Pablo
Wessig
*a,
Maik
Gerngroß
a,
Simon
Pape
a,
Philipp
Bruhns
a and
Jens
Weber‡
b
aInstitut für Chemie, Universität Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam, Germany. E-mail: wessig@uni-potsdam.de; Fax: +49-331977-5065; Tel: +49-331977-5401
bDepartment of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, Research Campus Golm, D-14476 Potsdam, Germany
First published on 3rd July 2014
New porous materials based on covalently connected monomers are presented. The key step of the synthesis is an acetalisation reaction. In previous years we used acetalisation reactions extensively to build up various molecular rods. Based on this approach, investigations towards porous polymeric materials were conducted by us. Here we wish to present the results of these studies in the synthesis of 1D polyacetals and porous 3D polyacetals. By scrambling experiments with 1D acetals we could prove that exchange reactions occur between different building blocks (evidenced by MALDI-TOF mass spectrometry). Based on these results we synthesized porous 3D polyacetals under the same mild conditions.
A common feature of all microporous polymers is the necessity of highly rigid strands and chains in order to prevent pore collapse. To ensure high microporosity in the final materials they need to feature either regular sites of contortion (such as a spirocenter or a tetraphenyl methane building block) to prevent dense chain packing or they need to be synthesized under well- controlled phase separation conditions (porogenic solvent) in order to fix the loose structure instead of forming dense polymer phases.
In the geometric sense, many of the above mentioned organic frameworks could be imagined to be composed of more or less long stiff molecular rods. Such molecules are characterized by a high aspect ratio and conformational rigidity and are the object of intense research efforts. Numerous applications both in life- and material-sciences were indeed already developed, which were summarized in some reviews.15 Basically, molecular rods may be subdivided in conjugated and non-conjugated rods. Whereas the former are important for organic electronics, the latter are predestined for biological and biochemical applications. Some years ago we developed a new class of non-conjugated molecular rods whose key elements are ketal structures. We therefore called these rods oligospiroketal (OSK) rods.16–22 The general structure of OSK rods (A) is depicted in Fig. 1. The backbone of the rods consists of spirocyclically joined 1,3-dioxane and cyclohexane rings, which are formed by ketalization of cyclohexan-1,4-dione E (or synthetic equivalents) with pentaerythritol B. Terminal functionalities are introduced by piperidine-4-one building blocks D. Furthermore, solubility enhancing building blocks (so-called “sleeves”, C) are often used due to the scarce solubility of longer OSK rods (Fig. 1).17,20 Meanwhile, we developed a series of mainly biochemical applications of OSK rods.18,19,21,22
Building on the experience with OSK rods and their structural similarity to the basic requirements of microporous polymers, we report here on some novel porous materials with polyacetals (PA) structure whose common structural element is the 2,4,8,10-tetraoxaspiro[5.5]undecane (TOSU) skeleton. Therefore we call these structures TOSU-PA. The synthesis of the rods is analyzed in detail before the synthesis of microporous structures and their characterization by means of gas adsorption is discussed.
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Scheme 1 Synthesis of TOSU-polyacetals and structure of building blocks 1a–e (the TOSU skeleton is marked blue in the formula of TOSU-PA). |
The next lower peaks (marked with ●) have a difference of 118 Dalton relative to the main peaks and can be explained by replacement of one of the formyl groups in TOSU-PA-a-A by a 5,5-bis-hydroxymethyl-1,3-dioxan-2-yl (BHMD) group (TOSU-PA-a-B). An analogous composition is observed in the case pentyl-substituted polymer TOSU-PA-b. The dependency of the degree of polymerization on the concentration of the monomers and/or the catalyst was analyzed subsequently. All synthesized polymers TOSU-PA-a, b, sc (sc = scrambling, Scheme 3) were analyzed by MALDI-TOF-MS. Minimum influence of the monomer concentration on the degree of polymerization was found (Fig. 3). Monomer concentrations ranging from 6.67 mmol L−1 up to 40 mmol L−1 of monomer 1a yielded rather similar MALDI-TOF-MS spectra. Furthermore, we varied the concentration of the catalyst TMS-OTf and observed once more no significant influence on the degree of polymerization (see ESI†). We also investigated the monomer molecular ratio. But contrary to expectations that polycondesations gave the best turnover to high molecular weights at a monomer molar ratio of 1:
1 we determined for our soluble systems that a monomer molar ratio of 1a/b
:
2 = 1
:
1.5 gave the highest molecular weights, i.e. largest number of repeating units according to MALDI-TOF analysis.
