Ultrafast Biodegradation Pathway of Polyimides Using Aromatic Diamine with Two Spiro Moieties Derived from Camphor

As a key structural element, polyimides (PIs) are garnering attention as high-functional polymers in the microelectronics and optical fields. Recently, their structural behaviour renders PIs as promising candidates for the formation of charge-transfer complex (CTC). Unfortunate drawbacks of PI-based CTCs are in their insolubility and low degradability. A novel organic diamine monomer bridges this gap. Based on these fundamental phenomena, we demonstrate an environmentally-friendly camphor aromatic diamine (CAD diamine). The efforts described herein shows excellent solubility and thermal stability by introducing two spiro moieties. Ultrafast degradable PIs were further prepared from CAD diamine, which not only prevented the polymer deterioration but also reported ultrafast degradability in a sufficient manner.


Synthesis of Bio-Based Monomers and Polyimides
Camphor Aromatic Diacetal (CAD Diacetal) [1,3]dioxole] (abbreviated as CAD diacetal) was synthesized by reacting camphorquinone and catechol. A three-neck flask was filled with 10 g of camphorquinone, 2.5 equiv of catechol, 5 mol % of PTSA and, 10 mol % of P 2 O 5 as the catalyst, and 200 ml of toluene as the solvent. The mixture was refluxed for 24 h with a Dean-Stark apparatus. After the reaction, the organic layer was extracted with benzene and washed with brine. The organic layer was dried over MgSO 4 , and the solvent was evaporated using a rotary evaporator. The crude product was purified by column chromatography (volume ratio of eluents V EA : V hexane = 1 : 3, R f = 0.8, silica gel), yielding the product as a white solid.

Camphor Aromatic Dinitro (CAD Dinitro)
To a solution of camphor aromatic diacetal (1.50 g, 4.28 mmol) in dichloromethane (15 ml) was added 12 mL of 60% aqueous HNO 3 in dichloromethane (15 mL) at keep the temperature at about 25 °C. The layers were separated, and the aqueous layer was extracted with EA. The combined organic layers were washed with brine, dried (MgSO 4 ), and concentrated in vacuo to give CAD dinitro. The crude product was purified by column chromatography (eluent V EA : V hexane = 1:3, R f = 0.7, silica gel), yielding the product as a yellow solid.

Camphor Aromatic Diamine (CAD Diamine)
Dinitro CAD (2.00 g, 4.54 mmol) in ethyl acetate (40 ml) was added to tin(II) chloride dehydrate (10.00 g, 0.04 mol) at 50 °C. The pH was adjusted to 2.0 by adding 5% of aqueous HCI solution. The combined organic layers were washed with brine and dried with MgSO 4 . The solvent was evaporated using a rotary evaporator. Crude product was purified with column chromatography (eluent V MC :

Homopolyimide as a Reference
In a 50 mL three-necked flask, 2 mmol TFMB was added in approximately 9.14 mL of N, N-dimethylformamide (DMF), stirred until TFMB was dissolved completely. Then the same molar amount of 6FDA was added and the mixture was stirred for 48 h at ambient temperature under nitrogen atmosphere. 4 mmol pyridine (P) and 8 mmol acetic anhydride (A) were added into the above PAA solution, the mixture reacted for 24 h to yield homogeneous solution. Then the solution was poured slowly into methanol for precipitation. The solid was washed thoroughly with methanol, and then redissolved in DMF and precipitated in methanol again to obtain a purified sample. Finally, it was imidized and dried in a vacuum tube at 80 °C.

Degradable Copolyimides
In a 50 mL three-necked flask, 2 mmol TFMB was added in approximately 9.14 mL of N, Ndimethylformamide (DMF), and stirried with a CAD monomer until the given compositions were dissolved completely. Afterward, the same molar amount of 6FDA was added and the mixture was stirred for 48 h at ambient temperature under nitrogen atmosphere. 4 mmol pyridine (P) and 8 mmol acetic anhydride (A) were added for 24 h to yield a homogeneous solution. The solution was poured slowly into methanol for precipitation. The solid was washed thoroughly with methanol, and then redissolved in DMF and precipitated in methanol again to obtain a purified sample. Finally, it was imidized and dried in a vacuum tube at 80 °C. 1

Hydrolysis Procedures
15 mg of PAxCADyI powder was dissolved in a deuterium buffer solution and tetrahydrofuran (THF) (volume ratio, V buffer sol . : V THF = 3.5 : 6.5). The polymers were dried under atmospheric pressure and 80 ℃, for 72 h and finally remained between 15 and 20 mg. The polymers were immersed in 3.5 mL of a citric acid buffer (pH 2.0) at 60 ℃. After the scheduled periods, the films were rinsed with distilled water, dried. The molecular weight of the hydrolyzed polymer was analyzed by Gel permeation chromatography (GPC) and repeatedly confirmed.

