**2.6 PIs derived from peptides and amino acids**

A monoamine-substituted version of the above-mentioned, microorganismderived building tool was used to develop an aromatic amino acid, 4-amino-Lphenylalanine. This was used as a core to build a cyclic dipeptide, 3,6-di(4 aminophenylmethyl)-2,5-diketopiperazine through iterative protection/coupling/ deprotection protocols [61].

Several bioPIs incorporating the diketopiperazine heterocyclic structure in the backbone were prepared by polycondensation of the peptide with commercially available aromatic dianhydrides.

Through a judicious design, the cyclic dipeptide monomer contains a centrosymmetric amide functionality in the diketopiperazine ring adjoined by two aromatic rings through a methylenic bridge. This type of architecture is able to induce auto-aggregation of the corresponding polymer chains through hydrogen bonding and π-π interactions. As a consequence, these bioPIs and their PAAs can selfassemble into nanosized conglomerates of various shapes (spheres, spiky balls, flakes, and rods), as observed by scanning electron microscopy (SEM) investigation. This particular feature (and its combination with the innate characteristics of any PI) enables these PIs suitable for applications such as fillers, heat-resistant superhydrophobic coatings, and ultralow-dielectric-constant films.

In addition, the peptide-based PIs display a high thermal resistance, particularly the one derived from pyromellitic dianhydride (PMDA), which showed the highest Td10 (around 432°C) and a Tg value above the decomposition temperature.

The literature provides few examples of other amino acids used as building blocks for bioPIs. For instance, isoleucine, valine, methionine, and phenylalanine were used together with benzimidazole pendant units to build four chiral diamines.

The diamines were polymerized with a particular dianhydride containing a pendant trifluoromethyl segment to attain optically active, aromatic bioPIs.

This resulted in amorphous, versatile PIs with improved solubility and molar masses above 124 kDa. The materials showed accessible Tg (142–165°C), suitable mechanical resilience (97 MPa ultimate strength), and a rather lower thermal stability (Td10 around 255°C) as compared to the PIs presented herein. They were further involved in sol-gel procedures to produce Ti bionanocomposites with improved UV absorption and enhanced gas permeability [62].

### **2.7 PIs derived from bio-based adenine**

Adenine, as an amine-substituted purine, is one of the four building blocks of the DNA's double helix supramolecular structure and therefore an important naturally abundant structural framework for multiple hydrogen bonds [63].

An adenine-containing diamine was synthesized through the nucleophilic substitution of biomass adenine with 4-chloronitrobenzene and the subsequent reduction of the nitro groups. The adenine-containing diamine was then polycondensed in a single-step reaction with the widely used 4,4<sup>0</sup> -(hexafluoroisopropylidene) diphthalic anhydride (6FDA) to obtain a new bioPI.

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

The unique conjugated heterocyclic structure of adenine and its propensity toward hydrogen-bonding interactions generated a bioPI with excellent solubility and outstanding combined (thermal and mechanical) performance. The Tg value was higher than 350°C and the Td10 over 500°C (both in nitrogen and air). The tensile strength was up to 144 MPa, and the elastic modulus exceeded 4.1 GPa. A low dielectric constant of 2.8 (measured at 10 MHz) adds an important feature to this biobased high-performance material. Similar results were obtained when the diamine was combined with another widely employed commercial dianhydride, ODPA [64].

The same adenine-based diamine was used together with PMDA to obtain a new bioPI and study its thermal expansion conduct along the in-plane direction. The PI films proved a rare in-plane thermal contraction feature which maintained its negative nature even above Tg. Infrared spectroscopy studies showed a mutation of the adeninepowered hydrogen bonding from a purine type to carbonyl-based interactions [65].
