pump, a Waters 2414 refractive index detector, and three Waters Styragel HT columns (HT2, HT3, HT4) thermostated at 35 °C. THF was used as the eluent at a flow rate of 1.0 mL/min for a total time of 36 minutes. Calibration was made against linear polystyrene standards. The obtained data were processed on professional software. 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were obtained on a Bruker Avance-400 spectrometer. NMR chemical shifts are reported in ppm with tetramethylsilane (TMS) as the internal reference. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra were obtained on a Bruker Autoflex III mass spectrometer equipped with a 355 nm nitrogen laser. α-Cyano-4-hydroxycinnamic acid was used as the matrix and reflective and linear positive ion modes were used. Synthesis of Diene Compounds A1 and A2 To a 25 mL round bottom flask containing 10-undecylenic acid (2.208 g, 12 mmol) in THF or CH2Cl2 (2 mL) was added 1, 6-diisocyanohexane (0.680 g, 5 mmol) and phenylacetaldehyde (1.440g, 12 mmol). The mixture was stirred at 30°C for 24 h. The reaction mixture was then concentrated by removing the solvent under reduced pressure, the yellow viscous residue was purified by column 225 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


chromatography (EtOAc: petroleum ether, 1:5). Compound A1 was obtained in 85% yield as a white solid. 1H NMR (400 MHz, CDCl3), δ (TMS, ppm): 7.15-7.29 (m, 10H), 5.95 (s, 2H), 5.81 (m, 2H), 5.37 (t, 2H), 4.96 (m, 4H), 3.08-3.25 (m, 8H), 2.31 (m, 4H), 2.04 (q, 4H), 1.55 (m, 4H), 1.15-1.41 (m, 28H). Compound A2 was synthesized in a similar way except that 2-methylpropanal (0.865g, 12 mmol) was used instead of phenylacetaldehyde. This compound was obtained as a white solid in 87% yield after column purification. 1H NMR (400 MHz, CDCl3), δ (TMS, ppm): 6.05 (s, 2H), 5.81 (m, 2H), 5.05 (m, 2H), 4.96 (m, 4H), 3.26 (m, 4H), 2.42 (t, 4H), 2.30 (m, 2H), 2.04 (q, 4H), 1.66 (m, 4H), 1.50 (m, 4H), 1.25-1.41 (m, 24H), 0.94 (t, 12H). Synthesis of Monomers B1 and B2 To a 25 mL round bottom flask containing compound A1 (2.232g, 3 mmol) in THF (1 mL) was added 3-mercaptopropionic acid (3.184g, 30 mmol) and DMPA (0.0308g, 0.12 mmol). The mixture was exposed to a UV lamp (365nm) for 3 h. After that, the reaction mixture was concentrated by removing the solvent under reduced pressure, the brown residue was purified by gradient column chromatography (EtOAc: petroleum ether: acetic acid, from 1:7:0.45 to 1:2:0.17). Monomer B1 was obtained as a yellow viscous liquid in 94% yield. 1H NMR (400 MHz, CDCl3), δ (TMS, ppm): 7.15-7.28 (m, 10H), 6.09 (s, 2H), 5.38 (s, 2H), 3.08-3.25 (m, 8H), 2.78 (t, 4H), 2.65 (t, 4H), 2.53 (t, 4H), 2.32 (m, 4H), 2.10 (s, HOAc), 1.50-1.62 (m, 4H), 1.50 (m, 4H), 1.25-1.41 (m, 24H), 0.94 (t, 12H). MALDI-TOF-MS: M/z calcd for C52H80N2O10S2 [M+Na]+, 979.53; Found [M+Na]+, 979.5. Monomer B2 was synthesized in a similar way except that compound A2 (1.944g, 3 mmol) was used instead of compound A1. This compound was obtained in 86% yield as a pale yellow viscous liquid. 1H NMR (400 MHz, CDCl3), δ (TMS, ppm): 6.14 (s, 2H), 5.05 (m, 2H), 3.27 (m, 4H), 2.79(t, 4H), 2.65 (t, 4H), 2.54 (t, 4H), 2.42 (t, 4H), 2.30 (m, 2H), 2.10 (s, HOAc), 1.46-1.71 (m, 12H), 6.98, 1.43 (s, BHT), 1.25-1.41 (m, 28H), 0.94 (t, 12H). MALDI-TOF-MS: M/z calcd for C44H80N2O10S2 [M+Na]+, 883.53; Found [M+Na]+, 883.6. Passerini Polymerization and Synthesis of Polymers To a 10 mL round bottom flask containing monomer B1 (0.287 g, 0.3 mmol) in CH2Cl2 (3.3 mL) was added 1, 6-diisocyanohexane (0.0408 g, 0.3 mmol) and o-nitrobenzaldehyde (0.0997 g, 6.6 mmol). The mixture was stirred at 30°C for 48 h. and then was precipitated into cold diethyl ether (30 mL). This procedure was repeated for two more times to get a yellow solid, which was further vacuum dried to get polymer P1 in 65% yield. Polymer P2 was synthesized in a similar way by using monomer B1 (0.287 g, 0.3 mmol), 1, 6-diisocyanohexane (0.0408 g, 0.3 mmol) and 2-methylpropanal (0.0476 g, 6.6 mmol). After three precipitations in diethyl ether and vacuum dry, a yellow solid was obtained in 65% yield. Similarly, Polymer P3 was synthesized by using monomer B2 (0.258 g, 0.3 mmol), 1, 6-diisocyanohexane (0.0408 g, 0.3 mmol) and phenylacetaldehyde 226 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

