One-Step Approach to Amino-Functionalized Semiaromatic

Article pubs.acs.org/Macromolecules

One-Step Approach to Amino-Functionalized Semiaromatic Polyamides: Modification and Cross-Linking Olga Maiatska,† Jürgen Omeis,‡ and Helmut Ritter*,† †

Institute of Organic Chemistry and Macromolecular Chemistry, Heinrich Heine University of Duesseldorf, Universitaetsstrasse 1, 40225 Duesseldorf, Germany ‡ BYK-Chemie GmbH, Abelstrasse 45, 46483 Wesel, Germany S Supporting Information *

ABSTRACT: The new method for the one-step synthesis of semiaromatic polyamides bearing primary aromatic amine groups in the repeating units is presented. Various aliphatic and aromatic diamines were used: 2,2′-(ethylenedioxy)bis(ethylamine) (3a), Jeffamine ED-600 (3b), 4,4′-oxidianiline (3c), and p-phenylenediamine (3d). They react with bis(N-carboxyanhydrides) of aromatic β-amino acid (N-unsubstituted bis(benzoxazine-2,4-diones)) yielding the corresponding semiaromatic polyamides (4a−4c). The obtained free amino groups were modified with 2-isocyanatoethyl methacrylate. These methacryl-functionalized polyamides can be cross-linked in the presence of N,N-dimethylarylamide via free radical polymerization.

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INTRODUCTION Synthetic aromatic and aliphatic polyamides belong to the most important category of high performance polymeric materials. They exhibit outstanding mechanical properties, thermal stability, and flame-retardant characteristics and therefore have received numerous applications, e.g., in textiles, membranes, ropes, and many automotive uses.1−3 Most important aliphatic polyamides are synthesized via ring-opening polymerization of caprolactam4−9 and via polycondensation of C6 diacid and C6 diamine.10 Aromatic and semiaromatic polyamides are obtained via polycondensation of aromatic diacid chlorides and aliphatic or aromatic diamines, respectively.11−16 A large number of new polyamide preparation, functionalization, and modifications have been reported.17−26 For example, new polyamides based on diphenylaminoisosorbide,27 aromatic poly(ether amide)s containing benzimidazole groups and ethylene oxide sequences,28 polyamides containing heteroatoms,11,29−33 or heterocyclic units in in the main chain34 are also described. Recently, we reported the synthesis of terminally modified cycloaliphatic polyamide-3 via ring-opening polymerization of the N-carboxyanhydride of β-amino acid (β-NCA) in the presence of different alcohols as initiators and end groups.35 Principally, the β-NCA function easily reacts with aliphatic amines under release of CO2 in very good yields; we were encouraged to use this type of reaction for the synthesis of novel types of polyamides. However, up to now, the use of aromatic bis(N-carboxyanhydrides) of aromatic β-amino acids (bis-β-NCA’s) and aliphatic or aromatic diamines for synthesis © XXXX American Chemical Society

of polyamides bearing the free aromatic amino groups has not yet been described so far in the literature. Thus, the direct synthesis of partially aromatic polyamides from aromatic bis-βNCA and aliphatic or aromatic diamines and the use of the obtained free aromatic amino groups for further modifications are the subject of the present investigation.

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RESULTS AND DISCUSSION The synthesis of bis(N-carboxyanhydrides) isomers 2a−2c was conducted using 4,4′-oxydiphthalic anhydride 1 and trimethylsilyl azide (Scheme 1). For this type of reaction, a general mechanism36 was postulated (Scheme 2). The reaction proceeds via Curtius rearrangement37 of acyl azide 1b to yield isolated isocyanate intermediate 1c. The IR spectrum indicates a strong absorption band of the isocyanate group at 2263 cm−1. The cyclization of isocyanate 1c to bis(N-carboxyanhydride) 2 takes place after adding ethanol, while the trimethylsilyl protected carboxylic group is hydrolyzed. The structures of isomers 2a−2c were confirmed by IR, 1H NMR spectroscopy, and mass spectrometry (Supporting Information). The 1H NMR spectrum consist of signals for aromatic and amide protons. Isomer ratios (2a:2b:2c = 0.5:1:0.5) were obtained by use of 1H NMR spectroscopy. Received: October 30, 2015 Revised: January 18, 2016

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DOI: 10.1021/acs.macromol.5b02368 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of Bis(N-carboxyanhydrides) 2a−2c

Scheme 2. Postulated Mechanism of Anhydride with Trimethylsilyl Azide

Scheme 3. Polymerization of Bis-β-NCA’s 2a−2c to Semiaromatic Polyamides 4a−4d

The chemical structures of 4a−4d were verified by 1H NMR and IR spectroscopy. The 1H NMR spectrum of isomers 2a−2c (Supporting Information) shows 11 peaks for aromatic protons and one multiplet for −NH− protons of cyclic anhydrides (δ = 11.6−11.77 ppm), whereas polymers 4a−4d exhibit broad peaks for aromatic protons, a multiplet for amide protons (δ = 9.65−10.17 ppm) and no trace of −NH− protons of cyclic

