The ring-opening addition reaction of tri-aziridine (AZOH, AZ) and thiol-carboxylic acid (TGA, TLA, MPA) was conducted in EA (monomer concentration: 30 wt%) at room temperature. The reactions were promoted without catalyst and yielded the network polymers. The reactions of AZ and MPA formed cloudy gel while the other reactions successfully yielded porous polymer.
Molecular structure of the porous polymers was studied by FT-IR and CP/MAS NMR spectroscopy. Figure 1 (i) shows FT-IR spectra of AZOH, TGA, and AZOH-TGA porous polymer. An absorption peak derived from secondary amine, formed by ring opening addition of aziridine, was detected at 1578 cm− 1. The small peaks arising from aziridine at 3050 cm− 1 and thiol at 2560 cm− 1 disappeared in the spectrum of the porous polymers. The CP/MAS NMR spectrum of the AZOH-TGA porous polymer and the assignments are shown in Fig. 1 (ii). The peaks derived from β-amino ester and α-tioester indicated that the ring opening addition of aziridine occurred with both the carboxylic and thiol groups of TGA. The other porous polymers showed similar profiles in the FT-IR and CP/MAS NMR spectra.
As described above, the ring-opening addition reaction between the aziridine and carboxylic groups occurs easily under the mild conditions without catalyst due their inherently high reactivity. In connection with the present work, ring-opening addition reactions between the aziridine and the thiol groups have been reported. Leeuwen and co-workers reported a regio-selective addition of thiophenol and aliphatic aziridines derived from norephedrine without catalysts or bases [14]. Similar reactions were reported for thiophenol addition to non-activated aziridines to yield regio-selective b-amino sulfides [15, 16]. The addition of CF3SO3H, a strong protic acid [17], Lewis acids including ZnCl2, Zn (CF3SO3)2, Cu(CF3SO3)2, and Yb(CF3SO3)2 [18], boron trifluoride-diethyl etherate [19], and a organophosphine (tributylphosphine) [20], is known to accelerate the reaction.
Although the reaction between AZOH or AZ with 1,6-hexane dithiol (HDT), an alkane dithiol, was attempted to synthesize the network polymer, no reaction occurred. This result may indicate that the thiol group in thiol-carboxylic acid (TGA, TLA, MPA) is activated by the carboxylic group to facilitate the reaction. The reaction between the aziridine and thiol groups would be promoted by the interaction between the HOMO of the S atom and the LUMO of the aziridine. In order to obtain information on this aspect, molecular structures of TGA, TLA, MPA, and HDT were optimized by DFT calculations. Figure 2 shows the optimized structures with molecular orbital representation. In the structures of TGA, TLA, and MPA, the thiol group is located in the vicinity of the carbonyl group, and orbital-orbital interactions between S and C = O groups would be expected. In a sharp contrast, such interactions are absent for HDT. The higher reactivities of TGA, TLA, and MPA than HDT can thus be reasonably interpreted.
SEM images of the AZOH based porous polymers are shown in Fig. 3. The AZOH-TGA and AZOH-TLA porous polymers (monomer concentration: 30 wt%) showed the surface morphology composed by connected particles, whose average diameters were 2.7 µm or 2.6 µm, respectively. AZOH-MPA porous polymer was formed by the connected particles with larger size in comparison with those in the AZOH-TGA and AZOH-TLA porous polymers. The diameters of the particles in the AZOH-MPA porous polymer decreased with increasing in the monomer concentration in the reaction systems, as shown in Figs. 4 (c), (d), and (e). The expanded SEM image of AZOH-MPA porous polymer, prepared from the reaction solution with 40 wt% monomer concentration, cleared the porous morphology of co-continuous structure whose backbones were formed by connected small particles less than 0.2 µm in the diameter. Figure 4 shows the SEM images of AZ based porous polymers. The surface morphology of these porous polymers was like those of the corresponding porous polymers with AZOH, as shown in Fig. 3. The averaged diameter of the particles in the AZ-TGA or AZ-TLA porous polymer was 5.3 µm or 3.5 µm, respectively, which was slightly larger than that of the corresponding porous polymers with AZOH.
The morphology of these porous polymers should be formed by polymerization induced phase separation via spinodal decomposition (SD). A phase separation model of the present reaction systems via SD is illustrated in Scheme 2. The co-continuous monolithic structure is formed at early stage of the phase separation. The progress of the phase separation transfers the morphology from the co-continuous monolithic structure to the particles due to the interfacial tension of the droplets, and their size increase with progress of the phase separation. The half-fused particles were fixed at the transition stage from the co-continuous structure (early stage of SD) to the isolate particles (late stage of SD). The connected particles with small diameters, as shown in Fig. 3 (f), are formed at this transition stage. High monomer concentration (40 wt%) of AZOH-MPA system should preferentially accelerate the polymerization (fixation) rate in the phase separation, which yielded the porous polymer with the co-continuous structure, as illustrated in Scheme 2 (i). The size of the particles should be depended on the phase separation morphology at the fixation period. In the case of the AZOH based porous polymers obtained from the reaction systems with 30 wt% of the monomer concentration, Figs. 3 (a), (b), and (d), the AZOH-MPA porous polymer showed larger averaged particle size, as shown in Fig. 3 (d). One explanation of the result is that the lower crosslinking density derived from longer molecular length of MPA would decelerate the polymerization (fixation). As the result, the relative phase separation rate would increase in the reaction, and the phase separation at the later stage composed by larger particle size should be formed, as illustrated in Scheme 2 (iii).