(0.0792 g, 6.6 mmol). After three precipitations in diethyl ether and vacuum dry, a yellow solid was obtained in 67% yield.

Results and Discussion


Synthesis of Two α, ω-Diene Compounds by P-3CR Reaction P-3CR reaction of an isocyanide with an aldehyde and a carboxylic acid forms an α-acyloxy carboxamide (24). This reaction can tolerate many functional groups like alkyne, alkene or azide groups. This reaction was recently developed as a polymerization method to prepare polyesters, polyamides and poly(ester-amide)s by using two components as bifunctional compounds (25–27). In our previous report, we used a polymer supported dicarboxylic acid for the preparation of sequence-regulated macromonomers via consecutive P-3CR with a protected isocyanide (23). Though separation is easy, it needs deprotection to liberate the terminal carboxylic acid for further P-3CR. To overcome this limit, we selected the thiol-ene reaction to transfer the terminal alkylene groups into carboxylic acids (28, 29). First, we prepared compound A1 by the P-3CR of 10-undecylenic acid, 1, 6-diisocyanohexane and phenylacetaldehyde at room temperature. The reaction was followed by NMR, and after 12 h, all the 1, 6-diisocyanohexane had been converted. After column purification, compound A1 was obtained as a white solid in 90% yield. It has two side chain benzyl groups originated from phenylacetaldehyde. The 1H NMR spectrum of compound A1 is shown in Figure 1. All the expected proton signals can be well resolved, and the integration ratio of these peaks also confirmed the integrity of the expected structure. Alternatively, compound A2 was obtained in a similar way by the P-3CR of 10-undecylenic acid, 1, 6-diisocyanohexane and 2- methylpropanal. The 1H NMR spectrum of compound A2 (Figure 1B) also confirmed the expected structure. This compound has two isopropyl side groups from 2-methylpropanal. Synthesis of Two Monomers by Thiol-ene Reaction The thiol-ene reaction is an efficient click reaction that has been widely used in polymer science (28, 29). First, we carried out the reaction of compound A1 and 3-mercaptopropionic acid with DMPA as an UV initiator. To ensure that all the terminal alkylene groups in compound A1 can be transferred, 3-mercaptopropionic acid was used in a 1.5-fold excess. In addition, low concentration of photo-initiator (4 mol% of compound A1) was used to suppress any side reactions (30). The final product was separated by column chromatography; however, due to the existence of two carboxylic acid groups, the absorption of this compound on silica column is very strong which makes the separation very difficult. Finally, by using a mixture of eluent and also change the composition of the eluent gradually, we successfully got the desired compound. The 1H NMR spectrum of monomer B1 is shown in Figure 2, it can be seen that the terminal alkylene protons at δ=5.81 and 5.96 ppm in compound A1 disappeared completely, indicating the full conversion of the alkylene groups. Meanwhile, other expected signals were all clearly resolved and the integration ratio was also in accordance with the expected structure. 227 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Figure 1. 1H NMR spectra of compounds A1 and A2 in CDCl3

Figure 2. 1H NMR spectrum of monomer B1 in CDCl3. 228 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Monomer B2 was obtained in a similar way and the NMR spectrum shown in Figure 3 confirmed the structure.

Figure 3. 1H NMR spectrum of monomer B2 in CDCl3.