The polycondensation of bis-β-NCA’s 2a−2c (Scheme 3) was accomplished by using of 2,2′-(ethylenedioxy)bis(ethylamine) (3a), Jeffamine ED-600 (3b), 4,4′-oxydianiline (3c), and p-phenylenediamine (3d). During the polymerization, a release of carbon dioxide was observed. The obtained polymers 4a−4d were soluble in DMF and DMSO. B

DOI: 10.1021/acs.macromol.5b02368 Macromolecules XXXX, XXX, XXX−XXX

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anhydrides The infrared spectra of monomers 2a−2c and polymer 4c are compared in Figure 1. The CO bond stretching at 1747 and 1693 cm−1 is clearly observed for 2, whereas polymer 4c exhibits a CO absorption at 1650 (amide I) and 1528 cm−1 (amide II). Molecular weights of the polymers 4a−4d were determined by use of GPC (Table 1). The polymers exhibit dispersity between 1.5 and 2.1, which is typical for condensation polymerization in lower molecular weight region. The thermal properties of polyamides 4a−4d were evaluated using differential scanning calorimetry (DSC) measurements (Table 1). Because of the oxidianiline units, 4c exhibits a higher glass transition than the polymers 4a and 4b. Unfortunately, the Tg value of the fully aromatic polyamide 4d could not be detected using standard DSC. In order to demonstrate the reactivity of free aromatic amino group of polyamide 4c, addition reaction with 2-isocyanatoethyl methacrylate has been carried out (Scheme 4), followed by free radical curing of the compound 5c to yield cross-linked polyamide (Scheme 5). The structure of 5c was confirmed by 1H NMR spectroscopy. The 1H NMR spectrum of 5c (Figure 2) consist of multiplets for aromatic and amide protons (δ = 10.4, 9.8, 9.3, 8.2−6.7 ppm), two multiplets for protons of the double bond (δ = 6.1, 5.6 ppm), two multiplets for methylene protons (δ = 4.1, 3.3 ppm), and one multiplet for methyl protons (δ = 1.8 ppm). The cross-linking of 5c in the presence of N,N-dimethylacrylamide comonomer and AIBN as radical initiator (Scheme 5) was investigated through rheological measurements in oscillatory mode at 62 °C. Since the methacryl ester function and N,N-dimethylacrylamide have a similar reactivity, it can be expected that a statistical cross-linked structure is obtained. Furthermore, the polymer 5c can be solved in 6 molar excess of N,N-dimethylacrylamide. A nearly quantitative conversion of methacryl groups is assumed. After 20 min of the initiation phase, a rapid increase of storage and loss modulus values was observed. After about 10 min, a polymer layer was cured (Figure 2, Supporting Information). The storage modulus G′ of 6 × 106 Pa was recorded at a constant shear rate 10 rad/s and a shear deformation γ of 0.1%. During this period most of the network points were formed. The thin disk obtained was mostly transparent and slightly brittle (Figure 3).

Figure 1. IR spectrum of 2 and polyamide 4c.

Table 1. Characterization of Polymers 4a−4d sample

Mwa [g mol−1]

Db

Tgc [°C]

4a 4b 4c 4d

16000 27000 11700 7000

1.6 2.1 1.7 1.5

97 −3 197 -*

a

Determined by GPC (DMF) with polystyrene standards. bD = Mw/ Mn. cDetermined by DSC (heating rate: 10 K min−1). *Could not be detected

Scheme 4. Reaction of Polyamide 4c with 2-Isocyanatoethyl Methacrylate

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CONCLUSION

Aromatic polyamides are of great practical importance. They find applications e.g. in high performance fibers and films. It can be concluded from the results described above that bis(benzoxazine-2,4-diones) as reactive educts is a promising example for the access to new types of functionalized semiaromatic polyamides under mild conditions. It was Scheme 5. Cross-Linking of Polyamide 5c in the Presence of N,N-Dimethylacrylamide in the Presence of Free Radical Initiator AIBN

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DOI: 10.1021/acs.macromol.5b02368 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. 1H NMR spectrum of polymer 5c.

Notes

The authors declare no competing financial interest.

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Figure 3. Photograph of the obtained cross-linked sample.

demonstrated that various aliphatic and aromatic diamine are suitable to create corresponding linear polyamides bearing free amino groups under release of CO2. The free amino groups are good candidates for further modifications. It was shown that aliphatic isocyanate derivatives like 2-isocyanatoethyl methacrylate fully react with these amino groups. This reaction can be used e.g. to prepare cross-linked materials. A potential application like membrane technology is the subject of further investigations.

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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.5b02368. Description of the syntheses and spectroscopic data of the obtained compounds (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Fax (+49) 211 8115840; e-mail [email protected] (H.R.). D

DOI: 10.1021/acs.macromol.5b02368 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.5b02368 Macromolecules XXXX, XXX, XXX−XXX