**Abstract**

The development of high-performance bio-based polyimides (PIs) seems a difficult task due to the incompatibility between petrochemical-derived, aromatic monomers and renewable, natural resources. Moreover, their production usually implies less eco-friendly experimental conditions, especially in terms of solvents and thermal conditions. In this chapter, we touch some of the most significant research endeavors that were devoted in the last decade to engineering naturally derived PI building blocks based on nontoxic, bio-renewable feedstocks. In most cases, the structural motifs of natural products are modified toward amine functionalities that are then used in classical or nonconventional methods for PI synthesis. We follow their evolution as viable alternatives to traditional starting compounds and prove they are able to generate eco-friendly PI materials that retain a combination of high-performance characteristics, or even bring some novel, enhanced features to the field. At the same time, serious progress has been made in the field of nonconventional synthetic and processing options for the development of PI-based materials. Greener experimental conditions such as ionic liquids, supercritical fluids, microwaves, and geothermal techniques represent feasible routes and reduce the negative environmental footprint of PIs' development. We also approach some insights regarding the sustainability, degradation, and recycling of PI-based materials.

**Keywords:** bio-based polyimides, renewable monomers, polyimide recycling, ionic liquids, supercritical fluids, microwave synthesis, geothermal synthesis

## **1. Introduction**

Long-established, commercial or innovative, tailor-made, functional polyimides (PIs) represent a benchmark for high-performance polymers and demonstrate many advantages, which include excellent thermo-oxidative stability, chemical inertness, high mechanical resistance, high dielectric strength, as well as a remarkable combination of thermal, mechanical, and electrical insulating properties [1–4]. Therefore, these heterocyclic polymers have found applications in many advanced technology industries, such as aviation, spaceflight, microelectronics (printed and integrated circuit board, flexible chip carriers) [5], composite materials [6], automotive, packaging industries, or separation membranes [7]. Moreover, PIs' biocompatibility was heavily explored in the last two decades and polyimide-based materials entered the high-demanding area of biomedical applications, such as cell

substrates, retina stimulation implants, cortical recordings, neural stimulation devices, and others [8].

This outstanding collection of traits mainly comes from their highly symmetrical backbones, extreme structural rigidity, and intra- and inter-chain hydrogen bonding. However, these features also imply some intrinsic weaknesses such as poor solubility in organic media, low thermal coefficient, poor corona resistance, relatively high thermal expansivity and, very important from the industrial point of view, difficult and expensive processability, which therefore restrict some of their potential applications [9–11].

PIs usually consist of linear, planar, (fully or partially) aromatic architectures that contain several flexible (bulky or pendant), heteroaliphatic structural elements which can provide additional features [12–14]. Completely aliphatic and crosslinked versions are also available.

Traditionally, linear PIs are prepared by a classical, two-step polycondensation method via a soluble poly(amic acid) (PAA) intermediate [15, 16].

The overall process displays a stepwise mechanism (a polyaddition alternative is also scarcely used in some labs) and runs at a rate dictated by the reactivity of the starting compounds. The formation of the PAA precursor is usually performed in common dipolar amidic solvents such as DMF (N, N-dimethylformamide), DMAc (N, N-dimethylacetamide), NMP (N-methyl-2-pyrrolidinone), TMU (1,1,3,3 tetramethylurea) which act as Lewis "bases," or in other nonamidic solvents such as DMSO (dimethyl sulfoxide) and m-cresol at low temperatures (from room temperature up to 80–100°C, depending on the basicity of the aromatic amines).

The cyclodehydration is conducted either by thermal or chemical imidization, both pathways being effective for either soluble or insoluble PIs. The former implies the stepwise heating of PAAs at various temperatures (usually from 100°C up to 250°C, depending on the polyimide's structure and the solvent used during synthesis) for various periods (mainly 1 h at each temperature). The latter involves the use of a chemical dehydrating agent of acidic (acetic anhydride, trifluoroacetic anhydride, and formic acid) or aminic (pyridine and trialkyl amines) nature to promote ring closure at temperatures between 20 and 80°C [17].

A one-step synthetic pathway is also used for the small-scale, in-house preparation of soluble polyimidic materials. This is based on the polycondensation of the monomers at high temperatures without the isolation of the PAA precursor [18].

Besides the widely employed reaction of (at least) a dianhydride with (at least) a diamine, an alternative that uses a dianhydride and diisocyanate is also known [19–21].

In the case of cross-linked PIs, the synthesis follows the same routes as above, with the usual additional presence of a diamine or triamine as a cross-linker in the PAA preparation stage [22].

Other functions may be also used in the development of cross-linked PIs, such as a diphenylethynylene structure along the main chain [23], or the use of a benzoxazine-containing monomer [24]. However, it must be noted that the threedimensional, cross-linked structures cannot be recycled.

Given the experimental conditions employed in the synthesis of heterocyclic polymers in general, PIs included, it is difficult to envisage synthetic pathways or experimental conditions obeying the strict criteria of green chemistry, ecological synthesis, or eco-friendly polymers.

However, due to their specific macromolecular architectures, linearity, and rigid arrangement, PIs can be already considered as long-life, and therefore sustainable materials, that are however not produced by eco-friendly methods, especially in terms of solvents (some of them are not so green) and thermal conditions (which imply a high energy intake).

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

At this point, a special comment must be inscribed to the starting building blocks. The majority of dianhydride- and diamine-type monomers used in the synthesis of PIs are produced using petroleum-based chemicals as the raw materials. Furthermore, some of these monomers (especially some diamines like methylene dianiline, or the less used isocyanates) and their intermediates are highly toxic, carcinogenic, or endocrine-disrupting [25]. Therefore, considerable research endeavors have been devoted to achieving benign polyimide building blocks based on nontoxic, bio-renewable feedstocks. At the same time, long-term efforts were (and still are, as it will be detailed later on) dedicated to finding less toxic experimental conditions and synthetic pathways.

Considering all of the above, it seems difficult at first to come close to the Holy Grail of bio-based and/or green polyimide chemistry. However, in this chapter, we seek to demonstrate that there are some viable, strong alternatives to the traditional polyimide building blocks or classical experimental conditions. Moreover, these can unlock more eco-friendly, bio-based PI materials (even by using green chemistry routes) which maintain the same combination of highperformance characteristics and even bring some new or enhanced features to the field. We will mainly focus on bio-based polyimides developed in the last 10 years from natural, renewable resources and will also touch the basics of some less harmful synthetic platforms like synthesis in ionic liquids, supercritical fluids, microwave conditions, and other nonconventional methods. Some interesting insights regarding the sustainability, degradation, and recycling of PI-based materials are also provided.