Measurements
1 H-NMR spectra were recorded on a Mercury Plus spectrometer at 25 °C operating at 600 MHz. Samples were dissolved in a mixture of deuterated trifluoroacetic acid and chloroform (1:9). The thermal stability of the polymers was determined by thermogravimetric analysis (TGA) with an SDT Q6000 apparatus from TA Instruments. The samples were heated from 30 to 800 °C at a heating rate of 10 °C/ min under a nitrogen flow of 100 mL/min. Thermal data acquisition was carried out using the Thermal Analysis software from TA Instruments. GPC was performed on a Water 1515 isocratic HPLC pump and a refractive index detector (Water 2414) at 40 °C. The samples were eluted with tetrahydrofuran using Agilent PL gel 5 μm Mixed-C Column 7.5 × 300 mm and PS standard was used to establish a calibration curve. 2D-NMR spectra were recorded on a Bruker AVANCE NMR with Temperature BBI operating at 400 MHz. Samples were dissolved in a mixture of deuterated trifluoroacetic acid and chloroform (1:9). Scheme 1 (A) Synthesis of bio-based monomers; CAD diacetal, CAD dinitro and CAD diamine from camphorquinone and (B) polymerization of PA x CAD y I copolyimides from a bio-monomer CAD diamine, 4,4′-(hexafluoroisopropylidene) phthalic anhydride (6FDA) and 2,2'-bis(trifluoromethyl) benzidine (TFMB). (x and y : feeding ratio of TFMB to CAD diamine, x = 100, 95, 90, y = 0, 5, 10) gives preliminary confirmation that CAD dinitro was successfully synthesized. The precise molecular structure of CAD dinitro was further confirmed by 1 H NMR, as shown in Fig. 1 C. The proton signals observed in the region of 1.10-1.23 ppm in this spectrum are attributed to the protons of CAD diacetal, which is in good agreement with the 1 H NMR spectrum of other Camphor compounds that have been previously reported in the literature.

CAD Diamine
We initiated our studies using a CAD diamine, which was obtained from dinitro CAD through a reduction method as depicted in Scheme 1 A. The ketal groups hydrolyze rapidly in acidic conditions [23]. Given that, the structure in which the ketal group is introduced into the polyimide main chain can increase the degradability of the existing PI. The proposed method is also effective with acid-labile ketal groups and free of hazardous reagents.
The CAD diamine monomer was synthesized with a conventional reduction method using SnCl 2 without further purification of the dinitro CAD. [24] The results from 1 H and 2D COSY NMR spectra of CAD diamine, the aromatic and diamine protons at i, i' and j, j' show dihedral coupling with the protons at l, l' and k, k', respectively (Fig. 1B  C). This confirms the presence of two aromatic rings on the rigid CAD diamine moiety. Because of the rigid CAD diamine structure, protons at d, d' and e, e' reflect different chemical shifts and proton h coupled with only one hydrogen. Here we found four multiplets at 2.13, 2.04, 1.76, and 1.58 and a doublet of at f (2.39 ppm). As the mono ketal or non-reacted compounds have different shifts of h protons by deshielding the effect of carbonyl groups, this hints that the

CAD Diacetal
The new bio-based CAD diacetal was prepared from DLcamphorquinone (CQ) following the chemical synthetic route depicted in Scheme 1 A. The acetalization reaction of each acetal group on CQ yielded two aromatic groups by one-pot synthesis. The CAD diacetal is an air-stable white solid. Recent bio-based monomers for improvement of the thermal properties of conventional polymers have several steps for preparation, but CAD diacetal, as it is synthesized in one-step, needs no protection and deprotection steps [22]. Furthermore, the organic acid catalyst was simply removed by washing a work-up process. CAD diacetal was designed to improve the reactivity leading to a high molecular weights polymer. Thermal properties are easily tuned by feed composition ratio of copolymerization.