P-3CR Polymerization of Monomers With the two monomers in hand, we studied the Passerini multi-component polymerization (MCP) of the two diacid monomers with 1, 6-diisocyanohexane in the presence of different monoaldehydes. As already revealed in our previous paper (26), the Passerini MCP is a step-wise polymerization, the two difunctional monomers should be in a 1:1 stoichiometry, but the monoaldehyde component can be in an excess to both increase the polymerization rate and the molecular weights of the final polymers. Other conditions, like solvents and monomer concentrations may also affect the polymerization. Therefore, we studied in detail the Passerini MCP of monomer B1, with 1, 6-diisocyanohexane and o-nitrophenyl benzaldehyde. Initially, the polymerization was carried out in CH2Cl2, and the concentration of monomer B1 was 0.5 M, 1.0 M, and 2.0 M respectively (Table 1, entry 1, 2, 5). The crude GPC traces of the three polymerizations are shown in Figure 4. It can be concluded that when the polymerization was conducted at 1.0 M, polymers with the highest molecular weights can be obtained. This is reasonable, because at higher monomer concentration, the viscosity of the polymerization system is rather high, thus limiting the final conversion of the end groups. The polymerizations at 1.0 M were also conducted in THF and CHCl3 (Table 1, entry 3, 4), the molecular weights of the polymers obtained in these two solvents were all lower than that in CH2Cl2. These results were all in accordance with our previous report, thus further confirming that the Passerini MCP should be carried out in less polar solvent at an appropriate monomer concentration to afford high molecular weight polymers. From the GPC traces (Shown in Figure 4) , we could 229 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

see the existence of oligomers; this is also the feature of step-wise polymerization. Nevertheless, after three precipitations in ethyl ether, polymer P1 was recovered as a white solid in 66% yield due to the removal of oligomers. Polymer yields for these polymerizations are quite similar. The highest molecular weight of polymer P1 was 16.5 kDa (Table 1, entry 2).


Table 1. Synthesis of Sequence Regulated Polymersa Entry

Polymer

Conc. /Mb

Solvent

Yield/%d

Mn/kDac

PDIc

1

P1

0.5

CH2Cl2

66

10.1

1.59

2

P1

1.0

CH2Cl2

64

16.5

1.75

3

P1

1.0

CHCl3

63

9.2

1.56

4

P1

1.0

THF

62

8.2

1.50

5

P1

2.0

CH2Cl2

65

9.1

1.59

6

P2

1.0

CH2Cl2

68

12.6

1.61

7

P3

1.0

CH2Cl2

58

15.5

1.66

a

At 30 °C in CH2Cl2 for 48h; b the concentration of monomer B1 or B2. Concentration of 1, 6-diisocyanohexane was equal to the monomer, while that of the aldehyde was 2.4 times in molar ratio. c Molecular weights and PDIs were measured by GPC with THF as eluent and calibrated with PS standards. d Yields were determined after precipitation and vacuum dryness.

Figure 4. GPC traces for the Passerini MCP of monomer B1 with 1, 6-diisocyanohexane and o-nitrophynyl aldehyde.

230 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


One polymer sample P1 was characterized by 1H NMR (Figure 5). The peak g is a newly formed peak after polymerization. The integration ratio of this peak with peak i (one of the characteristic of monomer B1) is 1.01:1.00, demonstrating the integrity of the repeating unit of the expected polymer. Thus, this polymer P1 contains an AABB side group sequence in the repeating units, where A is a benzyl group and B is an o-nitrophenyl group.

Figure 5. 1H NMR spectrum of polymer P1 in CDCl3

To regulate the side groups of polymers, monomer B1 was polymerized with 1, 6-diisocyanohexane and 2-methylpropanal to get another polymer P2 in a similar way. This polymer was obtained in 68% yield with a molecular weight of 12.6 kDa (Table 1, entry 6). The 1H NMR spectrum of polymer P2 is shown in Figure 6. Again, the newly formed characteristic peak (peak e) was clearly resolved, and the integration ratio of this peak to the characteristic peak in the monomer (peak d) was 1.00:1.02, demonstrating the integrity of the structure. Thus, this polymer P2 contains an AABB side group sequence in the repeating units, where A is a benzyl group and B is an isopropyl group. Furthermore, by polymerizing monomer B2 with 1, 6-diisocyanohexane and phenylacetaldehyde, we obtained polymer P3 in 58% yield with an average molecular weight of 15.5 kDa (Table 1, entry 7). The 1H NMR spectrum of polymer P3 is shown in Figure 7. It contains an AABB side group sequence in the repeating units, where A is a benzyl group and B is an isopropyl group.



Figure 6. 1H NMR spectrum of polymer P2 in CDCl3.

Figure 7. 1H NMR spectrum of polymer P3 in CDCl3.

Conclusions We demonstrated that orthogonal Passerini three-component reaction, thiolene reaction and Passerini MCP can be a facile synthetic strategy toward polymers containing poly(ester-amide) segments with ordered side group sequence. The Passerini three-component reaction was used to introduce side chains, the thiol-ene reaction was used to extend the main chain and transfer the dienes to dicarboxylic acid groups. The final Passerini MCP was used to synthesize a polymer, and at 232 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

the same time add two more side groups in a controlled way. In principle, this method can be extended to sequence-regulated polymers with more side groups via repeating the Passerini three-component reaction and the thiol-ene reaction.

Acknowledgments


This work was supported in part by the National Natural Science Foundation of China (No. 21090351 and 21225416).

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Sequence-Regulated Polymers via Combination of Orthogonal

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