*2.1.1 PIs derived from cardanol*

Cardanol is a phenolic lipid obtained from an anacardic acid, the main component of cashew nutshell liquid, and the starting point in the development of many interesting organic building blocks [36].

A partially bio-based aromatic diamine, namely 1,1-bis(4-aminophenyl)-3 pentadecylcyclohexane, was synthesized starting from cardanol via a 3-pentadecylcyclohexanone intermediate in the presence of aniline and aniline hydrochloride under reflux conditions.

This bulky, unsymmetrical diamine was used in the development of processable PIs with reasonably high molar masses in the range of 15–32 kDa [36].

The obtained polyimides and copolyimides were soluble in common organic solvents and maintained high thermal stability, with a glass transition temperature (Tg) in the 160–250°C framework (depending on the dianhydride partner) and a 10% mass loss temperature (Td10) recorded above 500°C.

### *2.1.2 PIs derived from isomannide*

• Another approach used biomass isomannide to develop both bio-based diamine and dianhydride monomers and optically transparent bioPIs therefrom [37]. The two hydroxylic groups of isomannide were used as the starting points of various chemical transformations to obtain one cycloaliphatic and two semiaromatic diamines, together with a flexible dianhydride.

The diamines were used in combination with the bio-based isomannide anhydride or with the commercial 4,4-oxydiphthalic anhydride (ODPA) to produce completely or partially bio-based PIs through the classic two-step method. The ordered arrangement of the isomannide heterocycle conveyed a certain degree of crystallization in the PI framework and afforded materials only soluble in common, high boiling point solvents.

The relatively rigid alicyclic isomannide imparted good optical transparency (transmittances above 80% at 450 nm), reasonably high thermal resistance (Tg between 227 and 264°C, Td10 greater than 430°C in nitrogen) and outstanding mechanical features (tensile strength above 90 MPa, elongation at break over 6%). Surprisingly, the use of the fully alicyclic monomer generated PIs of superior thermal stability when compared to the ones containing the semi-aromatic bio-based diamines.

Two of the isomannide-based diamines were further used in both their isomeric forms in combination with other six commercial dianhydrides. As expected, similar results were obtained: soluble, processable bioPIs with thermo-mechanical stability comparable with those of analogous petroleum-based PIs. They also provide the additional features of optical transparency and optical activity which qualify them for applications like liquid crystal alignment and solar cells [38, 39].

### *2.1.3 PIs derived from myo-inositol (MI)*

Myo-inositol or cyclohexanehexol is a naturally occurring cyclohexane decorated with six hydroxylic groups that is widely encountered in animals and plants. One of the many stereoisomers of inositol, it is the most commonly used biologically active isomer [40].

Until now, a large number of bioactive molecules bearing inositol-derived components were used as building blocks in developing macromolecular backbones

*New High-Performance Materials: Bio-Based, Eco-Friendly Polyimides DOI: http://dx.doi.org/10.5772/intechopen.93340*

containing diols, like polyurethanes [41] or polyamides [42]. It was a matter of time until it would have been used to prepare polyimidic architectures through its incorporation in diamines [15, 16, 28, 40].

For example, myo-inositol was treated with 1,1-dimethoxycyclohexane to yield a heterocyclic compound with three alicyclic units, the central one bearing two free hydroxyl groups [42]. These hydroxyl moieties are further used to introduce two aromatic amine functions through substitution and subsequent reduction reactions [40].

The target diamine readily underwent polycondensation with widely used, commercial dianhydrides in microwave conditions [40].

The resulting bioPIs, whose main chains inherited the bulky 6-5-6-5-6 polyalicyclic system from the diamine monomers, are soluble in common organic solvents at room temperature, even in chloroform, dichloromethane, and acetone. They displayed quite respectable molar masses, between 40 and 99 kDa, and relatively narrow polydispersities, with PDI values below 1.6. All bioPI films displayed characteristic thermal stability and outstanding mechanical features (tensile modulus above 3.5 GPa), even higher than those of most petroleum-based PIs. Their most important traits are the optical ones (light coloration and high transmittance values [for a PI], above 82%) and are also derived from the voluminous cycloaliphatic segment and its ability to hamper the charge-transfer complex usually found in PI materials.