Mechanical properties of the porous polymers were investigated by compression test. Figure 5 shows stress-strain curves of AZOH-TGA, AZOH-TLA, and AZOH-MPA porous polymers prepared in the reaction systems with the monomer concentration of 30 wt%. The results are summarized in Table 1. The order of the Young’s modulus of the porous polymers is as follows: AZOH-TLA > AZOH-TGA > AZOH-MPA. The similar tendency was observed in AZ based porous polymers. Both the small particle size and high bulk density should increase the Young’s modulus of the porous polymers with TLA. Another possibility to explain the results would be derived from molecular structure of the thiol-carboxylic acid used. Existence of a methyl group in TLA hinders rotation of C-C linkage between the thiol and carboxylic acid and would induce rigidity in the network, which should heighten the Young’s modulus. The AZ-TLA and AZ-TGA porous polymers showed lower Young’s modulus in comparison with those of the corresponding AZOH based porous polymers despite the higher bulk density. The larger particle size in the AZ based porous polymers should decrease the Young's modulus. The Young’s modulus of the AZOH-MPA porous polymer increased with increasing of the monomer concentration in the reaction systems due to the decrease of the particle size and increase of the bulk density.
Table 1
Structure and mechanical properties of the tri-aziridine and thiol-carboxylic acid porous polymers
Run
|
Aziridine
|
Thiol-
carboxylic acid
|
Monomer
conc. a
[wt%]
|
Particle
size
[µm]
|
Bulk
density
[g/cm3]
|
Young's modulus [kPa]
|
Td5
[ºC]
|
Td50
[ºC]
|
1
|
AZOH
|
TGA
|
30
|
2.7
|
0.318
|
226
|
163
|
373
|
2
|
AZOH
|
TLA
|
30
|
2.6
|
0.375
|
685
|
174
|
375
|
3
|
AZOH
|
MPA
|
20
|
7.4
|
0.205
|
51.4
|
|
|
4
|
AZOH
|
MPA
|
30
|
6.2
|
0.348
|
75.4
|
184
|
348
|
5
|
AZOH
|
MPA
|
40
|
< 0.2
|
0.579
|
414
|
|
|
6
|
AZ
|
TGA
|
30
|
5.3
|
0.388
|
96.3
|
162
|
342
|
7
|
AZ
|
TLA
|
30
|
3.5
|
0.457
|
136
|
168
|
347
|
a Monomer concentration in the reaction solution.
Thermal stability of the porous polymers was investigated by TGA (Fig. S1). Weight loss of all the porous polymers gradually began at round 160 ºC (5 wt% weight loss was attained at the temperatures ranged from about 160 ºC to 180 ºC). The weight loss promoted more than 200 ºC, and 50 wt% weight loss was attained at the temperatures ranged from about 350 ºC to 375 ºC. These phenomena should be derived from thermal-oxidative degradation of ester groups originated from tri-aziridine and thiol-carboxylic acid compounds.
The porous polymers absorbed various organic solvents. The porous polymers which absorbed methanol (MeOH) were gradually getting smaller. The weight of the porous polymers immersed in MeOH was traced (Fig. 6). The weight of the porous polymers increased by absorption of MeOH and reached the maximum values within a few days. After that, the weight of the porous polymer rapidly decreased, and all the porous polymers completely degraded within 50 days. The porous polymers obtained from the addition reaction of AZ and dicarboxylic acid were not degraded in MeOH [13]. The degradability of the present porous polymers should be derived from a-thioester group and d b-thioester groups in the polymer network, which were formed by the ring opening reaction between aziridine and thiol group of the thiol-carboxylic acid compounds [21–23]. The degradation rate of the AZOH based porous polymers is as follows: MPA > TLA > TGA. The high degradability of the AZOH-MPA porous polymer should be derived from lower crosslinking density in the polymer network owing to larger methylene length in MPA. Difference in the degradability between a-thioester group, derived from TGA or TLA, and b-thioester, derived from MPA, would be another possibility to explain the degradation rates. The AZ based porous polymers showed higher degradation rates in comparison with the corresponding AZOH based porous polymers. The affinity between the AZOH and MeOH might be higher than that of AZ and MeOH due to the OH group in AZOH. However, the degradation rates of the porous polymers showed opposite order to the affinity. One explanation of the results may be that smaller particles’ size, which provides larger surface area, in the AZOH based porous polymer accelerates the degradation. Lower bulk density, which means larger space, of the AZOH based porous polymer also would increase the degradation rates.
Molecular structure of the degradation products, after evaporation of MeOH following drying under the reduced pressure, of the porous polymers was studied by 1H NMR spectroscopy. The degradants were low viscous liquid and were easily soluble in CDCl3. The signals derived from methyl ester, thioester, and methylene adjacent to hydroxy group were detected in the NMR spectra (Fig. S2), suggesting that the degradation products contain alcohol compounds including (ii) in Fig. S2. These results indicate that the thioester groups in the polymer network were degraded by a transesterification reaction with MeOH under the ambient conditions. Degradation products having two or more -OH functions may possibly be used as monomers for the synthesis of polyurethanes and polyesters, which implies that the materials prepared in this work may be regarded as recyclable polymers.