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Fig. 3 MALDI-TOF MS spectra demonstrating the influence on the monomer concentration of polyacetal TOSU-PA-a. |
The acetalization reaction is basically a reversible reaction. Evidence came from a scrambling experiment, in which preformed OSKs having different alkyl side chain length (TOSU-PA-a and TOSU-PA-b) were reacted with 2 and the corresponding monomer 1b or 1a under Noyori conditions. If the reaction is indeed reversible, the final product should have both butyl and pentyl substituted units as repeating units in a single chain. This was indeed found and evidenced by MALDI-TOF-MS (see Fig. 4).
Fig. 4 shows a part of the resulting MALDI-TOF-MS spectrum around repeating units with n = 4 and n = 5. The significant new peaks around the main peak of TOSU-PA-a with a mass difference of 28 Dalton are the evidence of the exchange of the given butyl side chains in TOSU-PA-a with pentyl side chains of monomer 1b.
Fig. 5 shows the N2 (77.4 K) and CO2 (273 K) adsorption/desorption isotherms of TOSU-PA-c and TOSU-PA-d materials synthesized under varying conditions.
A first question relates to the used acetalization conditions. Fig. 5a shows clearly, that only materials synthesized under Noyori conditions (using DCM as solvent in first instance) yield porous materials. Classic conditions (i.e. refluxing benzene under continuous water removal and catalysis by 4-toluenesulfonic acid, pTSA) did not result in micro/mesoporous materials. This is in line with a report of Han et al., who could show that rather harsh conditions were needed to obtain microporous polyketals based on pTSA catalysis (180 °C, 3d, vacuum, closed flask).26
A closer look on the N2 adsorption/desorption isotherms shows that TOSU-PA-c shows some significant mesoporosity (indicated by the very steep volume increase at p/p0 ∼ 0.8 and the associated hysteresis) next to pronounced microporosity, which is evident from the large volume uptake at low relative pressures but also from more detailed isotherm analysis of this particular material (see ESI†). TOSU-PA-d does not show signs of mesoporosity. At this stage, we will not discuss the mesoporosity in detail as it seems to originate from the finer details of the phase separation, which might vary from batch to batch. Instead, we wish to discuss the microporosity in more detail, which is expected to originate from the rigid molecular structure of the used monomers and the linking chemistry (although phase separation might influence the final microporosity to some extent as well).
Associated with the microporosity of the materials, a non-closing hysteresis loop (i.e. adsorption and desorption branch do not unify even at very low relative pressure) is observed, which is a characteristic feature of many microporous polymers.27 The hysteresis was discussed in terms of the micropore connectivity and the softness of the materials and it was shown that CO2 adsorption at 273 K is another (more) useful tool for the analysis of microporous polymers. The materials were hence analysed by CO2 adsorption/desorption (see Fig. 5b for examples) as well and the impact of changed synthesis conditions was analysed as well.
In a first experiment, the CO2 adsorption capacity of TOSU-PA-c and TOSU-PA-d synthesized under Noyori conditions using either DCM or 1,2-dichlorobenzene (1,2-DCB) as solvent, was investigated (Fig. 5b). The results for DCM based systems are in line with the trend obtained by N2 physisorption at 77.4 K, i.e.TOSU-PA-c (DCM) and TOSU-PA-d (DCM) show high microporosity with the triformyl benzene based system having a higher porosity (CO2 uptake of 2.85 mmol g−1vs. 1.85 mmol g−1 at 273 K and 1 bar). The use of 1,2-dichlorobenzene (1,2-DCB) resulted finally in materials with lowered porosity. Interestingly, the porosity was quite comparable for both materials (TOSU-PA-c and TOSU-PA-d) in this case. This could relate to the fact that upon synthesis in 1,2-DCB no solid product precipitated from the reaction mixture (in contrast to the use of DCM), but just upon addition of ethanol. The materials obtained by this method could however not be dissolved after drying under high vacuum (see ESI† for details). The fact that no precipitate was formed initially indicates however a rather incomplete reaction, which in turn provides an explanation for the lowered porosity.28 It should be mentioned that 1,4-dichloro benzene (1,4-DCB) can basically also be used as solvent that allows the synthesis of microporous TOSU-PA-c. Its use under Noyori conditions is however not practically useful. This is due to its solid nature, which results in rather heterogenous conditions. 1,4-DCB was hence not investigated any further.