CAD Dinitro
The new bio-based CAD dinitro was synthesized by electrophilic aromatic substitution with benzenes at both ends of CAD diacetal (Scheme 1 A). Its molecular structure was first characterized by FT-IR. Compared with the FT-IR spectrum of CAD diacetal, a strong absorption band at 1527, 1342 cm − 1 and a strong and sharp absorption at 1300-1150 cm − 1 were detected in the FT-IR spectrum of CAD dinitro. These are assigned to the stretching vibrations of nitro groups and ether linkage of groups, respectively, indicating the nitro groups in CAD dinitro. The above result Fig. 1 Comparison of (A) FT-IR spectra (ATR), (B) 1 H-NMR and (C) 2D COSY NMR spectra (600 MHz, CDCl 3 ) of bio-based monomers; CAD diacetal, CAD dinitro and CAD diamine from camphorquinone. The peaks of amine group (-NH 2 ) appeared at 6.19-6.01 ppm in 1 H NMR and 3460, 3380, 1632 cm − 1 in FT-IR

Thermal Stability of Bio-Based Copolyimides
The thermal behavior of the PA x CAD y I copolyimides was measured by TGA, and the following results are shown in Fig. 3. The TGA curves show that reference PI and the PA x CAD y I copolyimides were considerably stable at high temperatures. The weight loss of copolyimides started above 530 ℃, and these copolymers also decomposed in one main step, leaving less than 10% residue after being heated.

Degradability of Bio-Based Copolyimides
The hydrolytic degradation of the bio-based copolyimides was observed by measuring the molecular weight by designated time frame (Fig. 4 A-4 C). The higher the proportion of the CAD diamine in the copolyimides, the more the molecular weight was reduced. This is because the PA x CAD y I copolyimides contained both ketal groups in the main polymer chain, and the hydrolysis of ketal bonds is faster than the hydrolysis of other bonds in acidic conditions ( Fig. 4D and E).

Conclusion
The novel ketal structure of diamine strongly influenced the degradability of PIs. Since the two spiro structures are located along the main chain of the final copolymer, complex peak originates from the mixture of diastereoisomeric compounds [25].

Bio-Based Copolyimides
PA x CAD y I copolyimides were prepared by a condensation polymerization process from the CAD diamine as depicted in Scheme 1B. The condensation polymerization process was followed by two-step processes. The first step is polycondensation, which was proceeded at a relatively low temperature. Thereafter, chemical imidization was followed to prevent the inevitable by-products. After purification by precipitating the polymer in methanol, the PIs were obtained. The chemical structures and composites of the PIs were analyzed by 1 H NMR, and their molecular weights were measured by GPC (Fig. 2 A). The molar ratios of the resulting PIs were early calculated by 1 H NMR spectra (Fig. 2B  C). In the 1 H NMR analysis of the copolyimides, all signals were assigned to different protons. The peaks of the CAD diamine moiety were assigned at 6.80-6.94 ppm and 2.44-1.10 ppm. The peaks of the 6FDA and TFMB were assigned at 8.12-7.51 ppm. Integration of the proton signal indicated the content of each moiety. The CAD diamine content in the copolyimides was slightly higher than their corresponding feedstock due to the reactivity of CAD diamine. during polymerization in proportion to the content of ketal groups, and decomposes rapidly due to a small amount of the CAD diamine content. The CAD diamine had excellent compatibility with current state-of-art PIs. Degradable the hydrolysis of the ketal group accelerates the decomposition of the polymer by cleavage of the main chain. It should be noted that the decomposition of the polymer can be easily adjusted according to the ratio of the monomer The two copolyimides contained acetal groups in the main chain. The higher the proportion of the CAD diamine in the copolyimides, the faster the molecular weight was reduced Fig. 3 (A) Thermal gravimetric analysis (TGA) and (B) differential thermal analysis (DTA) curves of PA x CAD y I copolyimides measured from 30 to 800 °C at 10 °C /min under nitrogen atmosphere; (C) Thermal stability of PA x CAD y I copolyimides. Temperature at which 5% weight loss (T 5% ) and temperature for maximum degradation rate (T d ) and remaining weight (W) at 800 o C were observed. The weight loss of copolyimides started above 530 o C, and these copolymers also decomposed in one main step, leaving less than 10% residue after being heated