### **2.2 PIs derived from anethole**

Anethole is a natural styrene analog that is widely found in various in essential oils from plants and can be used in a peculiar preparation protocol (polymerization of Wagner-Jauregg type monomers containing preformed imide rings) of bio-based PIs [43].

Starting from this phenylpropene derivative, a new multicyclic monomer containing Wagner-Jauregg type imide motifs was synthesized via cascade, double Diels-Alder reaction. The polycondensation reaction of this diphenol-type monomer between and decafluorobiphenyl yielded a colorless and transparent (86% transmittance at 450 nm) poly(ether imide) film.

The partial bioPI showed reasonably high molar mass (72 kDa, 1.6 dispersity), sound thermal stability (5% weight loss [Td5] starting above 410°C, Tg higher than 360°C) and above reasonable mechanical characteristics (1.90 GPa tensile strength, 53.3 MPa elastic modulus, 5.4% elongation). These traits make them eligible for various optoelectronic applications in harsh conditions.

### **2.3 PIs derived from vanillin and its derivatives**

Vanillin is one of the few commercially available, bio-based, aromatic compounds, and therefore, it is widely used in the polymer community dealing with bio-based macromolecular materials, PIs included [44, 45].

Vanillin was used as the starting point in the preparation of two dimers further used in two synthetic pathways for the development of bio-based aromatic diamines.

The first one involves three steps: phenol alkylation, oxidation of divanillin's aldehyde moieties, and subsequent reduction of the obtained oxime toward a methylated divanillylamine [46].

The second route uses the Curtius rearrangement and employs the synthesis of an acyl azide intermediate from divanillic acid, its transformation into a diisocyanate, and further hydrolysis toward another divanillin derivative decorated with two amino groups.

Another sequential synthetic scheme was used to prepare a diisocyanate starting from vanillic acid and its methylated dimer.

Therefore, these accessible synthetic transformations afford the preparation of a broad range of bio-based PI building blocks and corresponding polymers. One particularity of these procedures is that they unlock bio-based isocyanates which can be further used in combinations with dianhydrides to obtain bioPIs [45].

This series of partially bioPIs shows moderate molar masses (Mw from 49.5 to 75.8 kDa), characteristic polydispersity (2.0–2.5), acceptable solubility, and good thermal stability (Tg between 260 and 330°C, Td10 around 460°C).

The ability of vanillic acid to act as a versatile building block was explored even further together with another lignin-derivative, syringic acid, in the development of diacylhydrazides. These were then used in a two-step polycondensation procedure with aromatic anhydrides to generate partially bio-based poly(amide imide)s. The new materials provided transparent, flexible, and tough films with high thermal stability [47].

A simpler synthetic pathway can be also used to transform vanillin into an asymmetric diamine.

The aminic building block was then used to build partially bio-based PIs with four conventional, commercial dianhydrides and the results were compared with an analogous series based on a symmetric aromatic amine.

As expected, the use of the asymmetric structural moiety hampers the formation of charge-transfer complexes while vanillin's aromatic structure maintains backbone rigidity. This is translated into improved solubility, optical transparency, and hydrophobicity, without any negative impact upon thermal or mechanical stability [48].

The vanillin route was also pursued to develop a bio-based diamine containing aromatic, pyridine, and aliphatic structural moieties. This was used in combination with an alicyclic dianhydride, Epiclon, to obtain a semi-aromatic bioPI through the two-stage route.

The resulting flexible PI backbone afforded easily processable films with improved solubility and proper chemical and thermal stability (Td10 around 317°C). The PAA precursor was also employed to prepare a series of Ag nanocomposites by sonication. The casted films displayed a homogeneous dispersion of Ag nanoparticles within the bioPI matrix due to the high compatibility of the composites' elements. This resulted in Ag-induced, partial crystallinity, improved thermal properties (Td10 from 317°C up to 357°C, depending on the Ag amount), and antibacterial activity against *E. coli* [49].

### **2.4 PIs derived from camphor**

Another bio-renewable, natural resource, camphor, a waxy, alicyclic forestry product was used in its (D) form to produce two diamine building blocks via an accessible synthetic route [50, 51]. The diamine was further condensed with two aromatic dianhydrides to prepare semi-alicyclic PIs with high solubility and optical transparency by the conventional two-step method.