Finally, monomer 1e was used to produce TOSU-PA-e. 1e is basically a somewhat less reactive, but more readily available keto-based monomer. Reaction in 1,2-DCB yielded a moderately microporous polymer, while reaction in DCM led surprisingly to a lowered porosity (see ESI† for isotherm data) This might be related to the different solubility of the keto monomer 1e and oligomers build from it, compared to the formyl based monomer 1d. Table 1 summarizes the CO2 uptake for all materials as well as the respective surface areas and pore volume data obtained by analysis of the CO2 data by GCMC methodology,29 or by analysis of the N2 data by QSDFT analysis.30
Material | S BET a/m2 g−1 | V pore a/cm3 g−1 | CO2 capacityb/mmol g−1 |
---|---|---|---|
a From N2 data at 77.4 K, pore volume determined from uptake at p/p0 = 0.95. b At 273.15 K and 1 bar. | |||
TOSU-PA-1c (DCM) | 310 | 0.340 | 2.81 |
TOSU-PA-1c (1,2-DCB) | n.d. | n.d. | 1.52 |
TOSU-PA-1c (pTSA) | 22 | n.d. | n.d. |
TOSU-PA-1d (DCM) | 233 | 0.128 | 1.85 |
TOSU-PA-1d (1,2-DCB) | n.d. | n.d. | 1.47 |
TOSU-PA-1d (pTSA) | 25 | n.d | 0.67 |
TOSU-PA-1e (DCM) | 14 | n.d. | 0.71 |
TOSU-PA-1e(1,2-DCB) | 38 | n.d | 1.38 |
The structures of prepared TOSU-PAs were confirmed by FT-IR spectroscopy. In all spectra of the materials, bands that can be attributed to the typical bands of the monomers e.g. carbonyl stretching of aldehyde and acetyl monomers at 1690 cm−1 and the C–H stretching at 2950 cm−1 (Fig. S6–S10, ESI†) are observed. The new C–O–C–O–C stretching vibration at 1160 cm−1 indicates a successful acetalization.
Finally, thermal analysis of selected materials indicated a reasonable stability (see ESI†). The onset of decomposition under air was found at temperatures larger than 200 °C. Decomposition took place in a number of distinct steps, which can be related to the chemical diversity of aliphatic and aromatic units, connected by heteroatoms. Full decomposition was usually observed under air atmosphere at temperatures of ∼650 °C, indicating that no inorganic residues are contained within the materials. The thermal stability under nitrogen atmosphere was tested for a single material (TOSU-PA-e) as well. The onset of decomposition was again found at T > 250 °C, which is not very much different compared to decomposition under air, indicating that the initial steps are thermolytic cleavage of covalent bonds. Again, multiple further decomposition steps were found, however the material cannot be decomposed completely under N2 atmosphere and 32% residue (most probably carbon materials) are found at T ∼ 1000 °C.
It should be noted that the obtained TOSU-PAs are totally amorphous, concluded from Wide-Angle X-ray Scattering (WAXS) measurements (see ESI†).
The same procedure is followed for the preparation of TOSU-PA-b.
The same reaction conditions in 1,2-DCB as solvent did not lead to a solid, but with the addition of ethanol after reaction there could be obtained a gray precipitate which was filtered and washed with sodium bicarbonate, water, ethyl acetate, acetone, dichloromethane and dried in high vacuum.
The same procedure is followed for the preparation of TOSU-PA-d–e.
Fourier-Transform Infrared (FTIR) spectroscopy was conducted using a Perkin Elmer UATR Two machine with single reflection diamond. Wide-Angle X-ray Scattering (WAXS) was measured using a FR590 diffractometer from ENRAF NONIUS (Cu-K-alpha).
Thermogravimetric analysis (TGA) was performed under synthetic air or nitrogen flow using a NETZSCH TG209-F1. A heating rate of 10 K min−1 was used.
Footnotes |
† Electronic supplementary information (ESI) available: Additional synthetic procedures and analytical data (IR, TGA, gas adsorption analysis). See DOI: 10.1039/c4ra04437a |
‡ Present address: Hochschule Zittau/Görlitz, Fachgruppe Chemie, Theodor-Körner-Allee 16, D-02763 Zittau, Germany. E-mail: j.weber@hszg.de; Fax: +49-3583-61-1740; Tel: +49-3583-61-1705. |
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