The partially bio-based PIs were comparable to their fully aromatic counterparts (based on the combination of the similar rigid diamines with the same dianhydrides) in terms of solubility, optical features, thermal, and mechanical resilience. They provided transmittance values between 75 and 81% at 500 nm, Td10 from 390 to 519°C, tensile strength above 110 MPa, and elastic modulus in the range 2.4–3.24 GPa.

### **2.5 PIs derived from** *Escherichia coli*

Bio-available aromatic diamines were derived from genetically manipulated *Escherichia coli*, through a photodimer of an aromatic amino acid, namely

*New High-Performance Materials: Bio-Based, Eco-Friendly Polyimides DOI: http://dx.doi.org/10.5772/intechopen.93340*

4-aminocinnamic acid. This was produced by the fermentation and bioconversion of the genetically engineered microorganism from glucose via 4-aminophenylalanine [52].

The diamine displaying an alicyclic structure sandwiched between two aromatic rings was synthesized via a 4,40 -diamino-α-truxillic acid dihydrochloride, obtained through a [2+2] photocycloaddition of the microorganism-derived monoamine's hydrochloride salt.

This new building block was polycondensed afterward with different common dianhydrides following the two-stage procedure.

The bio-based PAAs displayed high molar masses, between 250 and 400 kDa, while the inadequate solubility of the thermally cyclized PIs precluded molecular mass determination. These bio-based PI films showed ultrahigh thermal resistance with Td10 values over 425°C and no Tg values below 350°C, which are the highest thermal features of all bio-based plastics reported thus far. They also showed high tensile strength and Young's moduli, excellent transparency, and high refractive indices, and adequate cell compatibility [53].

The transparent bioPI based on 1,2,3,4-cyclobutanetetracarboxylic dianhydride showed electrical insulative properties comparable to those of Kapton®, the most used PI dielectric. As in the case of the commercial PI, the volume resistivity proved to be directly connected to the annealing time and water uptake [54].

The precursor of this bioPI was used to prepare silica hybrids by sol-gel polycondensation with silicon alkoxide and in vacuo thermal annealing. The method generated transparent, thermo-mechanically robust films with excellent electrical stability [55].

The applicative potential of this bioPI was explored even further, by developing bionanohybrids through the sputtering of ITO nanolayers on the functionalized, reactive surface of a bioPI. The obtained materials displayed thermal, mechanical, electrical, optical, and adhesions performances comparable to or superior (especially in terms of optical transparency, ITO adherence, and device resistivity) to the extensively employed Kapton® PI film. This allowed the development of flexible, robust, and transparent electrodes for high tech electronic devices [56].

Other cycloaliphatic dianhydrides were also polymerized with the bio-based diamine coming from 4-aminocinnamic acid to draw a correlation between the PIs' solubility and the structural motifs belonging to the dianhydride.

The study concluded that a lower cycloaliphatic ring strain determines PI microstructures with improved flexibility and reduced Tg. All semi-aromatic bioPIs maintained relatively high molar masses (50–80 kDa) and displayed improved solubility and processability while preserving high thermal stability (Td10 temperatures above 375°C) [57, 58]. One of them was also used to develop highly transparent, flexible TiO2 and ZrO2 hybrid films that display the basic features of memory devices with tunable memory properties.

Ductile bioPI films were obtained from the same renewable semi-aromatic diamine by copolymerization with different binary mixtures of the aforementioned dianhydrides [59]. The copolymers displayed high-performance features comparable to those of the Kapton® film. For example, they maintained a high thermal resistance (Td10 values above 406°C and Tg over 208°C), improved tensile strength and elongation at break, and a Young's modulus around 4 GPa. Also, the copolymers showed adhesion strength to the carbon plate in the range of 0.22– 4.47 MPa, which is similar to that of cyanoacrylate-based superglues [59].

The same bio-based, exotic building block, 4-aminocinnamic acid, was employed to develop two other diamines, 4, 4<sup>0</sup> -diaminostylbene and its hydrogenated version, by using Grubb's olefin metathesis as a key step.

The two diamines were further polymerized via two-stage polycondensation with the same dianhydrides as the photodimer ester coming from 4-aminocinnamic acid. High molecular masses PAAs were obtained both as films and fibrils and their thermal imidization afforded partially bio-based PIs with high thermal stability and mechanical properties superior to Kapton® [60].
